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

annotate Doc/tutorial/datastructures.rst @ 83447:678564fc0962 3.3

#17789: test_random now works with unittest test discovery. Patch by Zachary Ware.

author	Ezio Melotti <ezio.melotti@gmail.com>
date	Fri, 19 Apr 2013 05:45:27 +0300
parents	8a5a68d988f9
children	d9b58229aabe

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
72808 352d075839f7 Closes #12192: Document that mutating list methods do not return the instance (original patch by Mike Hoy). Georg Brandl <georg@python.org> parents: 68257 diff changeset	22 Add an item to the end of the list. Equivalent to ``a[len(a):] = [x]``.
42604 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
72808 352d075839f7 Closes #12192: Document that mutating list methods do not return the instance (original patch by Mike Hoy). Georg Brandl <georg@python.org> parents: 68257 diff changeset	28 Extend the list by appending all the items in the given list. Equivalent to
42604 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
72808 352d075839f7 Closes #12192: Document that mutating list methods do not return the instance (original patch by Mike Hoy). Georg Brandl <georg@python.org> parents: 68257 diff changeset	43 Remove the first item from the list whose value is x. It is an error if
352d075839f7 Closes #12192: Document that mutating list methods do not return the instance (original patch by Mike Hoy). Georg Brandl <georg@python.org> parents: 68257 diff changeset	44 there is no such item.
42604 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
72808 352d075839f7 Closes #12192: Document that mutating list methods do not return the instance (original patch by Mike Hoy). Georg Brandl <georg@python.org> parents: 68257 diff changeset	73 Sort the items of the list in place.
42604 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
72808 352d075839f7 Closes #12192: Document that mutating list methods do not return the instance (original patch by Mike Hoy). Georg Brandl <georg@python.org> parents: 68257 diff changeset	79 Reverse the elements of the list in place.
352d075839f7 Closes #12192: Document that mutating list methods do not return the instance (original patch by Mike Hoy). Georg Brandl <georg@python.org> parents: 68257 diff changeset	80
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	81
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	82 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	83
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	84 >>> 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	85 >>> 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	86 2 1 0
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	87 >>> a.insert(2, -1)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	88 >>> a.append(333)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	89 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	90 [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	91 >>> a.index(333)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	92 1
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	93 >>> a.remove(333)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	94 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	95 [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	96 >>> a.reverse()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	97 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	98 [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	99 >>> a.sort()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	100 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	101 [-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	102
72808 352d075839f7 Closes #12192: Document that mutating list methods do not return the instance (original patch by Mike Hoy). Georg Brandl <georg@python.org> parents: 68257 diff changeset	103 You might have noticed that methods like ``insert``, ``remove`` or ``sort`` that
352d075839f7 Closes #12192: Document that mutating list methods do not return the instance (original patch by Mike Hoy). Georg Brandl <georg@python.org> parents: 68257 diff changeset	104 modify the list have no return value printed -- they return ``None``. [1]_ This
352d075839f7 Closes #12192: Document that mutating list methods do not return the instance (original patch by Mike Hoy). Georg Brandl <georg@python.org> parents: 68257 diff changeset	105 is a design principle for all mutable data structures in Python.
352d075839f7 Closes #12192: Document that mutating list methods do not return the instance (original patch by Mike Hoy). Georg Brandl <georg@python.org> parents: 68257 diff changeset	106
42604 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 .. _tut-lists-as-stacks:
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 Using Lists as Stacks
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	111 ---------------------
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	112
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	113 .. sectionauthor:: Ka-Ping Yee <ping@lfw.org>
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	114
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 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	117 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	118 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	119 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	120
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	121 >>> stack = [3, 4, 5]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	122 >>> stack.append(6)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	123 >>> stack.append(7)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	124 >>> stack
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	125 [3, 4, 5, 6, 7]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	126 >>> stack.pop()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	127 7
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	128 >>> stack
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	129 [3, 4, 5, 6]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	130 >>> stack.pop()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	131 6
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	132 >>> stack.pop()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	133 5
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	134 >>> stack
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	135 [3, 4]
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 .. _tut-lists-as-queues:
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 Using Lists as Queues
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	141 ---------------------
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	142
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	143 .. sectionauthor:: Ka-Ping Yee <ping@lfw.org>
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	144
60103 e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	145 It is also possible to use a list as a queue, where the first element added is
e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	146 the first element retrieved ("first-in, first-out"); however, lists are not
e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	147 efficient for this purpose. While appends and pops from the end of list are
e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	148 fast, doing inserts or pops from the beginning of a list is slow (because all
e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	149 of the other elements have to be shifted by one).
