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

annotate Doc/tutorial/inputoutput.rst @ 47426:4dedef4c0dfd

Blocked revisions 63863 via svnmerge ........ r63863 | benjamin.peterson | 2008-06-01 14:01:25 -0500 (Sun, 01 Jun 2008) | 2 lines add a warning about bgen being removed ........

author	Benjamin Peterson <benjamin@python.org>
date	Sun, 01 Jun 2008 19:04:01 +0000
parents	f28cbab11acd
children	b51cd5d3795c

rev	line source
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	1 .. _tut-io:
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 Input and Output
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 There are several ways to present the output of a program; data can be printed
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	8 in a human-readable form, or written to a file for future use. This chapter will
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	9 discuss some of the possibilities.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	10
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 .. _tut-formatting:
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 Fancier Output Formatting
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	15 =========================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	16
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	17 So far we've encountered two ways of writing values: expression statements and
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	18 the :func:`print` function. (A third way is using the :meth:`write` method
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	19 of file objects; the standard output file can be referenced as ``sys.stdout``.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	20 See the Library Reference for more information on this.)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	21
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	22 .. index:: module: string
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 Often you'll want more control over the formatting of your output than simply
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	25 printing space-separated values. There are two ways to format your output; the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	26 first way is to do all the string handling yourself; using string slicing and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	27 concatenation operations you can create any layout you can imagine. The
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	28 standard module :mod:`string` contains some useful operations for padding
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	29 strings to a given column width; these will be discussed shortly. The second
47283 f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	30 way is to use the :meth:`str.format` method.
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	31
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	32 The :mod:`string` module contains a class Template which offers yet another way
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	33 to substitute values into strings.
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 One question remains, of course: how do you convert values to strings? Luckily,
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	36 Python has ways to convert any value to a string: pass it to the :func:`repr`
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	37 or :func:`str` functions. Reverse quotes (``````) are equivalent to
47283 f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	38 :func:`repr`, but they are no longer used in modern Python code and are removed
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	39 in future versions of the language.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	40
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	41 The :func:`str` function is meant to return representations of values which are
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	42 fairly human-readable, while :func:`repr` is meant to generate representations
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	43 which can be read by the interpreter (or will force a :exc:`SyntaxError` if
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	44 there is not equivalent syntax). For objects which don't have a particular
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	45 representation for human consumption, :func:`str` will return the same value as
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	46 :func:`repr`. Many values, such as numbers or structures like lists and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	47 dictionaries, have the same representation using either function. Strings and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	48 floating point numbers, in particular, have two distinct representations.
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 Some examples::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	51
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	52 >>> s = 'Hello, world.'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	53 >>> str(s)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	54 'Hello, world.'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	55 >>> repr(s)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	56 "'Hello, world.'"
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	57 >>> str(0.1)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	58 '0.1'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	59 >>> repr(0.1)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	60 '0.10000000000000001'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	61 >>> x = 10 * 3.25
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	62 >>> y = 200 * 200
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	63 >>> s = 'The value of x is ' + repr(x) + ', and y is ' + repr(y) + '...'
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	64 >>> print(s)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	65 The value of x is 32.5, and y is 40000...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	66 >>> # The repr() of a string adds string quotes and backslashes:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	67 ... hello = 'hello, world\n'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	68 >>> hellos = repr(hello)
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	69 >>> print(hellos)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	70 'hello, world\n'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	71 >>> # The argument to repr() may be any Python object:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	72 ... repr((x, y, ('spam', 'eggs')))
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	73 "(32.5, 40000, ('spam', 'eggs'))"
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	74 >>> # reverse quotes are convenient in interactive sessions:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	75 ... `x, y, ('spam', 'eggs')`
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	76 "(32.5, 40000, ('spam', 'eggs'))"
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	77
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	78 Here are two ways to write a table of squares and cubes::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	79
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	80 >>> for x in range(1, 11):
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43115 diff changeset	81 ... print(repr(x).rjust(2), repr(x*x).rjust(3), end=' ')
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	82 ... # Note use of 'end' on previous line
95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	83 ... print(repr(xxx).rjust(4))
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	84 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	85 1 1 1
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	86 2 4 8
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	87 3 9 27
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	88 4 16 64
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	89 5 25 125
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	90 6 36 216
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	91 7 49 343
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	92 8 64 512
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	93 9 81 729
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	94 10 100 1000
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	95
43201 4902671dd678 Tutorial formatting patch by Robin Stocker. Georg Brandl <georg@python.org> parents: 43115 diff changeset	96 >>> for x in range(1, 11):
47283 f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	97 ... print('{0:2d} {1:3d} {2:4d}'.format(x, xx, xx*x))
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	98 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	99 1 1 1
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	100 2 4 8
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	101 3 9 27
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	102 4 16 64
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	103 5 25 125
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	104 6 36 216
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	105 7 49 343
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	106 8 64 512
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	107 9 81 729
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	108 10 100 1000
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 (Note that in the first example, one space between each column was added by the
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	111 way :func:`print` works: it always adds spaces between its arguments.)
