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

annotate Doc/tutorial/datastructures.rst @ 72990:8564d2b240b6

default - Fix closes Issue12529 - cgi.parse_header failure on double quotes and semicolons. Patch by Ben Darnell and Petri Lehtinen.

author	Senthil Kumaran <senthil@uthcode.com>
date	Thu, 20 Oct 2011 01:06:59 +0800
parents	352d075839f7
children	37094a1454ab

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
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	171 List comprehensions provide a concise way to create lists from sequences.
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	172 Common applications are to make lists where each element is the result of
50514 c3d3b760d8bf Remove trailing whitespace. Georg Brandl <georg@python.org> parents: 50176 diff changeset	173 some operations applied to each member of the sequence, or to create a
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	174 subsequence of those elements that satisfy a certain condition.
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	175
54630 2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	176 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	177 by a :keyword:`for` clause, then zero or more :keyword:`for` or :keyword:`if`
2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	178 clauses. The result will be a list resulting from evaluating the expression in
2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	179 the context of the :keyword:`for` and :keyword:`if` clauses which follow it. If
2795d6fb21e2 Elaborate encoding recommendations, and fix ambiguous wording for list comprehensions. Georg Brandl <georg@python.org> parents: 52892 diff changeset	180 the expression would evaluate to a tuple, it must be parenthesized.
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	181
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	182 Here we take a list of numbers and return a list of three times each number::
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	183
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	184 >>> vec = [2, 4, 6]
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	185 >>> [3*x for x in vec]
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	186 [6, 12, 18]
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	187
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	188 Now we get a little fancier::
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	189
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43117 diff changeset	190 >>> [[x, x**2] for x in vec]
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	191 [[2, 4], [4, 16], [6, 36]]
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	192
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	193 Here we apply a method call to each item in a sequence::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	194
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	195 >>> freshfruit = [' banana', ' loganberry ', 'passion fruit ']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	196 >>> [weapon.strip() for weapon in freshfruit]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	197 ['banana', 'loganberry', 'passion fruit']
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	198
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43117 diff changeset	199 Using the :keyword:`if` clause we can filter the stream::
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	200
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	201 >>> [3*x for x in vec if x > 3]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	202 [12, 18]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	203 >>> [3*x for x in vec if x < 2]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	204 []
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	205
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	206 Tuples can often be created without their parentheses, but not here::
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	207
50515 0e05b6ca6afd Merged revisions 68221 via svnmerge from Georg Brandl <georg@python.org> parents: 50514 diff changeset	208 >>> [x, x**2 for x in vec] # error - parens required for tuples
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	209 File "<stdin>", line 1, in ?
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	210 [x, x**2 for x in vec]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	211 ^
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	212 SyntaxError: invalid syntax
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	213 >>> [(x, x**2) for x in vec]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	214 [(2, 4), (4, 16), (6, 36)]
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	215
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43117 diff changeset	216 Here are some nested for loops and other fancy behavior::
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	217
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	218 >>> vec1 = [2, 4, 6]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	219 >>> vec2 = [4, 3, -9]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	220 >>> [x*y for x in vec1 for y in vec2]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	221 [8, 6, -18, 16, 12, -36, 24, 18, -54]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	222 >>> [x+y for x in vec1 for y in vec2]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	223 [6, 5, -7, 8, 7, -5, 10, 9, -3]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	224 >>> [vec1[i]*vec2[i] for i in range(len(vec1))]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	225 [8, 12, -54]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	226
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	227 List comprehensions can be applied to complex expressions and nested functions::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	228
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	229 >>> [str(round(355/113, i)) for i in range(1, 6)]
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	230 ['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	231
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	232
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	233 Nested List Comprehensions
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	234 --------------------------
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	235
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	236 If you've got the stomach for it, list comprehensions can be nested. They are a
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	237 powerful tool but -- like all powerful tools -- they need to be used carefully,
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	238 if at all.