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	150
60103 e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	151 To implement a queue, use :class:`collections.deque` which was designed to
e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	152 have fast appends and pops from both ends. For example::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	153
60103 e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	154 >>> from collections import deque
e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	155 >>> queue = deque(["Eric", "John", "Michael"])
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	156 >>> queue.append("Terry") # Terry arrives
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	157 >>> queue.append("Graham") # Graham arrives
60103 e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	158 >>> queue.popleft() # The first to arrive now leaves
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	159 'Eric'
60103 e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	160 >>> queue.popleft() # The second to arrive now leaves
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	161 'John'
60103 e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	162 >>> queue # Remaining queue in order of arrival
e4f969012e68 Merged revisions 79525 via svnmerge from Ezio Melotti <ezio.melotti@gmail.com> parents: 54773 diff changeset	163 deque(['Michael', 'Terry', 'Graham'])
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	164
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	165
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	166 .. _tut-listcomps:
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	167
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	168 List Comprehensions
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	169 -------------------
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	170
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	171 List comprehensions provide a concise way to create lists.
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	172 Common applications are to make new lists where each element is the result of
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	173 some operations applied to each member of another sequence or iterable, or to
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	174 create a subsequence of those elements that satisfy a certain condition.
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	175
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	176 For example, assume we want to create a list of squares, like::
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	177
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	178 >>> squares = []
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	179 >>> for x in range(10):
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	180 ... squares.append(x**2)
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	181 ...
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	182 >>> squares
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	183 [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	184
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	185 We can obtain the same result with::
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	186
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	187 squares = [x**2 for x in range(10)]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	188
79211 6c96878eb729 Close issue #16073: fix map() example in list comprehension documentation. Chris Jerdonek <chris.jerdonek@gmail.com> parents: 78512 diff changeset	189 This is also equivalent to ``squares = list(map(lambda x: x**2, range(10)))``,
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	190 but it's more concise and readable.
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	191
54630 2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	192 A list comprehension consists of brackets containing an expression followed
2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	193 by a :keyword:`for` clause, then zero or more :keyword:`for` or :keyword:`if`
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	194 clauses. The result will be a new list resulting from evaluating the expression
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	195 in the context of the :keyword:`for` and :keyword:`if` clauses which follow it.
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	196 For example, this listcomp combines the elements of two lists if they are not
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	197 equal::
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	198
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	199 >>> [(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	200 [(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	201
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	202 and it's equivalent to::
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	203
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	204 >>> combs = []
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	205 >>> for x in [1,2,3]:
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	206 ... for y in [3,1,4]:
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	207 ... if x != y:
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	208 ... combs.append((x, y))
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	209 ...
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	210 >>> combs
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	211 [(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	212
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	213 Note how the order of the :keyword:`for` and :keyword:`if` statements is the
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	214 same in both these snippets.
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	215
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	216 If the expression is a tuple (e.g. the ``(x, y)`` in the previous example),
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	217 it must be parenthesized. ::
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	218
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	219 >>> vec = [-4, -2, 0, 2, 4]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	220 >>> # create a new list with the values doubled
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	221 >>> [x*2 for x in vec]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	222 [-8, -4, 0, 4, 8]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	223 >>> # filter the list to exclude negative numbers
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	224 >>> [x for x in vec if x >= 0]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	225 [0, 2, 4]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	226 >>> # apply a function to all the elements
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	227 >>> [abs(x) for x in vec]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	228 [4, 2, 0, 2, 4]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	229 >>> # call a method on each element
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	230 >>> freshfruit = [' banana', ' loganberry ', 'passion fruit ']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	231 >>> [weapon.strip() for weapon in freshfruit]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	232 ['banana', 'loganberry', 'passion fruit']
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	233 >>> # create a list of 2-tuples like (number, square)
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	234 >>> [(x, x**2) for x in range(6)]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	235 [(0, 0), (1, 1), (2, 4), (3, 9), (4, 16), (5, 25)]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	236 >>> # the tuple must be parenthesized, otherwise an error is raised
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	237 >>> [x, x**2 for x in range(6)]
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	238 File "<stdin>", line 1, in ?
