cpython-lru_cache-weakref: Doc/tutorial/datastructures.rst annotate

annotate Doc/tutorial/datastructures.rst @ 79675:c0a2294de787 2.7

Issue #16199: Backport 2to3 typo fix from 3.2.

author	Chris Jerdonek <chris.jerdonek@gmail.com>
date	Thu, 11 Oct 2012 16:15:46 -0700
parents	bb63919cde6e
children	dc006b6212e7

rev	line source
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	1 .. _tut-structures:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	2
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	3 ***************
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	4 Data Structures
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	5 ***************
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	6
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	7 This chapter describes some things you've learned about already in more detail,
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	8 and adds some new things as well.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	9
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	10
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	11 .. _tut-morelists:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	12
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	13 More on Lists
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	14 =============
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	15
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	16 The list data type has some more methods. Here are all of the methods of list
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	17 objects:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	18
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	19
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	20 .. method:: list.append(x)
45972 c45a45669017 Mark the descitems in the tutorial as "noindex" so that :meth: cross-refs don't link to them. Georg Brandl <georg@python.org> parents: 44383 diff changeset	21 :noindex:
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	22
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	23 Add an item to the end of the list; equivalent to ``a[len(a):] = [x]``.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	24
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	25
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	26 .. method:: list.extend(L)
45972 c45a45669017 Mark the descitems in the tutorial as "noindex" so that :meth: cross-refs don't link to them. Georg Brandl <georg@python.org> parents: 44383 diff changeset	27 :noindex:
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	28
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	29 Extend the list by appending all the items in the given list; equivalent to
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	30 ``a[len(a):] = L``.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	31
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	32
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	33 .. method:: list.insert(i, x)
45972 c45a45669017 Mark the descitems in the tutorial as "noindex" so that :meth: cross-refs don't link to them. Georg Brandl <georg@python.org> parents: 44383 diff changeset	34 :noindex:
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	35
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	36 Insert an item at a given position. The first argument is the index of the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	37 element before which to insert, so ``a.insert(0, x)`` inserts at the front of
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	38 the list, and ``a.insert(len(a), x)`` is equivalent to ``a.append(x)``.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	39
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	40
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	41 .. method:: list.remove(x)
45972 c45a45669017 Mark the descitems in the tutorial as "noindex" so that :meth: cross-refs don't link to them. Georg Brandl <georg@python.org> parents: 44383 diff changeset	42 :noindex:
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	43
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	44 Remove the first item from the list whose value is x. It is an error if there
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	45 is no such item.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	46
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	47
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	48 .. method:: list.pop([i])
45972 c45a45669017 Mark the descitems in the tutorial as "noindex" so that :meth: cross-refs don't link to them. Georg Brandl <georg@python.org> parents: 44383 diff changeset	49 :noindex:
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	50
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	51 Remove the item at the given position in the list, and return it. If no index
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	52 is specified, ``a.pop()`` removes and returns the last item in the list. (The
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	53 square brackets around the i in the method signature denote that the parameter
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	54 is optional, not that you should type square brackets at that position. You
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	55 will see this notation frequently in the Python Library Reference.)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	56
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	57
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	58 .. method:: list.index(x)
45972 c45a45669017 Mark the descitems in the tutorial as "noindex" so that :meth: cross-refs don't link to them. Georg Brandl <georg@python.org> parents: 44383 diff changeset	59 :noindex:
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	60
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	61 Return the index in the list of the first item whose value is x. It is an
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	62 error if there is no such item.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	63
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	64
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	65 .. method:: list.count(x)
45972 c45a45669017 Mark the descitems in the tutorial as "noindex" so that :meth: cross-refs don't link to them. Georg Brandl <georg@python.org> parents: 44383 diff changeset	66 :noindex:
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	67
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	68 Return the number of times x appears in the list.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	69
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	70
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	71 .. method:: list.sort()
45972 c45a45669017 Mark the descitems in the tutorial as "noindex" so that :meth: cross-refs don't link to them. Georg Brandl <georg@python.org> parents: 44383 diff changeset	72 :noindex:
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	73
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	74 Sort the items of the list, in place.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	75
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	76
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	77 .. method:: list.reverse()
45972 c45a45669017 Mark the descitems in the tutorial as "noindex" so that :meth: cross-refs don't link to them. Georg Brandl <georg@python.org> parents: 44383 diff changeset	78 :noindex:
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	79
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	80 Reverse the elements of the list, in place.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	81
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	82 An example that uses most of the list methods::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	83
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	84 >>> a = [66.25, 333, 333, 1, 1234.5]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	85 >>> print a.count(333), a.count(66.25), a.count('x')
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	86 2 1 0
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	87 >>> a.insert(2, -1)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	88 >>> a.append(333)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	89 >>> a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	90 [66.25, 333, -1, 333, 1, 1234.5, 333]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	91 >>> a.index(333)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	92 1
