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

annotate Doc/tutorial/datastructures.rst @ 60008:0dfd62076ded

author	Benjamin Peterson <benjamin@python.org>
date	Thu, 25 Mar 2010 01:11:10 +0000
parents	575ec773dda4
children	fd5c0ea2870b

rev	line source
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	1 .. _tut-structures:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	2
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	3 ***************
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	4 Data Structures
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	5 ***************
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	6
9e1529bf0442 Move the 3k 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,
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	8 and adds some new things as well.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	9
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	10 .. _tut-morelists:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	11
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	12 More on Lists
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	13 =============
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	14
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	15 The list data type has some more methods. Here are all of the methods of list
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	16 objects:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	17
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	18
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	19 .. method:: list.append(x)
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	20 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	21
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	22 Add an item to the end of the list; equivalent to ``a[len(a):] = [x]``.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	23
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	24
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	25 .. method:: list.extend(L)
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	26 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	27
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	28 Extend the list by appending all the items in the given list; equivalent to
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	29 ``a[len(a):] = L``.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	30
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	31
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	32 .. method:: list.insert(i, x)
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	33 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	34
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	35 Insert an item at a given position. The first argument is the index of the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	36 element before which to insert, so ``a.insert(0, x)`` inserts at the front of
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	37 the list, and ``a.insert(len(a), x)`` is equivalent to ``a.append(x)``.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	38
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	39
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	40 .. method:: list.remove(x)
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	41 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	42
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	43 Remove the first item from the list whose value is x. It is an error if there
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	44 is no such item.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	45
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	46
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	47 .. method:: list.pop([i])
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	48 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	49
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	50 Remove the item at the given position in the list, and return it. If no index
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	51 is specified, ``a.pop()`` removes and returns the last item in the list. (The
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	52 square brackets around the i in the method signature denote that the parameter
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	53 is optional, not that you should type square brackets at that position. You
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	54 will see this notation frequently in the Python Library Reference.)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	55
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	56
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	57 .. method:: list.index(x)
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	58 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	59
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	60 Return the index in the list of the first item whose value is x. It is an
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	61 error if there is no such item.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	62
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	63
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	64 .. method:: list.count(x)
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	65 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	66
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	67 Return the number of times x appears in the list.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	68
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	69
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	70 .. method:: list.sort()
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	71 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	72
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	73 Sort the items of the list, in place.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	74
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	75
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	76 .. method:: list.reverse()
45996 376b5cb8a014 Merged revisions 61687-61688,61696,61700,61704-61705,61707-61709,61711-61712,61714-61716,61718-61722 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 45084 diff changeset	77 :noindex:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	78
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	79 Reverse the elements of the list, in place.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	80
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	81 An example that uses most of the list methods::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	82
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	83 >>> a = [66.25, 333, 333, 1, 1234.5]
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	84 >>> print(a.count(333), a.count(66.25), a.count('x'))
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	85 2 1 0
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	86 >>> a.insert(2, -1)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	87 >>> a.append(333)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	88 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	89 [66.25, 333, -1, 333, 1, 1234.5, 333]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	90 >>> a.index(333)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	91 1
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	92 >>> a.remove(333)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	93 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	94 [66.25, -1, 333, 1, 1234.5, 333]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	95 >>> a.reverse()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	96 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	97 [333, 1234.5, 1, 333, -1, 66.25]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	98 >>> a.sort()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	99 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	100 [-1, 1, 66.25, 333, 333, 1234.5]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	101
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	102
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	103 .. _tut-lists-as-stacks:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	104
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	105 Using Lists as Stacks
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	106 ---------------------
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	107
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	108 .. sectionauthor:: Ka-Ping Yee <ping@lfw.org>
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	109
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	110