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	239 [x, x**2 for x in range(6)]
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	240 ^
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	241 SyntaxError: invalid syntax
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	242 >>> # flatten a list using a listcomp with two 'for'
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	243 >>> vec = [[1,2,3], [4,5,6], [7,8,9]]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	244 >>> [num for elem in vec for num in elem]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	245 [1, 2, 3, 4, 5, 6, 7, 8, 9]
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	246
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	247 List comprehensions can contain complex expressions and nested functions::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	248
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	249 >>> from math import pi
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	250 >>> [str(round(pi, 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	251 ['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	252
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	253 Nested List Comprehensions
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	254 --------------------------
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	255
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	256 The initial expression in a list comprehension can be any arbitrary expression,
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	257 including another list comprehension.
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	258
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	259 Consider the following example of a 3x4 matrix implemented as a list of
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	260 3 lists of length 4::
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	261
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	262 >>> matrix = [
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	263 ... [1, 2, 3, 4],
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	264 ... [5, 6, 7, 8],
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	265 ... [9, 10, 11, 12],
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	266 ... ]
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	267
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	268 The following list comprehension will transpose rows and columns::
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	269
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	270 >>> [[row[i] for row in matrix] for i in range(4)]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	271 [[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	272
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	273 As we saw in the previous section, the nested listcomp is evaluated in
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	274 the context of the :keyword:`for` that follows it, so this example is
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	275 equivalent to::
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	276
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	277 >>> transposed = []
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	278 >>> for i in range(4):
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	279 ... transposed.append([row[i] for row in matrix])
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	280 ...
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	281 >>> transposed
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	282 [[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	283
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	284 which, in turn, is the same as::
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	285
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	286 >>> transposed = []
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	287 >>> for i in range(4):
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	288 ... # the following 3 lines implement the nested listcomp
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	289 ... transposed_row = []
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	290 ... for row in matrix:
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	291 ... transposed_row.append(row[i])
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	292 ... transposed.append(transposed_row)
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	293 ...
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	294 >>> transposed
ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	295 [[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	296
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	297 In the real world, you should prefer built-in functions to complex flow statements.
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	298 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	299
78512 233673503217 zip() returns an iterator, make a list() of it; thanks to Martin from docs@ Sandro Tosi <sandro.tosi@gmail.com> parents: 77496 diff changeset	300 >>> list(zip(*matrix))
73952 ad5c70296c7b #13549: improve tutorial section about listcomps. Ezio Melotti <ezio.melotti@gmail.com> parents: 68257 diff changeset	301 [(1, 5, 9), (2, 6, 10), (3, 7, 11), (4, 8, 12)]
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	302
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	303 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	304
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	305 .. _tut-del:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	306
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	307 The :keyword:`del` statement
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	308 ============================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	309
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	310 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	311 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	312 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	313 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	314 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	315
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	316 >>> 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	317 >>> del a[0]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	318 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	319 [1, 66.25, 333, 333, 1234.5]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	320 >>> del a[2:4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	321 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	322 [1, 66.25, 1234.5]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	323 >>> del a[:]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	324 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	325 []
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	326
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	327 :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	328
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	329 >>> del a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	330
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	331 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	332 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	333
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	334
49180 c87b84db244c Forward-port of r66447. Georg Brandl <georg@python.org> parents: 48665 diff changeset	335 .. _tut-tuples:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	336
44396 6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	337 Tuples and Sequences
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	338 ====================
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	339
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	340 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	341 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	342 :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	343 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	344 tuple.
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	345
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	346 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	347
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	348 >>> t = 12345, 54321, 'hello!'
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	349 >>> t[0]
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	350 12345
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	351 >>> t
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	352 (12345, 54321, 'hello!')
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	353 >>> # Tuples may be nested:
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	354 ... u = t, (1, 2, 3, 4, 5)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	355 >>> u
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	356 ((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))
77496 3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	357 >>> # Tuples are immutable:
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	358 ... t[0] = 88888
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	359 Traceback (most recent call last):
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	360 File "<stdin>", line 1, in <module>
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	361 TypeError: 'tuple' object does not support item assignment
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	362 >>> # but they can contain mutable objects:
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	363 ... v = ([1, 2, 3], [3, 2, 1])
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	364 >>> v
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	365 ([1, 2, 3], [3, 2, 1])
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	366
44396 6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	367
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	368 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	369 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	370 parentheses, although often parentheses are necessary anyway (if the tuple is
77496 3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	371 part of a larger expression). It is not possible to assign to the individual
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	372 items of a tuple, however it is possible to create tuples which contain mutable
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	373 objects, such as lists.
