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

annotate Doc/tutorial/datastructures.rst @ 46948:df11aff092cd

Remove duplicated paragraph.

author	Georg Brandl <georg@python.org>
date	Mon, 12 May 2008 16:33:11 +0000
parents	376b5cb8a014
children	55d8c35322a7

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
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	156
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	157 List Comprehensions
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	158 -------------------
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	159
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	160 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	161 Common applications are to make lists where each element is the result of
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	162 some operations applied to each member of the sequence, or to create a
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	163 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	164
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	165
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	166 Each list comprehension consists of an expression followed by a :keyword:`for`
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	167 clause, then zero or more :keyword:`for` or :keyword:`if` clauses. The result
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	168 will be a list resulting from evaluating the expression in the context of the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	169 :keyword:`for` and :keyword:`if` clauses which follow it. If the expression
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	170 would evaluate to a tuple, it must be parenthesized.
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	171
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	172 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	173
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	174 >>> vec = [2, 4, 6]
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	175 >>> [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	176 [6, 12, 18]
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 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	179
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43117 diff changeset	180 >>> [[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	181 [[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	182
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	183 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	184
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	185 >>> freshfruit = [' banana', ' loganberry ', 'passion fruit ']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	186 >>> [weapon.strip() for weapon in freshfruit]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	187 ['banana', 'loganberry', 'passion fruit']
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	188
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43117 diff changeset	189 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	190
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	191 >>> [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	192 [12, 18]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	193 >>> [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	194 []
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	195
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	196 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	197
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	198 >>> [x, x**2 for x in vec] # error - parens required for tuples
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	199 File "<stdin>", line 1, in ?
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	200 [x, x**2 for x in vec]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	201 ^
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	202 SyntaxError: invalid syntax
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	203 >>> [(x, x**2) for x in vec]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	204 [(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	205
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43117 diff changeset	206 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	207
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	208 >>> vec1 = [2, 4, 6]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	209 >>> vec2 = [4, 3, -9]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	210 >>> [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	211 [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	212 >>> [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	213 [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	214 >>> [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	215 [8, 12, -54]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	216
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	217 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	218
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	219 >>> [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	220 ['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	221
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	222
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	223 Nested List Comprehensions
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	224 --------------------------
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	225
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	226 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	227 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	228 if at all.
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	229
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	230 Consider the following example of a 3x3 matrix held as a list containing three
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	231 lists, one list per row::
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	232
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	233 >>> mat = [
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	234 ... [1, 2, 3],
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	235 ... [4, 5, 6],
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	236 ... [7, 8, 9],
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	237 ... ]
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 Now, if you wanted to swap rows and columns, you could use a list
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	240 comprehension::
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	241
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	242 >>> print [[row[i] for row in mat] for i in [0, 1, 2]]
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	243 [[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	244
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	245 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	246
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	247 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	248 left.
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	249
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	250 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	251
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	252 for i in [0, 1, 2]:
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	253 for row in mat:
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	254 print row[i],
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	255 print
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	256
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	257 In real world, you should prefer builtin functions to complex flow statements.
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	258 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	259
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	260 >>> zip(*mat)
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	261 [(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	262
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	263 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	264
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	265 .. _tut-del:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	266
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	267 The :keyword:`del` statement
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	268 ============================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	269
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	270 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	271 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	272 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	273 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	274 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	275
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	276 >>> 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	277 >>> del a[0]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	278 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	279 [1, 66.25, 333, 333, 1234.5]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	280 >>> del a[2:4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	281 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	282 [1, 66.25, 1234.5]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	283 >>> del a[:]
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 []
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	286
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	287 :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	288
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
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	291 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	292 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	293
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
44396 6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	296 Tuples and Sequences
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	297 ====================
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	298
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	299 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	300 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	301 :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	302 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	303 tuple.
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 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	306
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	307 >>> t = 12345, 54321, 'hello!'
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	308 >>> t[0]
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	309 12345
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	310 >>> t