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	93 >>> a.remove(333)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	94 >>> a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	95 [66.25, -1, 333, 1, 1234.5, 333]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	96 >>> a.reverse()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	97 >>> a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	98 [333, 1234.5, 1, 333, -1, 66.25]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	99 >>> a.sort()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	100 >>> a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	101 [-1, 1, 66.25, 333, 333, 1234.5]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	102
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	103
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	104 .. _tut-lists-as-stacks:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	105
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	106 Using Lists as Stacks
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	107 ---------------------
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	108
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	109 .. sectionauthor:: Ka-Ping Yee <ping@lfw.org>
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	110
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	111
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	112 The list methods make it very easy to use a list as a stack, where the last
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	113 element added is the first element retrieved ("last-in, first-out"). To add an
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	114 item to the top of the stack, use :meth:`append`. To retrieve an item from the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	115 top of the stack, use :meth:`pop` without an explicit index. For example::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	116
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	117 >>> stack = [3, 4, 5]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	118 >>> stack.append(6)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	119 >>> stack.append(7)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	120 >>> stack
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	121 [3, 4, 5, 6, 7]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	122 >>> stack.pop()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	123 7
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	124 >>> stack
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	125 [3, 4, 5, 6]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	126 >>> stack.pop()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	127 6
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	128 >>> stack.pop()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	129 5
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	130 >>> stack
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	131 [3, 4]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	132
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	133
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	134 .. _tut-lists-as-queues:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	135
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	136 Using Lists as Queues
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	137 ---------------------
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	138
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	139 .. sectionauthor:: Ka-Ping Yee <ping@lfw.org>
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	140
56655 5d8b2d6dcd62 Update advice on how to implement a queue. Raymond Hettinger <python@rcn.com> parents: 55049 diff changeset	141 It is also possible to use a list as a queue, where the first element added is
5d8b2d6dcd62 Update advice on how to implement a queue. Raymond Hettinger <python@rcn.com> parents: 55049 diff changeset	142 the first element retrieved ("first-in, first-out"); however, lists are not
5d8b2d6dcd62 Update advice on how to implement a queue. Raymond Hettinger <python@rcn.com> parents: 55049 diff changeset	143 efficient for this purpose. While appends and pops from the end of list are
5d8b2d6dcd62 Update advice on how to implement a queue. Raymond Hettinger <python@rcn.com> parents: 55049 diff changeset	144 fast, doing inserts or pops from the beginning of a list is slow (because all
5d8b2d6dcd62 Update advice on how to implement a queue. Raymond Hettinger <python@rcn.com> parents: 55049 diff changeset	145 of the other elements have to be shifted by one).
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	146
56655 5d8b2d6dcd62 Update advice on how to implement a queue. Raymond Hettinger <python@rcn.com> parents: 55049 diff changeset	147 To implement a queue, use :class:`collections.deque` which was designed to
5d8b2d6dcd62 Update advice on how to implement a queue. Raymond Hettinger <python@rcn.com> parents: 55049 diff changeset	148 have fast appends and pops from both ends. For example::
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	149
56655 5d8b2d6dcd62 Update advice on how to implement a queue. Raymond Hettinger <python@rcn.com> parents: 55049 diff changeset	150 >>> from collections import deque
5d8b2d6dcd62 Update advice on how to implement a queue. Raymond Hettinger <python@rcn.com> parents: 55049 diff changeset	151 >>> queue = deque(["Eric", "John", "Michael"])
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	152 >>> queue.append("Terry") # Terry arrives
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	153 >>> queue.append("Graham") # Graham arrives
56655 5d8b2d6dcd62 Update advice on how to implement a queue. Raymond Hettinger <python@rcn.com> parents: 55049 diff changeset	154 >>> queue.popleft() # The first to arrive now leaves
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	155 'Eric'
56655 5d8b2d6dcd62 Update advice on how to implement a queue. Raymond Hettinger <python@rcn.com> parents: 55049 diff changeset	156 >>> queue.popleft() # The second to arrive now leaves
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	157 'John'
56655 5d8b2d6dcd62 Update advice on how to implement a queue. Raymond Hettinger <python@rcn.com> parents: 55049 diff changeset	158 >>> queue # Remaining queue in order of arrival
5d8b2d6dcd62 Update advice on how to implement a queue. Raymond Hettinger <python@rcn.com> parents: 55049 diff changeset	159 deque(['Michael', 'Terry', 'Graham'])
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	160
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	161
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	162 .. _tut-functional:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	163
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	164 Functional Programming Tools
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	165 ----------------------------
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	166
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	167 There are three built-in functions that are very useful when used with lists:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	168 :func:`filter`, :func:`map`, and :func:`reduce`.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	169
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	170 ``filter(function, sequence)`` returns a sequence consisting of those items from
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	171 the sequence for which ``function(item)`` is true. If sequence is a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	172 :class:`string` or :class:`tuple`, the result will be of the same type;
72513 f612b783bc17 Doc fix. Mathematically correct sentence. Senthil Kumaran <senthil@uthcode.com> parents: 68256 diff changeset	173 otherwise, it is always a :class:`list`. For example, to compute a sequence of
f612b783bc17 Doc fix. Mathematically correct sentence. Senthil Kumaran <senthil@uthcode.com> parents: 68256 diff changeset	174 numbers not divisible by 2 and 3::
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	175
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	176 >>> def f(x): return x % 2 != 0 and x % 3 != 0
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	177 ...