44396 6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	374
77496 3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	375 Though tuples may seem similar to lists, they are often used in different
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	376 situations and for different purposes.
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	377 Tuples are :term:`immutable`, and usually contain an heterogeneous sequence of
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	378 elements that are accessed via unpacking (see later in this section) or indexing
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	379 (or even by attribute in the case of :func:`namedtuples <collections.namedtuple>`).
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	380 Lists are :term:`mutable`, and their elements are usually homogeneous and are
3550416d83b3 #14840: Add a bit on the difference between tuples and lists. Initial patch by Zachary Ware. Ezio Melotti <ezio.melotti@gmail.com> parents: 73952 diff changeset	381 accessed by iterating over the list.
44396 6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	382
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	383 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	384 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	385 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	386 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	387 in parentheses). Ugly, but effective. For example::
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	388
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	389 >>> empty = ()
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	390 >>> singleton = 'hello', # <-- note trailing comma
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	391 >>> len(empty)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	392 0
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	393 >>> len(singleton)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	394 1
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	395 >>> singleton
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	396 ('hello',)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	397
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	398 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	399 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	400 The reverse operation is also possible::
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	401
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	402 >>> x, y, z = t
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	403
52892 792bb885029f Merged revisions 70712,70714,70764-70765,70769-70771,70773,70776-70777,70788-70789,70824,70828,70832,70836,70842,70851,70855,70857,70866-70872,70883,70885,70893-70894,70896-70897,70903,70905-70907,70915,70927,70933,70951,70960,70962-70964,70998,71001,71006,71008,71010-71011,71019,71037,71056,71094,71101-71103,71106,71119,71123,71149-71150,71203,71212,71214-71217,71221,71240 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 51776 diff changeset	404 This is called, appropriately enough, sequence unpacking and works for any
54630 2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	405 sequence on the right-hand side. Sequence unpacking requires that there are as
2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	406 many variables on the left side of the equals sign as there are elements in the
52892 792bb885029f Merged revisions 70712,70714,70764-70765,70769-70771,70773,70776-70777,70788-70789,70824,70828,70832,70836,70842,70851,70855,70857,70866-70872,70883,70885,70893-70894,70896-70897,70903,70905-70907,70915,70927,70933,70951,70960,70962-70964,70998,71001,71006,71008,71010-71011,71019,71037,71056,71094,71101-71103,71106,71119,71123,71149-71150,71203,71212,71214-71217,71221,71240 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 51776 diff changeset	407 sequence. Note that multiple assignment is really just a combination of tuple
792bb885029f Merged revisions 70712,70714,70764-70765,70769-70771,70773,70776-70777,70788-70789,70824,70828,70832,70836,70842,70851,70855,70857,70866-70872,70883,70885,70893-70894,70896-70897,70903,70905-70907,70915,70927,70933,70951,70960,70962-70964,70998,71001,71006,71008,71010-71011,71019,71037,71056,71094,71101-71103,71106,71119,71123,71149-71150,71203,71212,71214-71217,71221,71240 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 51776 diff changeset	408 packing and sequence unpacking.
44396 6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	409
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	410
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	411 .. _tut-sets:
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 Sets
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	414 ====
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 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	417 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	418 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	419 like union, intersection, difference, and symmetric difference.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	420
80460 49cf38158c8b Rephrase a sentence in the set and dict comprehensions tutorial page. Ezio Melotti <ezio.melotti@gmail.com> parents: 79727 diff changeset	421 Curly braces or the :func:`set` function can be used to create sets. Note: to
54632 255db70bb316 Fix markup. Georg Brandl <georg@python.org> parents: 54630 diff changeset	422 create an empty set you have to use ``set()``, not ``{}``; the latter creates an
255db70bb316 Fix markup. Georg Brandl <georg@python.org> parents: 54630 diff changeset	423 empty dictionary, a data structure that we discuss in the next section.