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	111 The list methods make it very easy to use a list as a stack, where the last
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	112 element added is the first element retrieved ("last-in, first-out"). To add an
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	113 item to the top of the stack, use :meth:`append`. To retrieve an item from the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	114 top of the stack, use :meth:`pop` without an explicit index. For example::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	115
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	116 >>> stack = [3, 4, 5]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	117 >>> stack.append(6)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	118 >>> stack.append(7)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	119 >>> stack
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	120 [3, 4, 5, 6, 7]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	121 >>> stack.pop()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	122 7
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	123 >>> stack
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	124 [3, 4, 5, 6]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	125 >>> stack.pop()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	126 6
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	127 >>> stack.pop()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	128 5
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	129 >>> stack
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	130 [3, 4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	131
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	132
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	133 .. _tut-lists-as-queues:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	134
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	135 Using Lists as Queues
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	136 ---------------------
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	137
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	138 .. sectionauthor:: Ka-Ping Yee <ping@lfw.org>
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	139
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	140
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	141 You can also use a list conveniently as a queue, where the first element added
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	142 is the first element retrieved ("first-in, first-out"). To add an item to the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	143 back of the queue, use :meth:`append`. To retrieve an item from the front of
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	144 the queue, use :meth:`pop` with ``0`` as the index. For example::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	145
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	146 >>> queue = ["Eric", "John", "Michael"]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	147 >>> queue.append("Terry") # Terry arrives
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	148 >>> queue.append("Graham") # Graham arrives
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	149 >>> queue.pop(0)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	150 'Eric'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	151 >>> queue.pop(0)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	152 'John'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	153 >>> queue
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	154 ['Michael', 'Terry', 'Graham']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	155
59800 575ec773dda4 Merged revisions 79179 via svnmerge from Georg Brandl <georg@python.org> parents: 54773 diff changeset	156 However, since lists are implemented as an array of elements, they are not the
575ec773dda4 Merged revisions 79179 via svnmerge from Georg Brandl <georg@python.org> parents: 54773 diff changeset	157 optimal data structure to use as a queue (the ``pop(0)`` needs to move all
575ec773dda4 Merged revisions 79179 via svnmerge from Georg Brandl <georg@python.org> parents: 54773 diff changeset	158 following elements). See :ref:`tut-list-tools` for a look at
575ec773dda4 Merged revisions 79179 via svnmerge from Georg Brandl <georg@python.org> parents: 54773 diff changeset	159 :class:`collections.deque`, which is designed to work efficiently as a queue.
575ec773dda4 Merged revisions 79179 via svnmerge from Georg Brandl <georg@python.org> parents: 54773 diff changeset	160
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	161
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	162 .. _tut-listcomps:
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	163
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	164 List Comprehensions
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	165 -------------------
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	166
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	167 List comprehensions provide a concise way to create lists from sequences.
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	168 Common applications are to make lists where each element is the result of
50514 c3d3b760d8bf Remove trailing whitespace. Georg Brandl <georg@python.org> parents: 50176 diff changeset	169 some operations applied to each member of the sequence, or to create a
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	170 subsequence of those elements that satisfy a certain condition.
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	171
54630 2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	172 A list comprehension consists of brackets containing an expression followed
2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	173 by a :keyword:`for` clause, then zero or more :keyword:`for` or :keyword:`if`
2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	174 clauses. The result will be a list resulting from evaluating the expression in
2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	175 the context of the :keyword:`for` and :keyword:`if` clauses which follow it. If
2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	176 the expression would evaluate to a tuple, it must be parenthesized.
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	177
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	178 Here we take a list of numbers and return a list of three times each number::
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	179
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	180 >>> vec = [2, 4, 6]
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	181 >>> [3*x for x in vec]
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	182 [6, 12, 18]
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	183
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	184 Now we get a little fancier::
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	185
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43117 diff changeset	186 >>> [[x, x**2] for x in vec]
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	187 [[2, 4], [4, 16], [6, 36]]
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	188
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	189 Here we apply a method call to each item in a sequence::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	190
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	191 >>> freshfruit = [' banana', ' loganberry ', 'passion fruit ']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	192 >>> [weapon.strip() for weapon in freshfruit]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	193 ['banana', 'loganberry', 'passion fruit']
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	194
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43117 diff changeset	195 Using the :keyword:`if` clause we can filter the stream::
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	196
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	197 >>> [3*x for x in vec if x > 3]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	198 [12, 18]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	199 >>> [3*x for x in vec if x < 2]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	200 []
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	201
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	202 Tuples can often be created without their parentheses, but not here::
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	203
50515 0e05b6ca6afd Merged revisions 68221 via svnmerge from Georg Brandl <georg@python.org> parents: 50514 diff changeset	204 >>> [x, x**2 for x in vec] # error - parens required for tuples
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	205 File "<stdin>", line 1, in ?