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	178 >>> filter(f, range(2, 25))
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	179 [5, 7, 11, 13, 17, 19, 23]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	180
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	181 ``map(function, sequence)`` calls ``function(item)`` for each of the sequence's
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	182 items and returns a list of the return values. For example, to compute some
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	183 cubes::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	184
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	185 >>> def cube(x): return xxx
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	186 ...
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	187 >>> map(cube, range(1, 11))
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	188 [1, 8, 27, 64, 125, 216, 343, 512, 729, 1000]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	189
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	190 More than one sequence may be passed; the function must then have as many
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	191 arguments as there are sequences and is called with the corresponding item from
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	192 each sequence (or ``None`` if some sequence is shorter than another). For
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	193 example::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	194
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	195 >>> seq = range(8)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	196 >>> def add(x, y): return x+y
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	197 ...
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	198 >>> map(add, seq, seq)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	199 [0, 2, 4, 6, 8, 10, 12, 14]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	200
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	201 ``reduce(function, sequence)`` returns a single value constructed by calling the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	202 binary function function on the first two items of the sequence, then on the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	203 result and the next item, and so on. For example, to compute the sum of the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	204 numbers 1 through 10::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	205
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	206 >>> def add(x,y): return x+y
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	207 ...
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	208 >>> reduce(add, range(1, 11))
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	209 55
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	210
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	211 If there's only one item in the sequence, its value is returned; if the sequence
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	212 is empty, an exception is raised.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	213
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	214 A third argument can be passed to indicate the starting value. In this case the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	215 starting value is returned for an empty sequence, and the function is first
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	216 applied to the starting value and the first sequence item, then to the result
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	217 and the next item, and so on. For example, ::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	218
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	219 >>> def sum(seq):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	220 ... def add(x,y): return x+y
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	221 ... return reduce(add, seq, 0)
50522 ee9ced92843a Merged revisions 68133-68134,68141-68142,68145-68146,68148-68149,68159-68162,68166,68171-68174,68179,68195-68196,68210,68214-68215,68217-68222 via svnmerge from Georg Brandl <georg@python.org> parents: 49182 diff changeset	222 ...
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	223 >>> sum(range(1, 11))
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	224 55
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	225 >>> sum([])
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	226 0
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	227
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	228 Don't use this example's definition of :func:`sum`: since summing numbers is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	229 such a common need, a built-in function ``sum(sequence)`` is already provided,
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	230 and works exactly like this.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	231
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	232 .. versionadded:: 2.3
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	233
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	234
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	235 List Comprehensions
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	236 -------------------
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	237
73951 132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	238 List comprehensions provide a concise way to create lists.
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	239 Common applications are to make new lists where each element is the result of
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	240 some operations applied to each member of another sequence or iterable, or to
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	241 create a subsequence of those elements that satisfy a certain condition.
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	242
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	243 For example, assume we want to create a list of squares, like::
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	244
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	245 >>> squares = []
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	246 >>> for x in range(10):
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	247 ... squares.append(x**2)
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	248 ...
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	249 >>> squares
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	250 [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	251
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	252 We can obtain the same result with::
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	253
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	254 squares = [x**2 for x in range(10)]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	255
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	256 This is also equivalent to ``squares = map(lambda x: x**2, range(10))``,
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	257 but it's more concise and readable.
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	258
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	259 A list comprehension consists of brackets containing an expression followed
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	260 by a :keyword:`for` clause, then zero or more :keyword:`for` or :keyword:`if`
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	261 clauses. The result will be a new list resulting from evaluating the expression
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	262 in the context of the :keyword:`for` and :keyword:`if` clauses which follow it.
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	263 For example, this listcomp combines the elements of two lists if they are not
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	264 equal::
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	265
73951 132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	266 >>> [(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	267 [(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	268
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	269 and it's equivalent to:
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	270
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	271 >>> combs = []
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	272 >>> for x in [1,2,3]:
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	273 ... for y in [3,1,4]:
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	274 ... if x != y:
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	275 ... combs.append((x, y))
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	276 ...
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	277 >>> combs
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	278 [(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	279
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	280 Note how the order of the :keyword:`for` and :keyword:`if` statements is the
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	281 same in both these snippets.