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	311 (12345, 54321, 'hello!')
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	312 >>> # Tuples may be nested:
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	313 ... u = t, (1, 2, 3, 4, 5)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	314 >>> u
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	315 ((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	316
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	317 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	318 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	319 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	320 part of a larger expression).
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	321
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	322 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	323 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	324 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	325 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	326 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	327
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	328 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	329 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	330 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	331 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	332 in parentheses). Ugly, but effective. For example::
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	333
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	334 >>> empty = ()
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	335 >>> singleton = 'hello', # <-- note trailing comma
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	336 >>> len(empty)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	337 0
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	338 >>> len(singleton)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	339 1
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	340 >>> singleton
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	341 ('hello',)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	342
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	343 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	344 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	345 The reverse operation is also possible::
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	346
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	347 >>> x, y, z = t
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	348
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	349 This is called, appropriately enough, sequence unpacking. Sequence unpacking
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	350 requires the list of variables on the left to have the same number of elements
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	351 as the length of the sequence. Note that multiple assignment is really just a
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	352 combination of tuple packing and sequence unpacking!
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 There is a small bit of asymmetry here: packing multiple values always creates
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	355 a tuple, and unpacking works for any sequence.
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	356
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	357 .. 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	358
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	359
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	360 .. _tut-sets:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	361
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	362 Sets
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	363 ====
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	364
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	365 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	366 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	367 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	368 like union, intersection, difference, and symmetric difference.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	369
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	370 Curly braces or the :func:`set` function can be use to create sets. Note:
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	371 To create an empty set you have to use set(), not {}; the latter creates
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	372 an empty dictionary, a data structure that we discuss in the next section.
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	373
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	374 Here is a brief demonstration::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	375
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	376 >>> 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	377 >>> print(basket)
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	378 {'orange', 'bananna', 'pear', 'apple'}
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	379 >>> 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	380 >>> 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	381 >>> fruit
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	382 {'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	383 >>> 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	384 >>> 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	385 {'orange', 'apple'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	386 >>> 'orange' in fruit # fast membership testing
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	387 True
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	388 >>> 'crabgrass' in fruit
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	389 False
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	390
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	391 >>> # 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	392 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	393 >>> a = set('abracadabra')
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	394 >>> b = set('alacazam')
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	395 >>> 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	396 {'a', 'r', 'b', 'c', 'd'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	397 >>> 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	398 {'r', 'd', 'b'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	399 >>> 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	400 {'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	401 >>> 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	402 {'a', 'c'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	403 >>> 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	404 {'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	405
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	406 Like for lists, there is a set comprehension syntax::
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	407
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	408 >>> 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	409 >>> 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	410 {'r', 'd'}
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	411
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	412
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	413
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	414 .. _tut-dictionaries:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	415
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	416 Dictionaries
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 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	420 :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	421 "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	422 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	423 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	424 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	425 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	426 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	427 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	428 :meth:`extend`.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	429
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	430 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	431 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	432 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	433 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	434 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	435
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	436 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	437 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	438 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	439 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	440 using a non-existent key.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	441
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	442 The :meth:`keys` method of a dictionary object returns a list of all the keys
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	443 used in the dictionary, in arbitrary order (if you want it sorted, just apply
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	444 the :meth:`sort` method to the list of keys). To check whether a single key is
43303 9176a53b4b62 Remove yet more references to has_key() methods. Collin Winter <collinw@gmail.com> parents: 43201 diff changeset	445 in the dictionary, use the :keyword:`in` keyword.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	446
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	447 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	448
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	449 >>> tel = {'jack': 4098, 'sape': 4139}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	450 >>> tel['guido'] = 4127
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	451 >>> tel
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	452 {'sape': 4139, 'guido': 4127, 'jack': 4098}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	453 >>> tel['jack']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	454 4098
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	455 >>> del tel['sape']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	456 >>> tel['irv'] = 4127
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	457 >>> tel
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	458 {'guido': 4127, 'irv': 4127, 'jack': 4098}
43117 7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	459 >>> list(tel.keys())
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	460 ['guido', 'irv', 'jack']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	461 >>> 'guido' in tel