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	206 [x, x**2 for x in vec]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	207 ^
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	208 SyntaxError: invalid syntax
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	209 >>> [(x, x**2) for x in vec]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	210 [(2, 4), (4, 16), (6, 36)]
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	211
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43117 diff changeset	212 Here are some nested for loops and other fancy behavior::
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	213
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	214 >>> vec1 = [2, 4, 6]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	215 >>> vec2 = [4, 3, -9]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	216 >>> [x*y for x in vec1 for y in vec2]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	217 [8, 6, -18, 16, 12, -36, 24, 18, -54]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	218 >>> [x+y for x in vec1 for y in vec2]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	219 [6, 5, -7, 8, 7, -5, 10, 9, -3]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	220 >>> [vec1[i]*vec2[i] for i in range(len(vec1))]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	221 [8, 12, -54]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	222
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	223 List comprehensions can be applied to complex expressions and nested functions::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	224
45084 38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	225 >>> [str(round(355/113, i)) for i in range(1, 6)]
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	226 ['3.1', '3.14', '3.142', '3.1416', '3.14159']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	227
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	228
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	229 Nested List Comprehensions
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	230 --------------------------
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	231
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	232 If you've got the stomach for it, list comprehensions can be nested. They are a
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	233 powerful tool but -- like all powerful tools -- they need to be used carefully,
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	234 if at all.
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	235
50514 c3d3b760d8bf Remove trailing whitespace. Georg Brandl <georg@python.org> parents: 50176 diff changeset	236 Consider the following example of a 3x3 matrix held as a list containing three
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	237 lists, one list per row::
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	238
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	239 >>> mat = [
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	240 ... [1, 2, 3],
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	241 ... [4, 5, 6],
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	242 ... [7, 8, 9],
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	243 ... ]
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	244
50514 c3d3b760d8bf Remove trailing whitespace. Georg Brandl <georg@python.org> parents: 50176 diff changeset	245 Now, if you wanted to swap rows and columns, you could use a list
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	246 comprehension::
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	247
46978 55d8c35322a7 Convert a lot of print statements to print functions in docstrings, Neal Norwitz <nnorwitz@gmail.com> parents: 46948 diff changeset	248 >>> print([[row[i] for row in mat] for i in [0, 1, 2]])
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	249 [[1, 4, 7], [2, 5, 8], [3, 6, 9]]
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	250
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	251 Special care has to be taken for the nested list comprehension:
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	252
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	253 To avoid apprehension when nesting list comprehensions, read from right to
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	254 left.
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	255
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	256 A more verbose version of this snippet shows the flow explicitly::
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	257
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	258 for i in [0, 1, 2]:
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	259 for row in mat:
46978 55d8c35322a7 Convert a lot of print statements to print functions in docstrings, Neal Norwitz <nnorwitz@gmail.com> parents: 46948 diff changeset	260 print(row[i], end="")
48438 a0882bfaf54d fix no-op in tutorial Benjamin Peterson <benjamin@python.org> parents: 47283 diff changeset	261 print()
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	262
51776 d06a3ebf306f Merged revisions 69846 via svnmerge from Mark Dickinson <dickinsm@gmail.com> parents: 50517 diff changeset	263 In real world, you should prefer built-in functions to complex flow statements.