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	282
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	283 If the expression is a tuple (e.g. the ``(x, y)`` in the previous example),
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	284 it must be parenthesized. ::
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	285
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	286 >>> vec = [-4, -2, 0, 2, 4]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	287 >>> # create a new list with the values doubled
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	288 >>> [x*2 for x in vec]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	289 [-8, -4, 0, 4, 8]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	290 >>> # filter the list to exclude negative numbers
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	291 >>> [x for x in vec if x >= 0]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	292 [0, 2, 4]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	293 >>> # apply a function to all the elements
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	294 >>> [abs(x) for x in vec]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	295 [4, 2, 0, 2, 4]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	296 >>> # call a method on each element
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	297 >>> freshfruit = [' banana', ' loganberry ', 'passion fruit ']
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	298 >>> [weapon.strip() for weapon in freshfruit]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	299 ['banana', 'loganberry', 'passion fruit']
73951 132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	300 >>> # create a list of 2-tuples like (number, square)
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	301 >>> [(x, x**2) for x in range(6)]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	302 [(0, 0), (1, 1), (2, 4), (3, 9), (4, 16), (5, 25)]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	303 >>> # the tuple must be parenthesized, otherwise an error is raised
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	304 >>> [x, x**2 for x in range(6)]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	305 File "<stdin>", line 1
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	306 [x, x**2 for x in range(6)]
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	307 ^
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	308 SyntaxError: invalid syntax
73951 132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	309 >>> # flatten a list using a listcomp with two 'for'
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	310 >>> vec = [[1,2,3], [4,5,6], [7,8,9]]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	311 >>> [num for elem in vec for num in elem]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	312 [1, 2, 3, 4, 5, 6, 7, 8, 9]
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	313
73951 132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	314 List comprehensions can contain complex expressions and nested functions::
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	315
73951 132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	316 >>> from math import pi
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	317 >>> [str(round(pi, i)) for i in range(1, 6)]
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	318 ['3.1', '3.14', '3.142', '3.1416', '3.14159']
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	319
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	320
44302 20ed1d415409 Add a section about nested listcomps to the tutorial. Georg Brandl <georg@python.org> parents: 42603 diff changeset	321 Nested List Comprehensions
73951 132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	322 ''''''''''''''''''''''''''
44302 20ed1d415409 Add a section about nested listcomps to the tutorial. Georg Brandl <georg@python.org> parents: 42603 diff changeset	323
73951 132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	324 The initial expression in a list comprehension can be any arbitrary expression,
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	325 including another list comprehension.
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	326
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	327 Consider the following example of a 3x4 matrix implemented as a list of
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	328 3 lists of length 4::
44302 20ed1d415409 Add a section about nested listcomps to the tutorial. Georg Brandl <georg@python.org> parents: 42603 diff changeset	329
73951 132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	330 >>> matrix = [
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	331 ... [1, 2, 3, 4],
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	332 ... [5, 6, 7, 8],
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	333 ... [9, 10, 11, 12],
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	334 ... ]
44302 20ed1d415409 Add a section about nested listcomps to the tutorial. Georg Brandl <georg@python.org> parents: 42603 diff changeset	335
73951 132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	336 The following list comprehension will transpose rows and columns::
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	337
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	338 >>> [[row[i] for row in matrix] for i in range(4)]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	339 [[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	340
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	341 As we saw in the previous section, the nested listcomp is evaluated in
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	342 the context of the :keyword:`for` that follows it, so this example is
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	343 equivalent to::
44302 20ed1d415409 Add a section about nested listcomps to the tutorial. Georg Brandl <georg@python.org> parents: 42603 diff changeset	344
73951 132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	345 >>> transposed = []
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	346 >>> for i in range(4):
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	347 ... transposed.append([row[i] for row in matrix])
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	348 ...
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	349 >>> transposed
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	350 [[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]
44302 20ed1d415409 Add a section about nested listcomps to the tutorial. Georg Brandl <georg@python.org> parents: 42603 diff changeset	351
73951 132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	352 which, in turn, is the same as::
44302 20ed1d415409 Add a section about nested listcomps to the tutorial. Georg Brandl <georg@python.org> parents: 42603 diff changeset	353
73951 132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	354 >>> transposed = []
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	355 >>> for i in range(4):
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	356 ... # the following 3 lines implement the nested listcomp
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	357 ... transposed_row = []
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	358 ... for row in matrix:
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	359 ... transposed_row.append(row[i])
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	360 ... transposed.append(transposed_row)
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	361 ...
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	362 >>> transposed
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	363 [[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]
44302 20ed1d415409 Add a section about nested listcomps to the tutorial. Georg Brandl <georg@python.org> parents: 42603 diff changeset	364
20ed1d415409 Add a section about nested listcomps to the tutorial. Georg Brandl <georg@python.org> parents: 42603 diff changeset	365
73951 132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	366 In the real world, you should prefer built-in functions to complex flow statements.
44302 20ed1d415409 Add a section about nested listcomps to the tutorial. Georg Brandl <georg@python.org> parents: 42603 diff changeset	367 The :func:`zip` function would do a great job for this use case::
20ed1d415409 Add a section about nested listcomps to the tutorial. Georg Brandl <georg@python.org> parents: 42603 diff changeset	368
73951 132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	369 >>> zip(*matrix)
132158b287d7 #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 72513 diff changeset	370 [(1, 5, 9), (2, 6, 10), (3, 7, 11), (4, 8, 12)]
44302 20ed1d415409 Add a section about nested listcomps to the tutorial. Georg Brandl <georg@python.org> parents: 42603 diff changeset	371
20ed1d415409 Add a section about nested listcomps to the tutorial. Georg Brandl <georg@python.org> parents: 42603 diff changeset	372 See :ref:`tut-unpacking-arguments` for details on the asterisk in this line.