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	424
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	425 Here is a brief demonstration::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	426
63813 28fef9e7c3bb Issue #4570: Clean-up tutorial example Raymond Hettinger <python@rcn.com> parents: 61802 diff changeset	427 >>> basket = {'apple', 'orange', 'apple', 'pear', 'orange', 'banana'}
28fef9e7c3bb Issue #4570: Clean-up tutorial example Raymond Hettinger <python@rcn.com> parents: 61802 diff changeset	428 >>> print(basket) # show that duplicates have been removed
66478 7281937b2d65 Merged revisions 86134,86315-86316,86390,86424-86425,86428,86550,86561-86562,86564-86565,86705,86708,86713 via svnmerge from Georg Brandl <georg@python.org> parents: 65119 diff changeset	429 {'orange', 'banana', 'pear', 'apple'}
63813 28fef9e7c3bb Issue #4570: Clean-up tutorial example Raymond Hettinger <python@rcn.com> parents: 61802 diff changeset	430 >>> 'orange' in basket # fast membership testing
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	431 True
63813 28fef9e7c3bb Issue #4570: Clean-up tutorial example Raymond Hettinger <python@rcn.com> parents: 61802 diff changeset	432 >>> 'crabgrass' in basket
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	433 False
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	434
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	435 >>> # 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	436 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	437 >>> a = set('abracadabra')
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	438 >>> b = set('alacazam')
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	439 >>> 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	440 {'a', 'r', 'b', 'c', 'd'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	441 >>> 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	442 {'r', 'd', 'b'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	443 >>> 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	444 {'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	445 >>> 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	446 {'a', 'c'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	447 >>> 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	448 {'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	449
80460 49cf38158c8b Rephrase a sentence in the set and dict comprehensions tutorial page. Ezio Melotti <ezio.melotti@gmail.com> parents: 79727 diff changeset	450 Similarly to :ref:`list comprehensions <tut-listcomps>`, set comprehensions
49cf38158c8b Rephrase a sentence in the set and dict comprehensions tutorial page. Ezio Melotti <ezio.melotti@gmail.com> parents: 79727 diff changeset	451 are also supported::
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	452
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	453 >>> 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	454 >>> 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	455 {'r', 'd'}
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	456
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	457
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	458 .. _tut-dictionaries:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	459
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	460 Dictionaries
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	461 ============
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	462
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	463 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	464 :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	465 "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	466 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	467 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	468 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	469 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	470 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	471 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	472 :meth:`extend`.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	473
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	474 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	475 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	476 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	477 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	478 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	479
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	480 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	481 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	482 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	483 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	484 using a non-existent key.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	485
50176 ea9c6026ae7e #4667: fix some 2.x leftovers in the tutorial. Georg Brandl <georg@python.org> parents: 50165 diff changeset	486 Performing ``list(d.keys())`` on a dictionary returns a list of all the keys
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	487 used in the dictionary, in arbitrary order (if you want it sorted, just use
72808 352d075839f7 Closes #12192: Document that mutating list methods do not return the instance (original patch by Mike Hoy). Georg Brandl <georg@python.org> parents: 68257 diff changeset	488 ``sorted(d.keys())`` instead). [2]_ To check whether a single key is in the
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	489 dictionary, use the :keyword:`in` keyword.