42604 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 This example demonstrates the :meth:`rjust` method of string objects, which
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	114 right-justifies a string in a field of a given width by padding it with spaces
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	115 on the left. There are similar methods :meth:`ljust` and :meth:`center`. These
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	116 methods do not write anything, they just return a new string. If the input
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	117 string is too long, they don't truncate it, but return it unchanged; this will
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	118 mess up your column lay-out but that's usually better than the alternative,
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	119 which would be lying about a value. (If you really want truncation you can
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	120 always add a slice operation, as in ``x.ljust(n)[:n]``.)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	121
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	122 There is another method, :meth:`zfill`, which pads a numeric string on the left
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	123 with zeros. It understands about plus and minus signs::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	124
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	125 >>> '12'.zfill(5)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	126 '00012'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	127 >>> '-3.14'.zfill(7)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	128 '-003.14'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	129 >>> '3.14159265359'.zfill(5)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	130 '3.14159265359'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	131
47283 f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	132 Basic usage of the :meth:`str.format` method looks like this::
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	133
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	134 >>> print 'We are the {0} who say "{1}!"'.format('knights', 'Ni')
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	135 We are the knights who say "Ni!"
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	136
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	137 The brackets and characters within them (called format fields) are replaced with
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	138 the objects passed into the format method. The number in the brackets refers to
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	139 the position of the object passed into the format method. ::
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	140
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	141 >>> print '{0} and {1}'.format('spam', 'eggs')
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	142 spam and eggs
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	143 >>> print '{1} and {0}'.format('spam', 'eggs')
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	144 eggs and spam
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	145
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	146 If keyword arguments are used in the format method, their values are referred to
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	147 by using the name of the argument. ::
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	148
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	149 >>> print 'This {food} is {adjective}.'.format(food='spam', adjective='absolutely horrible')
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	150 This spam is absolutely horrible.
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	151
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	152 Positional and keyword arguments can be arbitrarily combined::
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	153
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	154 >>> print 'The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred', other='Georg')
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	155 The story of Bill, Manfred, and Georg.
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	156
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	157 An optional ``':``` and format specifier can follow the field name. This also
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	158 greater control over how the value is formatted. The following example
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	159 truncates the Pi to three places after the decimal.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	160
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	161 >>> import math
47283 f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	162 >>> print('The value of PI is approximately {0:.3f}.'.format(math.pi))
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	163 The value of PI is approximately 3.142.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	164
47283 f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	165 Passing an integer after the ``':'`` will cause that field to be a minimum
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	166 number of characters wide. This is useful for making tables pretty.::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	167
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	168 >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	169 >>> for name, phone in table.items():
47283 f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	170 ... print('{0:10} ==> {1:10d}'.format(name, phone))
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	171 ...
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	172 Jack ==> 4098
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	173 Dcab ==> 7678
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	174 Sjoerd ==> 4127
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	175
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	176 If you have a really long format string that you don't want to split up, it
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	177 would be nice if you could reference the variables to be formatted by name
47283 f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	178 instead of by position. This can be done by simply passing the dict and using
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	179 square brackets ``'[]'`` to access the keys ::
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	180
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	181 >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
47283 f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	182 >>> print('Jack: {0[Jack]:d}; Sjoerd: {0[Sjoerd]:d}; Dcab: {0[Dcab]:d}'.format(table))
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	183 Jack: 4098; Sjoerd: 4127; Dcab: 8637678
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	184
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	185 This could also be done by passing the table as keyword arguments with the '**'
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	186 notation.::
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	187
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	188 >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	189 >>> print('Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table))
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	190 Jack: 4098; Sjoerd: 4127; Dcab: 8637678
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	191
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	192 This is particularly useful in combination with the new built-in :func:`vars`
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	193 function, which returns a dictionary containing all local variables.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	194
47283 f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	195 For a complete overview of string formating with :meth:`str.format`, see
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	196 :ref:`formatstrings`.