20ed1d415409 Add a section about nested listcomps to the tutorial. Georg Brandl <georg@python.org> parents: 42603 diff changeset	373
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	374 .. _tut-del:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	375
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	376 The :keyword:`del` statement
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	377 ============================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	378
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	379 There is a way to remove an item from a list given its index instead of its
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	380 value: the :keyword:`del` statement. This differs from the :meth:`pop` method
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	381 which returns a value. The :keyword:`del` statement can also be used to remove
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	382 slices from a list or clear the entire list (which we did earlier by assignment
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	383 of an empty list to the slice). For example::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	384
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	385 >>> a = [-1, 1, 66.25, 333, 333, 1234.5]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	386 >>> del a[0]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	387 >>> a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	388 [1, 66.25, 333, 333, 1234.5]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	389 >>> del a[2:4]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	390 >>> a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	391 [1, 66.25, 1234.5]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	392 >>> del a[:]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	393 >>> a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	394 []
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	395
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	396 :keyword:`del` can also be used to delete entire variables::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	397
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	398 >>> del a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	399
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	400 Referencing the name ``a`` hereafter is an error (at least until another value
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	401 is assigned to it). We'll find other uses for :keyword:`del` later.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	402
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	403
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	404 .. _tut-tuples:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	405
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	406 Tuples and Sequences
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	407 ====================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	408
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	409 We saw that lists and strings have many common properties, such as indexing and
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	410 slicing operations. They are two examples of sequence data types (see
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	411 :ref:`typesseq`). Since Python is an evolving language, other sequence data
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	412 types may be added. There is also another standard sequence data type: the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	413 tuple.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	414
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	415 A tuple consists of a number of values separated by commas, for instance::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	416
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	417 >>> t = 12345, 54321, 'hello!'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	418 >>> t[0]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	419 12345
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	420 >>> t
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	421 (12345, 54321, 'hello!')
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	422 >>> # Tuples may be nested:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	423 ... u = t, (1, 2, 3, 4, 5)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	424 >>> u
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	425 ((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))
77495 bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	426 >>> # Tuples are immutable:
bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	427 ... t[0] = 88888
bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	428 Traceback (most recent call last):
bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	429 File "<stdin>", line 1, in <module>
bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	430 TypeError: 'tuple' object does not support item assignment
bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	431 >>> # but they can contain mutable objects:
bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	432 ... v = ([1, 2, 3], [3, 2, 1])
bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	433 >>> v
bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	434 ([1, 2, 3], [3, 2, 1])
bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	435
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	436
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	437 As you see, on output tuples are always enclosed in parentheses, so that nested
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	438 tuples are interpreted correctly; they may be input with or without surrounding
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	439 parentheses, although often parentheses are necessary anyway (if the tuple is
77495 bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	440 part of a larger expression). It is not possible to assign to the individual
bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	441 items of a tuple, however it is possible to create tuples which contain mutable
bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	442 objects, such as lists.
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	443
77495 bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	444 Though tuples may seem similar to lists, they are often used in different
bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	445 situations and for different purposes.
bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	446 Tuples are :term:`immutable`, and usually contain an heterogeneous sequence of
bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	447 elements that are accessed via unpacking (see later in this section) or indexing
bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	448 (or even by attribute in the case of :func:`namedtuples <collections.namedtuple>`).
bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	449 Lists are :term:`mutable`, and their elements are usually homogeneous and are
bb63919cde6e #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 76513 diff changeset	450 accessed by iterating over the list.