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	239
50514 c3d3b760d8bf Remove trailing whitespace. Georg Brandl <georg@python.org> parents: 50176 diff changeset	240 Consider the following example of a 3x3 matrix held as a list containing three
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	241 lists, one list per row::
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	242
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	243 >>> mat = [
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	244 ... [1, 2, 3],
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	245 ... [4, 5, 6],
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	246 ... [7, 8, 9],
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	247 ... ]
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	248
50514 c3d3b760d8bf Remove trailing whitespace. Georg Brandl <georg@python.org> parents: 50176 diff changeset	249 Now, if you wanted to swap rows and columns, you could use a list
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	250 comprehension::
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	251
46978 55d8c35322a7 Convert a lot of print statements to print functions in docstrings, Neal Norwitz <nnorwitz@gmail.com> parents: 46948 diff changeset	252 >>> print([[row[i] for row in mat] for i in [0, 1, 2]])
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	253 [[1, 4, 7], [2, 5, 8], [3, 6, 9]]
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	254
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	255 Special care has to be taken for the nested list comprehension:
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	256
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	257 To avoid apprehension when nesting list comprehensions, read from right to
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	258 left.
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	259
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	260 A more verbose version of this snippet shows the flow explicitly::
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	261
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	262 for i in [0, 1, 2]:
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	263 for row in mat:
46978 55d8c35322a7 Convert a lot of print statements to print functions in docstrings, Neal Norwitz <nnorwitz@gmail.com> parents: 46948 diff changeset	264 print(row[i], end="")
48438 a0882bfaf54d fix no-op in tutorial Benjamin Peterson <benjamin@python.org> parents: 47283 diff changeset	265 print()
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	266
51776 d06a3ebf306f Merged revisions 69846 via svnmerge from Mark Dickinson <dickinsm@gmail.com> parents: 50517 diff changeset	267 In real world, you should prefer built-in functions to complex flow statements.
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	268 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	269
48665 b2c505d4a203 #3522: zip() returns an iterator. Georg Brandl <georg@python.org> parents: 48438 diff changeset	270 >>> list(zip(*mat))
44310 e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	271 [(1, 4, 7), (2, 5, 8), (3, 6, 9)]
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	272
e70acbf26cda Merged revisions 59488-59511 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43497 diff changeset	273 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	274
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	275 .. _tut-del:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	276
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	277 The :keyword:`del` statement
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	278 ============================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	279
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	280 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	281 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	282 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	283 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	284 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	285
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	286 >>> 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	287 >>> del a[0]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	288 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	289 [1, 66.25, 333, 333, 1234.5]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	290 >>> del a[2:4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	291 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	292 [1, 66.25, 1234.5]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	293 >>> del a[:]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	294 >>> a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	295 []
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	296
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	297 :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	298
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	299 >>> del a
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	300
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	301 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	302 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	303
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	304
49180 c87b84db244c Forward-port of r66447. Georg Brandl <georg@python.org> parents: 48665 diff changeset	305 .. _tut-tuples:
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	306
44396 6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	307 Tuples and Sequences
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	308 ====================
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	309
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	310 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	311 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	312 :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	313 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	314 tuple.
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	315
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	316 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	317
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	318 >>> t = 12345, 54321, 'hello!'
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	319 >>> t[0]
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	320 12345
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	321 >>> t
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	322 (12345, 54321, 'hello!')
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	323 >>> # Tuples may be nested:
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	324 ... u = t, (1, 2, 3, 4, 5)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	325 >>> u
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	326 ((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	327
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	328 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	329 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	330 parentheses, although often parentheses are necessary anyway (if the tuple is
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	331 part of a larger expression).
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	332
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	333 Tuples have many uses. For example: (x, y) coordinate pairs, employee records
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	334 from a database, etc. Tuples, like strings, are immutable: it is not possible
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	335 to assign to the individual items of a tuple (you can simulate much of the same
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	336 effect with slicing and concatenation, though). It is also possible to create
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	337 tuples which contain mutable objects, such as lists.
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	338
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	339 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	340 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	341 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	342 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	343 in parentheses). Ugly, but effective. For example::
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	344
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	345 >>> empty = ()
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	346 >>> singleton = 'hello', # <-- note trailing comma
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	347 >>> len(empty)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	348 0
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	349 >>> len(singleton)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	350 1
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	351 >>> singleton
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	352 ('hello',)
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	353
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	354 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	355 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	356 The reverse operation is also possible::
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	357
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	358 >>> x, y, z = t
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	359
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	360 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	361 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	362 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	363 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	364 packing and sequence unpacking.