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	264 The :func:`zip` function would do a great job for this use case::
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	265
48665 b2c505d4a203 #3522: zip() returns an iterator. Georg Brandl <georg@python.org> parents: 48438 diff changeset	266 >>> list(zip(*mat))
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	267 [(1, 4, 7), (2, 5, 8), (3, 6, 9)]
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	268
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	269 See :ref:`tut-unpacking-arguments` for details on the asterisk in this line.
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	270
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	271 .. _tut-del:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	272
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	273 The :keyword:`del` statement
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	274 ============================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	275
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	276 There is a way to remove an item from a list given its index instead of its
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	277 value: the :keyword:`del` statement. This differs from the :meth:`pop` method
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	278 which returns a value. The :keyword:`del` statement can also be used to remove
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	279 slices from a list or clear the entire list (which we did earlier by assignment
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	280 of an empty list to the slice). For example::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	281
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	282 >>> a = [-1, 1, 66.25, 333, 333, 1234.5]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	283 >>> del a[0]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	284 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	285 [1, 66.25, 333, 333, 1234.5]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	286 >>> del a[2:4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	287 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	288 [1, 66.25, 1234.5]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	289 >>> del a[:]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	290 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	291 []
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	292
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	293 :keyword:`del` can also be used to delete entire variables::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	294
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	295 >>> del a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	296
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	297 Referencing the name ``a`` hereafter is an error (at least until another value
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	298 is assigned to it). We'll find other uses for :keyword:`del` later.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	299
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	300
49180 c87b84db244c Forward-port of r66447. Georg Brandl <georg@python.org> parents: 48665 diff changeset	301 .. _tut-tuples:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	302
44396 6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	303 Tuples and Sequences
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	304 ====================
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	305
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	306 We saw that lists and strings have many common properties, such as indexing and
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	307 slicing operations. They are two examples of sequence data types (see
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	308 :ref:`typesseq`). Since Python is an evolving language, other sequence data
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	309 types may be added. There is also another standard sequence data type: the
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	310 tuple.
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	311
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	312 A tuple consists of a number of values separated by commas, for instance::
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	313
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	314 >>> t = 12345, 54321, 'hello!'
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	315 >>> t[0]
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	316 12345
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	317 >>> t
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	318 (12345, 54321, 'hello!')
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	319 >>> # Tuples may be nested:
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	320 ... u = t, (1, 2, 3, 4, 5)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	321 >>> u
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	322 ((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	323
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	324 As you see, on output tuples are always enclosed in parentheses, so that nested
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	325 tuples are interpreted correctly; they may be input with or without surrounding
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	326 parentheses, although often parentheses are necessary anyway (if the tuple is
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	327 part of a larger expression).
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	328
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	329 Tuples have many uses. For example: (x, y) coordinate pairs, employee records
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	330 from a database, etc. Tuples, like strings, are immutable: it is not possible
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	331 to assign to the individual items of a tuple (you can simulate much of the same
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	332 effect with slicing and concatenation, though). It is also possible to create
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	333 tuples which contain mutable objects, such as lists.
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	334
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	335 A special problem is the construction of tuples containing 0 or 1 items: the
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	336 syntax has some extra quirks to accommodate these. Empty tuples are constructed
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	337 by an empty pair of parentheses; a tuple with one item is constructed by
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	338 following a value with a comma (it is not sufficient to enclose a single value
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	339 in parentheses). Ugly, but effective. For example::
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	340
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	341 >>> empty = ()
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	342 >>> singleton = 'hello', # <-- note trailing comma
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	343 >>> len(empty)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	344 0
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	345 >>> len(singleton)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	346 1
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	347 >>> singleton
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	348 ('hello',)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	349
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	350 The statement ``t = 12345, 54321, 'hello!'`` is an example of tuple packing:
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	351 the values ``12345``, ``54321`` and ``'hello!'`` are packed together in a tuple.