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	451
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	452 A special problem is the construction of tuples containing 0 or 1 items: the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	453 syntax has some extra quirks to accommodate these. Empty tuples are constructed
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	454 by an empty pair of parentheses; a tuple with one item is constructed by
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	455 following a value with a comma (it is not sufficient to enclose a single value
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	456 in parentheses). Ugly, but effective. For example::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	457
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	458 >>> empty = ()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	459 >>> singleton = 'hello', # <-- note trailing comma
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	460 >>> len(empty)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	461 0
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	462 >>> len(singleton)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	463 1
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	464 >>> singleton
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	465 ('hello',)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	466
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	467 The statement ``t = 12345, 54321, 'hello!'`` is an example of tuple packing:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	468 the values ``12345``, ``54321`` and ``'hello!'`` are packed together in a tuple.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	469 The reverse operation is also possible::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	470
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	471 >>> x, y, z = t
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	472
52900 4acbbb3e65e2 Merged revisions 70866-70868,70870-70871,70893,70896,70902,70905,70907,70912,70915,70927,70933,70940,70944,70954,70963,70998,71056 via svnmerge from Georg Brandl <georg@python.org> parents: 50522 diff changeset	473 This is called, appropriately enough, sequence unpacking and works for any
4acbbb3e65e2 Merged revisions 70866-70868,70870-70871,70893,70896,70902,70905,70907,70912,70915,70927,70933,70940,70944,70954,70963,70998,71056 via svnmerge from Georg Brandl <georg@python.org> parents: 50522 diff changeset	474 sequence on the right-hand side. Sequence unpacking requires the list of
4acbbb3e65e2 Merged revisions 70866-70868,70870-70871,70893,70896,70902,70905,70907,70912,70915,70927,70933,70940,70944,70954,70963,70998,71056 via svnmerge from Georg Brandl <georg@python.org> parents: 50522 diff changeset	475 variables on the left to have the same number of elements as the length of the
4acbbb3e65e2 Merged revisions 70866-70868,70870-70871,70893,70896,70902,70905,70907,70912,70915,70927,70933,70940,70944,70954,70963,70998,71056 via svnmerge from Georg Brandl <georg@python.org> parents: 50522 diff changeset	476 sequence. Note that multiple assignment is really just a combination of tuple
4acbbb3e65e2 Merged revisions 70866-70868,70870-70871,70893,70896,70902,70905,70907,70912,70915,70927,70933,70940,70944,70954,70963,70998,71056 via svnmerge from Georg Brandl <georg@python.org> parents: 50522 diff changeset	477 packing and sequence unpacking.
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	478
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	479
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	480 .. _tut-sets:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	481
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	482 Sets
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	483 ====
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	484
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	485 Python also includes a data type for sets. A set is an unordered collection
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	486 with no duplicate elements. Basic uses include membership testing and
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	487 eliminating duplicate entries. Set objects also support mathematical operations
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	488 like union, intersection, difference, and symmetric difference.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	489
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	490 Here is a brief demonstration::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	491
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	492 >>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	493 >>> fruit = set(basket) # create a set without duplicates
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	494 >>> fruit
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	495 set(['orange', 'pear', 'apple', 'banana'])
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	496 >>> 'orange' in fruit # fast membership testing
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	497 True
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	498 >>> 'crabgrass' in fruit
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	499 False
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	500
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	501 >>> # Demonstrate set operations on unique letters from two words
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	502 ...
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	503 >>> a = set('abracadabra')
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	504 >>> b = set('alacazam')
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	505 >>> a # unique letters in a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	506 set(['a', 'r', 'b', 'c', 'd'])
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	507 >>> a - b # letters in a but not in b
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	508 set(['r', 'd', 'b'])
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	509 >>> a \| b # letters in either a or b
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	510 set(['a', 'c', 'r', 'd', 'b', 'm', 'z', 'l'])
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	511 >>> a & b # letters in both a and b
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	512 set(['a', 'c'])
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	513 >>> a ^ b # letters in a or b but not both
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	514 set(['r', 'd', 'b', 'm', 'z', 'l'])
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	515
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	516
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	517 .. _tut-dictionaries:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	518
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	519 Dictionaries
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	520 ============
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	521
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	522 Another useful data type built into Python is the dictionary (see
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	523 :ref:`typesmapping`). Dictionaries are sometimes found in other languages as
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	524 "associative memories" or "associative arrays". Unlike sequences, which are
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	525 indexed by a range of numbers, dictionaries are indexed by keys, which can be
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	526 any immutable type; strings and numbers can always be keys. Tuples can be used
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	527 as keys if they contain only strings, numbers, or tuples; if a tuple contains
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	528 any mutable object either directly or indirectly, it cannot be used as a key.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	529 You can't use lists as keys, since lists can be modified in place using index
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	530 assignments, slice assignments, or methods like :meth:`append` and
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	531 :meth:`extend`.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	532
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	533 It is best to think of a dictionary as an unordered set of key: value pairs,
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	534 with the requirement that the keys are unique (within one dictionary). A pair of
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	535 braces creates an empty dictionary: ``{}``. Placing a comma-separated list of
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	536 key:value pairs within the braces adds initial key:value pairs to the
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	537 dictionary; this is also the way dictionaries are written on output.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	538
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	539 The main operations on a dictionary are storing a value with some key and
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	540 extracting the value given the key. It is also possible to delete a key:value
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	541 pair with ``del``. If you store using a key that is already in use, the old
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	542 value associated with that key is forgotten. It is an error to extract a value
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	543 using a non-existent key.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	544
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	545 The :meth:`keys` method of a dictionary object returns a list of all the keys
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	546 used in the dictionary, in arbitrary order (if you want it sorted, just apply
65360 80fe066c135a #8267: Use sorted() to get a sorted list of dict keys. Georg Brandl <georg@python.org> parents: 60099 diff changeset	547 the :func:`sorted` function to it). To check whether a single key is in the
80fe066c135a #8267: Use sorted() to get a sorted list of dict keys. Georg Brandl <georg@python.org> parents: 60099 diff changeset	548 dictionary, use the :keyword:`in` keyword.