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	197
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	198
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	199 Old string formatting
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	200 ---------------------
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	201
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	202 The ``%`` operator can also be used for string formatting. It interprets the
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	203 left argument much like a :cfunc:`sprintf`\ -style format string to be applied
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	204 to the right argument, and returns the string resulting from this formatting
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	205 operation. For example::
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	206
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	207 >>> import math
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	208 >>> print 'The value of PI is approximately %5.3f.' % math.pi
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	209 The value of PI is approximately 3.142.
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	210
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	211 Since :meth:`str.format` is quite new, a lot of Python code still uses the ``%``
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	212 operator. However, because this old style of formatting will eventually removed
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	213 from the language :meth:`str.format` should generally be used.
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	214
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	215 More information can be found in the :ref:`old-string-formatting` section.
f28cbab11acd Merged revisions 63655 via svnmerge from Benjamin Peterson <benjamin@python.org> parents: 44999 diff changeset	216
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	217
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	218 .. _tut-files:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	219
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	220 Reading and Writing Files
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	221 =========================
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	222
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	223 .. index::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	224 builtin: open
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	225 object: file
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	226
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	227 :func:`open` returns a file object, and is most commonly used with two
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	228 arguments: ``open(filename, mode)``.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	229
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	230 ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	231
44396 6556f3f22c19 Merged revisions 59605-59624 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 43440 diff changeset	232 >>> f = open('/tmp/workfile', 'w')
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	233 >>> print(f)
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	234 <open file '/tmp/workfile', mode 'w' at 80a0960>
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	235
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	236 The first argument is a string containing the filename. The second argument is
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	237 another string containing a few characters describing the way in which the file
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	238 will be used. mode can be ``'r'`` when the file will only be read, ``'w'``
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	239 for only writing (an existing file with the same name will be erased), and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	240 ``'a'`` opens the file for appending; any data written to the file is
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	241 automatically added to the end. ``'r+'`` opens the file for both reading and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	242 writing. The mode argument is optional; ``'r'`` will be assumed if it's
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	243 omitted.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	244
44999 7c02182e2979 Merged revisions 60284-60349 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44396 diff changeset	245 On Windows and the Macintosh, ``'b'`` appended to the mode opens the file in
7c02182e2979 Merged revisions 60284-60349 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44396 diff changeset	246 binary mode, so there are also modes like ``'rb'``, ``'wb'``, and ``'r+b'``.
7c02182e2979 Merged revisions 60284-60349 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44396 diff changeset	247 Windows makes a distinction between text and binary files; the end-of-line
7c02182e2979 Merged revisions 60284-60349 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44396 diff changeset	248 characters in text files are automatically altered slightly when data is read or
7c02182e2979 Merged revisions 60284-60349 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44396 diff changeset	249 written. This behind-the-scenes modification to file data is fine for ASCII
7c02182e2979 Merged revisions 60284-60349 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44396 diff changeset	250 text files, but it'll corrupt binary data like that in :file:`JPEG` or
7c02182e2979 Merged revisions 60284-60349 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44396 diff changeset	251 :file:`EXE` files. Be very careful to use binary mode when reading and writing
7c02182e2979 Merged revisions 60284-60349 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44396 diff changeset	252 such files. On Unix, it doesn't hurt to append a ``'b'`` to the mode, so
7c02182e2979 Merged revisions 60284-60349 via svnmerge from Christian Heimes <christian@cheimes.de> parents: 44396 diff changeset	253 you can use it platform-independently for all binary files.
43440 8d7e93b47cab Clarify the difference between text and binary files. I'm not sure the Skip Montanaro <skip@pobox.com> parents: 43418 diff changeset	254
8d7e93b47cab Clarify the difference between text and binary files. I'm not sure the Skip Montanaro <skip@pobox.com> parents: 43418 diff changeset	255 This behind-the-scenes modification to file data is fine for text files, but
8d7e93b47cab Clarify the difference between text and binary files. I'm not sure the Skip Montanaro <skip@pobox.com> parents: 43418 diff changeset	256 will corrupt binary data like that in :file:`JPEG` or :file:`EXE` files. Be
8d7e93b47cab Clarify the difference between text and binary files. I'm not sure the Skip Montanaro <skip@pobox.com> parents: 43418 diff changeset	257 very careful to use binary mode when reading and writing such files.