44396 6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	365
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	366 .. XXX Add a bit on the difference between tuples and lists.
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	367
6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44310 diff changeset	368
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	369 .. _tut-sets:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	370
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	371 Sets
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	372 ====
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	373
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	374 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	375 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	376 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	377 like union, intersection, difference, and symmetric difference.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	378
65119 f00440af9413 Merged revisions 84249,84264,84326-84327,84407,84476,84480-84482,84484,84530-84531,84553,84619,84684,84915-84916 via svnmerge from Georg Brandl <georg@python.org> parents: 63813 diff changeset	379 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	380 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	381 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	382
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	383 Here is a brief demonstration::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	384
63813 28fef9e7c3bb Issue #4570: Clean-up tutorial example Raymond Hettinger <python@rcn.com> parents: 61802 diff changeset	385 >>> basket = {'apple', 'orange', 'apple', 'pear', 'orange', 'banana'}
28fef9e7c3bb Issue #4570: Clean-up tutorial example Raymond Hettinger <python@rcn.com> parents: 61802 diff changeset	386 >>> 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	387 {'orange', 'banana', 'pear', 'apple'}
63813 28fef9e7c3bb Issue #4570: Clean-up tutorial example Raymond Hettinger <python@rcn.com> parents: 61802 diff changeset	388 >>> 'orange' in basket # fast membership testing
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	389 True
63813 28fef9e7c3bb Issue #4570: Clean-up tutorial example Raymond Hettinger <python@rcn.com> parents: 61802 diff changeset	390 >>> 'crabgrass' in basket
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	391 False
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	392
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	393 >>> # 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	394 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	395 >>> a = set('abracadabra')
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	396 >>> b = set('alacazam')
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	397 >>> 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	398 {'a', 'r', 'b', 'c', 'd'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	399 >>> 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	400 {'r', 'd', 'b'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	401 >>> 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	402 {'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	403 >>> 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	404 {'a', 'c'}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	405 >>> 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	406 {'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	407
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	408 Like :ref:`for lists <tut-listcomps>`, there is a set comprehension syntax::
45084 38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	409
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	410 >>> 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	411 >>> 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	412 {'r', 'd'}
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	413
42604 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 .. _tut-dictionaries:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	417
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	418 Dictionaries
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	419 ============
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	420
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	421 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	422 :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	423 "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	424 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	425 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	426 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	427 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	428 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	429 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	430 :meth:`extend`.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	431
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	432 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	433 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	434 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	435 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	436 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	437
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	438 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	439 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	440 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	441 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	442 using a non-existent key.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	443
50176 ea9c6026ae7e #4667: fix some 2.x leftovers in the tutorial. Georg Brandl <georg@python.org> parents: 50165 diff changeset	444 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	445 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	446 ``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	447 dictionary, use the :keyword:`in` keyword.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	448
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	449 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	450
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	451 >>> tel = {'jack': 4098, 'sape': 4139}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	452 >>> tel['guido'] = 4127
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	453 >>> tel
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	454 {'sape': 4139, 'guido': 4127, 'jack': 4098}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	455 >>> tel['jack']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	456 4098
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	457 >>> del tel['sape']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	458 >>> tel['irv'] = 4127
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	459 >>> tel
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	460 {'guido': 4127, 'irv': 4127, 'jack': 4098}
43117 7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	461 >>> list(tel.keys())
50176 ea9c6026ae7e #4667: fix some 2.x leftovers in the tutorial. Georg Brandl <georg@python.org> parents: 50165 diff changeset	462 ['irv', 'guido', 'jack']
ea9c6026ae7e #4667: fix some 2.x leftovers in the tutorial. Georg Brandl <georg@python.org> parents: 50165 diff changeset	463 >>> sorted(tel.keys())
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	464 ['guido', 'irv', 'jack']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	465 >>> 'guido' in tel
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	466 True
43117 7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	467 >>> 'jack' not in tel
7ea1367ba275 A few more fixes to the tutorial Neal Norwitz <nnorwitz@gmail.com> parents: 43115 diff changeset	468 False
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	469
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	470 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	471 key-value pairs::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	472
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	473 >>> dict([('sape', 4139), ('guido', 4127), ('jack', 4098)])
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	474 {'sape': 4139, 'jack': 4098, 'guido': 4127}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	475
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	476 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	477 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	478
38604fa2a48f Update docs w.r.t. PEP 3100 changes -- patch for GHOP by Dan Finnie. Georg Brandl <georg@python.org> parents: 44396 diff changeset	479 >>> {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	480 {2: 4, 4: 16, 6: 36}
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	481
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	482 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	483 keyword arguments::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	484
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	485 >>> dict(sape=4139, guido=4127, jack=4098)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	486 {'sape': 4139, 'jack': 4098, 'guido': 4127}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	487
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	488
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	489 .. _tut-loopidioms:
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 Looping Techniques
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
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	494 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	495 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	496