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	352 The reverse operation is also possible::
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	353
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	354 >>> x, y, z = t
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	355
52892 792bb885029f Merged revisions 70712,70714,70764-70765,70769-70771,70773,70776-70777,70788-70789,70824,70828,70832,70836,70842,70851,70855,70857,70866-70872,70883,70885,70893-70894,70896-70897,70903,70905-70907,70915,70927,70933,70951,70960,70962-70964,70998,71001,71006,71008,71010-71011,71019,71037,71056,71094,71101-71103,71106,71119,71123,71149-71150,71203,71212,71214-71217,71221,71240 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 51776 diff changeset	356 This is called, appropriately enough, sequence unpacking and works for any
54630 2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	357 sequence on the right-hand side. Sequence unpacking requires that there are as
2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	358 many variables on the left side of the equals sign as there are elements in the
52892 792bb885029f Merged revisions 70712,70714,70764-70765,70769-70771,70773,70776-70777,70788-70789,70824,70828,70832,70836,70842,70851,70855,70857,70866-70872,70883,70885,70893-70894,70896-70897,70903,70905-70907,70915,70927,70933,70951,70960,70962-70964,70998,71001,71006,71008,71010-71011,71019,71037,71056,71094,71101-71103,71106,71119,71123,71149-71150,71203,71212,71214-71217,71221,71240 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 51776 diff changeset	359 sequence. Note that multiple assignment is really just a combination of tuple
792bb885029f Merged revisions 70712,70714,70764-70765,70769-70771,70773,70776-70777,70788-70789,70824,70828,70832,70836,70842,70851,70855,70857,70866-70872,70883,70885,70893-70894,70896-70897,70903,70905-70907,70915,70927,70933,70951,70960,70962-70964,70998,71001,71006,71008,71010-71011,71019,71037,71056,71094,71101-71103,71106,71119,71123,71149-71150,71203,71212,71214-71217,71221,71240 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 51776 diff changeset	360 packing and sequence unpacking.
44396 6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	361
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	362 .. XXX Add a bit on the difference between tuples and lists.
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	363
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	364
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	365 .. _tut-sets:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	366
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	367 Sets
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	368 ====
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	369
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	370 Python also includes a data type for sets. A set is an unordered collection
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	371 with no duplicate elements. Basic uses include membership testing and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	372 eliminating duplicate entries. Set objects also support mathematical operations
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	373 like union, intersection, difference, and symmetric difference.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	374
54632 255db70bb316 Fix markup. Georg Brandl <georg@python.org> parents: 54630 diff changeset	375 Curly braces or the :func:`set` function can be use to create sets. Note: To
255db70bb316 Fix markup. Georg Brandl <georg@python.org> parents: 54630 diff changeset	376 create an empty set you have to use ``set()``, not ``{}``; the latter creates an
255db70bb316 Fix markup. Georg Brandl <georg@python.org> parents: 54630 diff changeset	377 empty dictionary, a data structure that we discuss in the next section.
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	378
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	379 Here is a brief demonstration::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	380
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	381 >>> basket = {'apple', 'orange', 'apple', 'pear', 'orange', 'banana'}
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	382 >>> print(basket)
50073 60ee901b7352 Forward-port r67624 and r67627. Georg Brandl <georg@python.org> parents: 49180 diff changeset	383 {'orange', 'banana', 'pear', 'apple'}
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	384 >>> fruit = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	385 >>> fruit = set(basket) # create a set without duplicates
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	386 >>> fruit
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	387 {'orange', 'pear', 'apple', 'banana'}
45084 38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	388 >>> fruit = {'orange', 'apple'} # {} syntax is equivalent to [] for lists
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	389 >>> fruit
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	390 {'orange', 'apple'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	391 >>> 'orange' in fruit # fast membership testing
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	392 True
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	393 >>> 'crabgrass' in fruit
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	394 False
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	395
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	396 >>> # Demonstrate set operations on unique letters from two words
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	397 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	398 >>> a = set('abracadabra')
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	399 >>> b = set('alacazam')
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	400 >>> a # unique letters in a
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	401 {'a', 'r', 'b', 'c', 'd'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	402 >>> a - b # letters in a but not in b
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	403 {'r', 'd', 'b'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	404 >>> a \| b # letters in either a or b
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	405 {'a', 'c', 'r', 'd', 'b', 'm', 'z', 'l'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	406 >>> a & b # letters in both a and b