42604 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 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	492
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	493 >>> tel = {'jack': 4098, 'sape': 4139}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	494 >>> tel['guido'] = 4127
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	495 >>> tel
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	496 {'sape': 4139, 'guido': 4127, 'jack': 4098}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	497 >>> tel['jack']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	498 4098
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	499 >>> del tel['sape']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	500 >>> tel['irv'] = 4127
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	501 >>> tel
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	502 {'guido': 4127, 'irv': 4127, 'jack': 4098}
43117 7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	503 >>> list(tel.keys())
50176 ea9c6026ae7e #4667: fix some 2.x leftovers in the tutorial. Georg Brandl <georg@python.org> parents: 50165 diff changeset	504 ['irv', 'guido', 'jack']
ea9c6026ae7e #4667: fix some 2.x leftovers in the tutorial. Georg Brandl <georg@python.org> parents: 50165 diff changeset	505 >>> sorted(tel.keys())
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	506 ['guido', 'irv', 'jack']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	507 >>> 'guido' in tel
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	508 True
43117 7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	509 >>> 'jack' not in tel
7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	510 False
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	511
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	512 The :func:`dict` constructor builds dictionaries directly from sequences of
54773 f0195daadffb The key-value pairs can be lists or tuples or any iterable. Raymond Hettinger <python@rcn.com> parents: 54769 diff changeset	513 key-value pairs::
42604 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 >>> dict([('sape', 4139), ('guido', 4127), ('jack', 4098)])
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	516 {'sape': 4139, 'jack': 4098, 'guido': 4127}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	517
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	518 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	519 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	520
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	521 >>> {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	522 {2: 4, 4: 16, 6: 36}
42604 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 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	525 keyword arguments::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	526
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	527 >>> dict(sape=4139, guido=4127, jack=4098)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	528 {'sape': 4139, 'jack': 4098, 'guido': 4127}
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
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	531 .. _tut-loopidioms:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	532
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	533 Looping Techniques
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	534 ==================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	535
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	536 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	537 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	538
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	539 >>> 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	540 >>> 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	541 ... print(k, v)
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 gallahad the pure
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	544 robin the brave
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	545
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	546 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	547 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	548
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	549 >>> 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	550 ... print(i, v)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	551 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	552 0 tic
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	553 1 tac
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	554 2 toe
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	555
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	556 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	557 with the :func:`zip` function. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	558
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	559 >>> questions = ['name', 'quest', 'favorite color']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	560 >>> answers = ['lancelot', 'the holy grail', 'blue']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	561 >>> for q, a in zip(questions, answers):
47283 f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 46978 diff changeset	562 ... print('What is your {0}? It is {1}.'.format(q, a))
50517 83efc2467faf Fix a few remaining problems found by rstlint. Georg Brandl <georg@python.org> parents: 50515 diff changeset	563 ...
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	564 What is your name? It is lancelot.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	565 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	566 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	567
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	568 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	569 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	570
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43117 diff changeset	571 >>> 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	572 ... print(i)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	573 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	574 9
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	575 7
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	576 5
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	577 3
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	578 1
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 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	581 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	582
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	583 >>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	584 >>> 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	585 ... print(f)
50517 83efc2467faf Fix a few remaining problems found by rstlint. Georg Brandl <georg@python.org> parents: 50515 diff changeset	586 ...
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	587 apple
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	588 banana
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	589 orange
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	590 pear
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	591
79727 1f1bf6a3abbc Issue #16225: Add additional note to tutorial about changing sequence while looping. Chris Jerdonek <chris.jerdonek@gmail.com> parents: 79211 diff changeset	592 To change a sequence you are iterating over while inside the loop (for
1f1bf6a3abbc Issue #16225: Add additional note to tutorial about changing sequence while looping. Chris Jerdonek <chris.jerdonek@gmail.com> parents: 79211 diff changeset	593 example to duplicate certain items), it is recommended that you first make
1f1bf6a3abbc Issue #16225: Add additional note to tutorial about changing sequence while looping. Chris Jerdonek <chris.jerdonek@gmail.com> parents: 79211 diff changeset	594 a copy. Looping over a sequence does not implicitly make a copy. The slice
1f1bf6a3abbc Issue #16225: Add additional note to tutorial about changing sequence while looping. Chris Jerdonek <chris.jerdonek@gmail.com> parents: 79211 diff changeset	595 notation makes this especially convenient::
1f1bf6a3abbc Issue #16225: Add additional note to tutorial about changing sequence while looping. Chris Jerdonek <chris.jerdonek@gmail.com> parents: 79211 diff changeset	596
1f1bf6a3abbc Issue #16225: Add additional note to tutorial about changing sequence while looping. Chris Jerdonek <chris.jerdonek@gmail.com> parents: 79211 diff changeset	597 >>> words = ['cat', 'window', 'defenestrate']
1f1bf6a3abbc Issue #16225: Add additional note to tutorial about changing sequence while looping. Chris Jerdonek <chris.jerdonek@gmail.com> parents: 79211 diff changeset	598 >>> for w in words[:]: # Loop over a slice copy of the entire list.