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	549
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	550 Here is a small example using a dictionary::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	551
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	552 >>> tel = {'jack': 4098, 'sape': 4139}
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	553 >>> tel['guido'] = 4127
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	554 >>> tel
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	555 {'sape': 4139, 'guido': 4127, 'jack': 4098}
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	556 >>> tel['jack']
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	557 4098
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	558 >>> del tel['sape']
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	559 >>> tel['irv'] = 4127
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	560 >>> tel
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	561 {'guido': 4127, 'irv': 4127, 'jack': 4098}
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	562 >>> tel.keys()
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	563 ['guido', 'irv', 'jack']
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	564 >>> 'guido' in tel
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	565 True
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	566
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	567 The :func:`dict` constructor builds dictionaries directly from lists of
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	568 key-value pairs stored as tuples. When the pairs form a pattern, list
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	569 comprehensions can compactly specify the key-value list. ::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	570
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	571 >>> dict([('sape', 4139), ('guido', 4127), ('jack', 4098)])
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	572 {'sape': 4139, 'jack': 4098, 'guido': 4127}
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	573 >>> dict([(x, x**2) for x in (2, 4, 6)]) # use a list comprehension
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	574 {2: 4, 4: 16, 6: 36}
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	575
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	576 Later in the tutorial, we will learn about Generator Expressions which are even
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	577 better suited for the task of supplying key-values pairs to the :func:`dict`
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	578 constructor.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	579
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	580 When the keys are simple strings, it is sometimes easier to specify pairs using
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	581 keyword arguments::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	582
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	583 >>> dict(sape=4139, guido=4127, jack=4098)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	584 {'sape': 4139, 'jack': 4098, 'guido': 4127}
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	585
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	586
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	587 .. _tut-loopidioms:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	588
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	589 Looping Techniques
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	590 ==================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	591
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	592 When looping through a sequence, the position index and corresponding value can
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	593 be retrieved at the same time using the :func:`enumerate` function. ::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	594
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	595 >>> for i, v in enumerate(['tic', 'tac', 'toe']):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	596 ... print i, v
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	597 ...
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	598 0 tic
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	599 1 tac
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	600 2 toe
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	601
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	602 To loop over two or more sequences at the same time, the entries can be paired
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	603 with the :func:`zip` function. ::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	604
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	605 >>> questions = ['name', 'quest', 'favorite color']
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	606 >>> answers = ['lancelot', 'the holy grail', 'blue']
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	607 >>> for q, a in zip(questions, answers):
47282 70f9a5db279c update the tutorial to use str.format Benjamin Peterson <benjamin@python.org> parents: 46163 diff changeset	608 ... print 'What is your {0}? It is {1}.'.format(q, a)
50522 ee9ced92843a Merged revisions 68133-68134,68141-68142,68145-68146,68148-68149,68159-68162,68166,68171-68174,68179,68195-68196,68210,68214-68215,68217-68222 via svnmerge from Georg Brandl <georg@python.org> parents: 49182 diff changeset	609 ...
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	610 What is your name? It is lancelot.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	611 What is your quest? It is the holy grail.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	612 What is your favorite color? It is blue.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	613
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	614 To loop over a sequence in reverse, first specify the sequence in a forward
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	615 direction and then call the :func:`reversed` function. ::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	616
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	617 >>> for i in reversed(xrange(1,10,2)):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	618 ... print i
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	619 ...
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	620 9
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	621 7
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	622 5
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	623 3
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	624 1
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	625
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	626 To loop over a sequence in sorted order, use the :func:`sorted` function which
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	627 returns a new sorted list while leaving the source unaltered. ::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	628
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	629 >>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	630 >>> for f in sorted(set(basket)):
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	631 ... print f
50522 ee9ced92843a Merged revisions 68133-68134,68141-68142,68145-68146,68148-68149,68159-68162,68166,68171-68174,68179,68195-68196,68210,68214-68215,68217-68222 via svnmerge from Georg Brandl <georg@python.org> parents: 49182 diff changeset	632 ...
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	633 apple
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	634 banana
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	635 orange
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	636 pear
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	637
76513 e2a3260f1718 Reorder the entries to put the type specific technique last. Raymond Hettinger <python@rcn.com> parents: 73951 diff changeset	638 When looping through dictionaries, the key and corresponding value can be
e2a3260f1718 Reorder the entries to put the type specific technique last. Raymond Hettinger <python@rcn.com> parents: 73951 diff changeset	639 retrieved at the same time using the :meth:`iteritems` method. ::
e2a3260f1718 Reorder the entries to put the type specific technique last. Raymond Hettinger <python@rcn.com> parents: 73951 diff changeset	640
e2a3260f1718 Reorder the entries to put the type specific technique last. Raymond Hettinger <python@rcn.com> parents: 73951 diff changeset	641 >>> knights = {'gallahad': 'the pure', 'robin': 'the brave'}
e2a3260f1718 Reorder the entries to put the type specific technique last. Raymond Hettinger <python@rcn.com> parents: 73951 diff changeset	642 >>> for k, v in knights.iteritems():
e2a3260f1718 Reorder the entries to put the type specific technique last. Raymond Hettinger <python@rcn.com> parents: 73951 diff changeset	643 ... print k, v
e2a3260f1718 Reorder the entries to put the type specific technique last. Raymond Hettinger <python@rcn.com> parents: 73951 diff changeset	644 ...