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	407 {'a', 'c'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	408 >>> a ^ b # letters in a or b but not both
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	409 {'r', 'd', 'b', 'm', 'z', 'l'}
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	410
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	411 Like :ref:`for lists <tut-listcomps>`, there is a set comprehension syntax::
45084 38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	412
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	413 >>> a = {x for x in 'abracadabra' if x not in 'abc'}
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	414 >>> a
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	415 {'r', 'd'}
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	416
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	417
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	418
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	419 .. _tut-dictionaries:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	420
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	421 Dictionaries
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	422 ============
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	423
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	424 Another useful data type built into Python is the dictionary (see
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	425 :ref:`typesmapping`). Dictionaries are sometimes found in other languages as
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	426 "associative memories" or "associative arrays". Unlike sequences, which are
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	427 indexed by a range of numbers, dictionaries are indexed by keys, which can be
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	428 any immutable type; strings and numbers can always be keys. Tuples can be used
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	429 as keys if they contain only strings, numbers, or tuples; if a tuple contains
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	430 any mutable object either directly or indirectly, it cannot be used as a key.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	431 You can't use lists as keys, since lists can be modified in place using index
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	432 assignments, slice assignments, or methods like :meth:`append` and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	433 :meth:`extend`.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	434
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	435 It is best to think of a dictionary as an unordered set of key: value pairs,
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	436 with the requirement that the keys are unique (within one dictionary). A pair of
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	437 braces creates an empty dictionary: ``{}``. Placing a comma-separated list of
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	438 key:value pairs within the braces adds initial key:value pairs to the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	439 dictionary; this is also the way dictionaries are written on output.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	440
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	441 The main operations on a dictionary are storing a value with some key and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	442 extracting the value given the key. It is also possible to delete a key:value
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	443 pair with ``del``. If you store using a key that is already in use, the old
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	444 value associated with that key is forgotten. It is an error to extract a value
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	445 using a non-existent key.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	446
50176 ea9c6026ae7e #4667: fix some 2.x leftovers in the tutorial. Georg Brandl <georg@python.org> parents: 50165 diff changeset	447 Performing ``list(d.keys())`` on a dictionary returns a list of all the keys
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	448 used in the dictionary, in arbitrary order (if you want it sorted, just use
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	449 ``sorted(d.keys())`` instead). [1]_ To check whether a single key is in the
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	450 dictionary, use the :keyword:`in` keyword.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	451
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	452 Here is a small example using a dictionary::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	453
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	454 >>> tel = {'jack': 4098, 'sape': 4139}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	455 >>> tel['guido'] = 4127
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	456 >>> tel
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	457 {'sape': 4139, 'guido': 4127, 'jack': 4098}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	458 >>> tel['jack']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	459 4098
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	460 >>> del tel['sape']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	461 >>> tel['irv'] = 4127
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	462 >>> tel
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	463 {'guido': 4127, 'irv': 4127, 'jack': 4098}
43117 7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	464 >>> list(tel.keys())
50176 ea9c6026ae7e #4667: fix some 2.x leftovers in the tutorial. Georg Brandl <georg@python.org> parents: 50165 diff changeset	465 ['irv', 'guido', 'jack']
ea9c6026ae7e #4667: fix some 2.x leftovers in the tutorial. Georg Brandl <georg@python.org> parents: 50165 diff changeset	466 >>> sorted(tel.keys())
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	467 ['guido', 'irv', 'jack']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	468 >>> 'guido' in tel
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	469 True
43117 7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	470 >>> 'jack' not in tel
7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	471 False
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	472
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	473 The :func:`dict` constructor builds dictionaries directly from sequences of