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	497 >>> 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	498 >>> 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	499 ... print(k, v)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	500 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	501 gallahad the pure
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	502 robin the brave
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	503
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	504 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	505 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	506
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	507 >>> 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	508 ... print(i, v)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	509 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	510 0 tic
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	511 1 tac
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	512 2 toe
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	513
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	514 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	515 with the :func:`zip` function. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	516
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	517 >>> questions = ['name', 'quest', 'favorite color']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	518 >>> answers = ['lancelot', 'the holy grail', 'blue']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	519 >>> for q, a in zip(questions, answers):
47283 f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 46978 diff changeset	520 ... 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	521 ...
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	522 What is your name? It is lancelot.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	523 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	524 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	525
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	526 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	527 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	528
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43117 diff changeset	529 >>> 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	530 ... print(i)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	531 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	532 9
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	533 7
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	534 5
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	535 3
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	536 1
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	537
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	538 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	539 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	540
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	541 >>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	542 >>> 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	543 ... print(f)
50517 83efc2467faf Fix a few remaining problems found by rstlint. Georg Brandl <georg@python.org> parents: 50515 diff changeset	544 ...
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	545 apple
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	546 banana
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	547 orange
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	548 pear
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	549
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	550
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	551 .. _tut-conditions:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	552
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	553 More on Conditions
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	554 ==================
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 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	557 operators, not just comparisons.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	558
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	559 The 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	560 (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	561 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	562 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	563 lower than that of all numerical operators.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	564
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	565 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	566 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	567
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	568 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	569 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	570 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	571 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	572 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	573 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	574
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	575 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	576 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	577 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	578 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	579 ``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	580 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	581
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	582 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	583 to a variable. For example, ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	584
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	585 >>> string1, string2, string3 = '', 'Trondheim', 'Hammer Dance'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	586 >>> non_null = string1 or string2 or string3
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	587 >>> non_null
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	588 'Trondheim'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	589
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	590 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	591 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	592 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	593 intended.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	594
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	595
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	596 .. _tut-comparing:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	597
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	598 Comparing Sequences and Other Types
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	599 ===================================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	600
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	601 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	602 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	603 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	604 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	605 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	606 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	607 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	608 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	609 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	610 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	611 between sequences of the same type::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	612
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	613 (1, 2, 3) < (1, 2, 4)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	614 [1, 2, 3] < [1, 2, 4]
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	615 'ABC' < 'C' < 'Pascal' < 'Python'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	616 (1, 2, 3, 4) < (1, 2, 4)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	617 (1, 2) < (1, 2, -1)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	618 (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	619 (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	620
43497 6614d42d7f0a #1228: new comparison behavior. Georg Brandl <georg@python.org> parents: 43303 diff changeset	621 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	622 provided that the objects have appropriate comparison methods. For example,
6614d42d7f0a #1228: new comparison behavior. Georg Brandl <georg@python.org> parents: 43303 diff changeset	623 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	624 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	625 interpreter will raise a :exc:`TypeError` exception.
54769 eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	626
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	627
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	628 .. rubric:: Footnotes
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	629
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	630 .. [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	631 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	632
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	633 .. [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	634 supports operations like membership test and iteration, but its contents
eea4d237c16d Expand a bit on dict views. Georg Brandl <georg@python.org> parents: 54632 diff changeset	635 are not independent of the original dictionary -- it is only a view.

Mercurial > cpython-lru_cache-weakref

annotate Doc/tutorial/datastructures.rst @ 72990:8564d2b240b6