e2a3260f1718 Reorder the entries to put the type specific technique last. Raymond Hettinger <python@rcn.com> parents: 73951 diff changeset	645 gallahad the pure
e2a3260f1718 Reorder the entries to put the type specific technique last. Raymond Hettinger <python@rcn.com> parents: 73951 diff changeset	646 robin the brave
e2a3260f1718 Reorder the entries to put the type specific technique last. Raymond Hettinger <python@rcn.com> parents: 73951 diff changeset	647
42603 c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	648
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	649 .. _tut-conditions:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	650
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	651 More on Conditions
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	652 ==================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	653
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	654 The conditions used in ``while`` and ``if`` statements can contain any
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	655 operators, not just comparisons.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	656
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	657 The comparison operators ``in`` and ``not in`` check whether a value occurs
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	658 (does not occur) in a sequence. The operators ``is`` and ``is not`` compare
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	659 whether two objects are really the same object; this only matters for mutable
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	660 objects like lists. All comparison operators have the same priority, which is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	661 lower than that of all numerical operators.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	662
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	663 Comparisons can be chained. For example, ``a < b == c`` tests whether ``a`` is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	664 less than ``b`` and moreover ``b`` equals ``c``.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	665
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	666 Comparisons may be combined using the Boolean operators ``and`` and ``or``, and
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	667 the outcome of a comparison (or of any other Boolean expression) may be negated
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	668 with ``not``. These have lower priorities than comparison operators; between
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	669 them, ``not`` has the highest priority and ``or`` the lowest, so that ``A and
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	670 not B or C`` is equivalent to ``(A and (not B)) or C``. As always, parentheses
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	671 can be used to express the desired composition.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	672
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	673 The Boolean operators ``and`` and ``or`` are so-called short-circuit
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	674 operators: their arguments are evaluated from left to right, and evaluation
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	675 stops as soon as the outcome is determined. For example, if ``A`` and ``C`` are
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	676 true but ``B`` is false, ``A and B and C`` does not evaluate the expression
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	677 ``C``. When used as a general value and not as a Boolean, the return value of a
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	678 short-circuit operator is the last evaluated argument.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	679
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	680 It is possible to assign the result of a comparison or other Boolean expression
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	681 to a variable. For example, ::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	682
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	683 >>> string1, string2, string3 = '', 'Trondheim', 'Hammer Dance'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	684 >>> non_null = string1 or string2 or string3
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	685 >>> non_null
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	686 'Trondheim'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	687
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	688 Note that in Python, unlike C, assignment cannot occur inside expressions. C
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	689 programmers may grumble about this, but it avoids a common class of problems
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	690 encountered in C programs: typing ``=`` in an expression when ``==`` was
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	691 intended.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	692
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	693
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	694 .. _tut-comparing:
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	695
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	696 Comparing Sequences and Other Types
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	697 ===================================
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	698
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	699 Sequence objects may be compared to other objects with the same sequence type.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	700 The comparison uses lexicographical ordering: first the first two items are
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	701 compared, and if they differ this determines the outcome of the comparison; if
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	702 they are equal, the next two items are compared, and so on, until either
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	703 sequence is exhausted. If two items to be compared are themselves sequences of
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	704 the same type, the lexicographical comparison is carried out recursively. If
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	705 all items of two sequences compare equal, the sequences are considered equal.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	706 If one sequence is an initial sub-sequence of the other, the shorter sequence is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	707 the smaller (lesser) one. Lexicographical ordering for strings uses the ASCII
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	708 ordering for individual characters. Some examples of comparisons between
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	709 sequences of the same type::
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	710
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	711 (1, 2, 3) < (1, 2, 4)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	712 [1, 2, 3] < [1, 2, 4]
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	713 'ABC' < 'C' < 'Pascal' < 'Python'
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	714 (1, 2, 3, 4) < (1, 2, 4)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	715 (1, 2) < (1, 2, -1)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	716 (1, 2, 3) == (1.0, 2.0, 3.0)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	717 (1, 2, ('aa', 'ab')) < (1, 2, ('abc', 'a'), 4)
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	718
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	719 Note that comparing objects of different types is legal. The outcome is
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	720 deterministic but arbitrary: the types are ordered by their name. Thus, a list
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	721 is always smaller than a string, a string is always smaller than a tuple, etc.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	722 [#]_ Mixed numeric types are compared according to their numeric value, so 0
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	723 equals 0.0, etc.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	724
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	725
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	726 .. rubric:: Footnotes
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	727
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	728 .. [#] The rules for comparing objects of different types should not be relied upon;
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	729 they may change in a future version of the language.
c62f6669ae22 Move the 2.6 reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	730

Mercurial > cpython-lru_cache-weakref

annotate Doc/tutorial/datastructures.rst @ 79675:c0a2294de787 2.7