1f1bf6a3abbc Issue #16225: Add additional note to tutorial about changing sequence while looping. Chris Jerdonek <chris.jerdonek@gmail.com> parents: 79211 diff changeset	599 ... if len(w) > 6:
1f1bf6a3abbc Issue #16225: Add additional note to tutorial about changing sequence while looping. Chris Jerdonek <chris.jerdonek@gmail.com> parents: 79211 diff changeset	600 ... words.insert(0, w)
1f1bf6a3abbc Issue #16225: Add additional note to tutorial about changing sequence while looping. Chris Jerdonek <chris.jerdonek@gmail.com> parents: 79211 diff changeset	601 ...
1f1bf6a3abbc Issue #16225: Add additional note to tutorial about changing sequence while looping. Chris Jerdonek <chris.jerdonek@gmail.com> parents: 79211 diff changeset	602 >>> words
1f1bf6a3abbc Issue #16225: Add additional note to tutorial about changing sequence while looping. Chris Jerdonek <chris.jerdonek@gmail.com> parents: 79211 diff changeset	603 ['defenestrate', 'cat', 'window', 'defenestrate']
1f1bf6a3abbc Issue #16225: Add additional note to tutorial about changing sequence while looping. Chris Jerdonek <chris.jerdonek@gmail.com> parents: 79211 diff changeset	604
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	605
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	606 .. _tut-conditions:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	607
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	608 More on Conditions
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	609 ==================
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 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	612 operators, not just comparisons.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	613
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	614 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	615 (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	616 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	617 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	618 lower than that of all numerical operators.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	619
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	620 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	621 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	622
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	623 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	624 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	625 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	626 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	627 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	628 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	629
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	630 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	631 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	632 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	633 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	634 ``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	635 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	636
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	637 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	638 to a variable. For example, ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	639
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	640 >>> string1, string2, string3 = '', 'Trondheim', 'Hammer Dance'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	641 >>> non_null = string1 or string2 or string3
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	642 >>> non_null
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	643 'Trondheim'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	644
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	645 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	646 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	647 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	648 intended.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	649
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	650
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	651 .. _tut-comparing:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	652
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	653 Comparing Sequences and Other Types
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	654 ===================================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	655
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	656 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	657 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	658 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	659 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	660 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	661 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	662 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	663 If one sequence is an initial sub-sequence of the other, the shorter sequence is
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	664 the smaller (lesser) one. Lexicographical ordering for strings uses the Unicode
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	665 codepoint number to order individual characters. Some examples of comparisons
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	666 between sequences of the same type::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	667
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	668 (1, 2, 3) < (1, 2, 4)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	669 [1, 2, 3] < [1, 2, 4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	670 'ABC' < 'C' < 'Pascal' < 'Python'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	671 (1, 2, 3, 4) < (1, 2, 4)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	672 (1, 2) < (1, 2, -1)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	673 (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	674 (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	675
43497 6614d42d7f0a #1228: new comparison behavior. Georg Brandl <georg@python.org> parents: 43303 diff changeset	676 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	677 provided that the objects have appropriate comparison methods. For example,
6614d42d7f0a #1228: new comparison behavior. Georg Brandl <georg@python.org> parents: 43303 diff changeset	678 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	679 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	680 interpreter will raise a :exc:`TypeError` exception.
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	681
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	682
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	683 .. rubric:: Footnotes
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	684
72808 352d075839f7 Closes #12192: Document that mutating list methods do not return the instance (original patch by Mike Hoy). Georg Brandl <georg@python.org> parents: 68257 diff changeset	685 .. [1] Other languages may return the mutated object, which allows method
352d075839f7 Closes #12192: Document that mutating list methods do not return the instance (original patch by Mike Hoy). Georg Brandl <georg@python.org> parents: 68257 diff changeset	686 chaining, such as ``d->insert("a")->remove("b")->sort();``.
352d075839f7 Closes #12192: Document that mutating list methods do not return the instance (original patch by Mike Hoy). Georg Brandl <georg@python.org> parents: 68257 diff changeset	687
352d075839f7 Closes #12192: Document that mutating list methods do not return the instance (original patch by Mike Hoy). Georg Brandl <georg@python.org> parents: 68257 diff changeset	688 .. [2] Calling ``d.keys()`` will return a :dfn:`dictionary view` object. It
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	689 supports operations like membership test and iteration, but its contents
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	690 are not independent of the original dictionary -- it is only a view.

Mercurial > cpython-lru_cache-weakref

annotate Doc/tutorial/datastructures.rst @ 83447:678564fc0962 3.3