54773 f0195daadffb The key-value pairs can be lists or tuples or any iterable. Raymond Hettinger <python@rcn.com> parents: 54769 diff changeset	474 key-value pairs::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	475
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	476 >>> dict([('sape', 4139), ('guido', 4127), ('jack', 4098)])
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	477 {'sape': 4139, 'jack': 4098, 'guido': 4127}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	478
45084 38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	479 In addition, dict comprehensions can be used to create dictionaries from
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	480 arbitrary key and value expressions::
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	481
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	482 >>> {x: x**2 for x in (2, 4, 6)}
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	483 {2: 4, 4: 16, 6: 36}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	484
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	485 When the keys are simple strings, it is sometimes easier to specify pairs using
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	486 keyword arguments::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	487
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	488 >>> dict(sape=4139, guido=4127, jack=4098)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	489 {'sape': 4139, 'jack': 4098, 'guido': 4127}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	490
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	491
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	492 .. _tut-loopidioms:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	493
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	494 Looping Techniques
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	495 ==================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	496
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	497 When looping through dictionaries, the key and corresponding value can be
43117 7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	498 retrieved at the same time using the :meth:`items` method. ::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	499
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	500 >>> knights = {'gallahad': 'the pure', 'robin': 'the brave'}
43117 7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	501 >>> for k, v in knights.items():
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	502 ... print(k, v)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	503 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	504 gallahad the pure
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	505 robin the brave
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	506
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	507 When looping through a sequence, the position index and corresponding value can
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	508 be retrieved at the same time using the :func:`enumerate` function. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	509
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	510 >>> for i, v in enumerate(['tic', 'tac', 'toe']):
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	511 ... print(i, v)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	512 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	513 0 tic
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	514 1 tac
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	515 2 toe
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	516
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	517 To loop over two or more sequences at the same time, the entries can be paired
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	518 with the :func:`zip` function. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	519
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	520 >>> questions = ['name', 'quest', 'favorite color']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	521 >>> answers = ['lancelot', 'the holy grail', 'blue']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	522 >>> for q, a in zip(questions, answers):
47283 f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 46978 diff changeset	523 ... print('What is your {0}? It is {1}.'.format(q, a))
50517 83efc2467faf Fix a few remaining problems found by rstlint. Georg Brandl <georg@python.org> parents: 50515 diff changeset	524 ...
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	525 What is your name? It is lancelot.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	526 What is your quest? It is the holy grail.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	527 What is your favorite color? It is blue.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	528
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	529 To loop over a sequence in reverse, first specify the sequence in a forward
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	530 direction and then call the :func:`reversed` function. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	531
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43117 diff changeset	532 >>> for i in reversed(range(1, 10, 2)):
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	533 ... print(i)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	534 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	535 9
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	536 7
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	537 5
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	538 3
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	539 1
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	540
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	541 To loop over a sequence in sorted order, use the :func:`sorted` function which
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	542 returns a new sorted list while leaving the source unaltered. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	543
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	544 >>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	545 >>> for f in sorted(set(basket)):
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	546 ... print(f)
50517 83efc2467faf Fix a few remaining problems found by rstlint. Georg Brandl <georg@python.org> parents: 50515 diff changeset	547 ...