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	462 True
43117 7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	463 >>> 'jack' not in tel
7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	464 False
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	465
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	466 The :func:`dict` constructor builds dictionaries directly from lists of
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	467 key-value pairs stored as tuples. When the pairs form a pattern, list
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	468 comprehensions can compactly specify the key-value list. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	469
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	470 >>> dict([('sape', 4139), ('guido', 4127), ('jack', 4098)])
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	471 {'sape': 4139, 'jack': 4098, 'guido': 4127}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	472
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	473 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	474 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	475
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	476 >>> {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	477 {2: 4, 4: 16, 6: 36}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	478
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	479 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	480 keyword arguments::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	481
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	482 >>> dict(sape=4139, guido=4127, jack=4098)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	483 {'sape': 4139, 'jack': 4098, 'guido': 4127}
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
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	486 .. XXX Find out the right way to do these DUBOIS
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	487 .. _tut-loopidioms:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	488
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	489 Looping Techniques
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 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	493 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	494
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	495 >>> 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	496 >>> 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	497 ... print(k, v)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	498 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	499 gallahad the pure
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	500 robin the brave
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	501
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	502 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	503 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	504
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	505 >>> 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	506 ... print(i, v)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	507 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	508 0 tic
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	509 1 tac
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	510 2 toe
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	511
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	512 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	513 with the :func:`zip` function. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	514
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	515 >>> questions = ['name', 'quest', 'favorite color']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	516 >>> answers = ['lancelot', 'the holy grail', 'blue']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	517 >>> for q, a in zip(questions, answers):
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	518 ... print('What is your %s? It is %s.' % (q, a))
42604 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 What is your name? It is lancelot.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	521 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	522 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	523
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	524 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	525 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	526
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43117 diff changeset	527 >>> 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	528 ... print(i)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	529 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	530 9
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	531 7
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	532 5
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	533 3
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	534 1
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	535
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	536 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	537 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	538
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	539 >>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	540 >>> 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	541 ... print(f)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	542 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	543 apple
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	544 banana
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	545 orange
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	546 pear
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	547
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	548
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	549 .. _tut-conditions:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	550
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	551 More on Conditions
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 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	555 operators, not just comparisons.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	556
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	557 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	558 (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	559 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	560 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	561 lower than that of all numerical operators.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	562
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	563 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	564 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	565
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	566 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	567 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	568 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	569 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	570 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	571 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	572
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	573 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	574 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	575 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	576 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	577 ``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	578 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	579
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	580 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	581 to a variable. For example, ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	582
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	583 >>> string1, string2, string3 = '', 'Trondheim', 'Hammer Dance'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	584 >>> non_null = string1 or string2 or string3
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	585 >>> non_null
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	586 'Trondheim'
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 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	589 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	590 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	591 intended.
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
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	594 .. _tut-comparing:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	595
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	596 Comparing Sequences and Other Types
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 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	600 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	601 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	602 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	603 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	604 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	605 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	606 If one sequence is an initial sub-sequence of the other, the shorter sequence is
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	607 the smaller (lesser) one. Lexicographical ordering for strings uses the ASCII
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	608 ordering for individual characters. Some examples of comparisons between
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	609 sequences of the same type::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	610
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	611 (1, 2, 3) < (1, 2, 4)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	612 [1, 2, 3] < [1, 2, 4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	613 'ABC' < 'C' < 'Pascal' < 'Python'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	614 (1, 2, 3, 4) < (1, 2, 4)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	615 (1, 2) < (1, 2, -1)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	616 (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	617 (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	618
43497 6614d42d7f0a #1228: new comparison behavior. Georg Brandl <georg@python.org> parents: 43303 diff changeset	619 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	620 provided that the objects have appropriate comparison methods. For example,
6614d42d7f0a #1228: new comparison behavior. Georg Brandl <georg@python.org> parents: 43303 diff changeset	621 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	622 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	623 interpreter will raise a :exc:`TypeError` exception.

Mercurial > cpython-lru_cache-weakref

annotate Doc/tutorial/datastructures.rst @ 46948:df11aff092cd