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	548 apple
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	549 banana
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	550 orange
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	551 pear
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	552
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	553
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	554 .. _tut-conditions:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	555
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	556 More on Conditions
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	557 ==================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	558
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	559 The conditions used in ``while`` and ``if`` statements can contain any
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	560 operators, not just comparisons.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	561
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	562 The comparison operators ``in`` and ``not in`` check whether a value occurs
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	563 (does not occur) in a sequence. The operators ``is`` and ``is not`` compare
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	564 whether two objects are really the same object; this only matters for mutable
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	565 objects like lists. All comparison operators have the same priority, which is
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	566 lower than that of all numerical operators.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	567
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	568 Comparisons can be chained. For example, ``a < b == c`` tests whether ``a`` is
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	569 less than ``b`` and moreover ``b`` equals ``c``.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	570
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	571 Comparisons may be combined using the Boolean operators ``and`` and ``or``, and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	572 the outcome of a comparison (or of any other Boolean expression) may be negated
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	573 with ``not``. These have lower priorities than comparison operators; between
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	574 them, ``not`` has the highest priority and ``or`` the lowest, so that ``A and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	575 not B or C`` is equivalent to ``(A and (not B)) or C``. As always, parentheses
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	576 can be used to express the desired composition.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	577
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	578 The Boolean operators ``and`` and ``or`` are so-called short-circuit
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	579 operators: their arguments are evaluated from left to right, and evaluation
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	580 stops as soon as the outcome is determined. For example, if ``A`` and ``C`` are
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	581 true but ``B`` is false, ``A and B and C`` does not evaluate the expression
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	582 ``C``. When used as a general value and not as a Boolean, the return value of a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	583 short-circuit operator is the last evaluated argument.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	584
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	585 It is possible to assign the result of a comparison or other Boolean expression
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	586 to a variable. For example, ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	587
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	588 >>> string1, string2, string3 = '', 'Trondheim', 'Hammer Dance'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	589 >>> non_null = string1 or string2 or string3
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	590 >>> non_null
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	591 'Trondheim'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	592
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	593 Note that in Python, unlike C, assignment cannot occur inside expressions. C
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	594 programmers may grumble about this, but it avoids a common class of problems
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	595 encountered in C programs: typing ``=`` in an expression when ``==`` was
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	596 intended.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	597
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	598
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	599 .. _tut-comparing:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	600
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	601 Comparing Sequences and Other Types
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	602 ===================================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	603
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	604 Sequence objects may be compared to other objects with the same sequence type.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	605 The comparison uses lexicographical ordering: first the first two items are
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	606 compared, and if they differ this determines the outcome of the comparison; if
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	607 they are equal, the next two items are compared, and so on, until either
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	608 sequence is exhausted. If two items to be compared are themselves sequences of
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	609 the same type, the lexicographical comparison is carried out recursively. If
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	610 all items of two sequences compare equal, the sequences are considered equal.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	611 If one sequence is an initial sub-sequence of the other, the shorter sequence is
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	612 the smaller (lesser) one. Lexicographical ordering for strings uses the Unicode
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	613 codepoint number to order individual characters. Some examples of comparisons
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	614 between sequences of the same type::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	615
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	616 (1, 2, 3) < (1, 2, 4)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	617 [1, 2, 3] < [1, 2, 4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	618 'ABC' < 'C' < 'Pascal' < 'Python'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	619 (1, 2, 3, 4) < (1, 2, 4)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	620 (1, 2) < (1, 2, -1)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	621 (1, 2, 3) == (1.0, 2.0, 3.0)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	622 (1, 2, ('aa', 'ab')) < (1, 2, ('abc', 'a'), 4)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	623
43497 6614d42d7f0a #1228: new comparison behavior. Georg Brandl <georg@python.org> parents: 43303 diff changeset	624 Note that comparing objects of different types with ``<`` or ``>`` is legal
6614d42d7f0a #1228: new comparison behavior. Georg Brandl <georg@python.org> parents: 43303 diff changeset	625 provided that the objects have appropriate comparison methods. For example,
6614d42d7f0a #1228: new comparison behavior. Georg Brandl <georg@python.org> parents: 43303 diff changeset	626 mixed numeric types are compared according to their numeric value, so 0 equals
6614d42d7f0a #1228: new comparison behavior. Georg Brandl <georg@python.org> parents: 43303 diff changeset	627 0.0, etc. Otherwise, rather than providing an arbitrary ordering, the
6614d42d7f0a #1228: new comparison behavior. Georg Brandl <georg@python.org> parents: 43303 diff changeset	628 interpreter will raise a :exc:`TypeError` exception.
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	629
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	630
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	631 .. rubric:: Footnotes
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	632
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	633 .. [1] Calling ``d.keys()`` will return a :dfn:`dictionary view` object. It
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	634 supports operations like membership test and iteration, but its contents
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	635 are not independent of the original dictionary -- it is only a view.

Mercurial > cpython-lru_cache-weakref

annotate Doc/tutorial/datastructures.rst @ 60008:0dfd62076ded