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	258
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	259
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	260 .. _tut-filemethods:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	261
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	262 Methods of File Objects
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	263 -----------------------
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	264
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	265 The rest of the examples in this section will assume that a file object called
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	266 ``f`` has already been created.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	267
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	268 To read a file's contents, call ``f.read(size)``, which reads some quantity of
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	269 data and returns it as a string. size is an optional numeric argument. When
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	270 size is omitted or negative, the entire contents of the file will be read and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	271 returned; it's your problem if the file is twice as large as your machine's
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	272 memory. Otherwise, at most size bytes are read and returned. If the end of
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	273 the file has been reached, ``f.read()`` will return an empty string (``""``).
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	274 ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	275
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	276 >>> f.read()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	277 'This is the entire file.\n'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	278 >>> f.read()
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
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	281 ``f.readline()`` reads a single line from the file; a newline character (``\n``)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	282 is left at the end of the string, and is only omitted on the last line of the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	283 file if the file doesn't end in a newline. This makes the return value
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	284 unambiguous; if ``f.readline()`` returns an empty string, the end of the file
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	285 has been reached, while a blank line is represented by ``'\n'``, a string
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	286 containing only a single newline. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	287
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	288 >>> f.readline()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	289 'This is the first line of the file.\n'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	290 >>> f.readline()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	291 'Second line of the file\n'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	292 >>> f.readline()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	293 ''
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	294
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	295 ``f.readlines()`` returns a list containing all the lines of data in the file.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	296 If given an optional parameter sizehint, it reads that many bytes from the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	297 file and enough more to complete a line, and returns the lines from that. This
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	298 is often used to allow efficient reading of a large file by lines, but without
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	299 having to load the entire file in memory. Only complete lines will be returned.
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
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	302 >>> f.readlines()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	303 ['This is the first line of the file.\n', 'Second line of the file\n']
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	304
43418 0c8fec76111c Merged revisions 58211-58220 via svnmerge from Thomas Wouters <thomas@python.org> parents: 43219 diff changeset	305 An alternative approach to reading lines is to loop over the file object. This is
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	306 memory efficient, fast, and leads to simpler code::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	307
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	308 >>> for line in f:
43115 95d6fe2e8e0e Tutorial update for 3.0 by Paul Dubois. Guido van Rossum <guido@python.org> parents: 42604 diff changeset	309 print(line, end='')
42604 9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	310
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	311 This is the first line of the file.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	312 Second line of the file
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	313
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	314 The alternative approach is simpler but does not provide as fine-grained
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	315 control. Since the two approaches manage line buffering differently, they
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	316 should not be mixed.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	317
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	318 ``f.write(string)`` writes the contents of string to the file, returning
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	319 ``None``. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	320
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	321 >>> f.write('This is a test\n')
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	322
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	323 To write something other than a string, it needs to be converted to a string
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	324 first::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	325
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	326 >>> value = ('the answer', 42)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	327 >>> s = str(value)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	328 >>> f.write(s)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	329
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	330 ``f.tell()`` returns an integer giving the file object's current position in the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	331 file, measured in bytes from the beginning of the file. To change the file
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	332 object's position, use ``f.seek(offset, from_what)``. The position is computed
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	333 from adding offset to a reference point; the reference point is selected by
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	334 the from_what argument. A from_what value of 0 measures from the beginning
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	335 of the file, 1 uses the current file position, and 2 uses the end of the file as
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	336 the reference point. from_what can be omitted and defaults to 0, using the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	337 beginning of the file as the reference point. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	338
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	339 >>> f = open('/tmp/workfile', 'r+')
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	340 >>> f.write('0123456789abcdef')
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	341 >>> f.seek(5) # Go to the 6th byte in the file
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	342 >>> f.read(1)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	343 '5'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	344 >>> f.seek(-3, 2) # Go to the 3rd byte before the end
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	345 >>> f.read(1)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	346 'd'
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	347
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	348 When you're done with a file, call ``f.close()`` to close it and free up any
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	349 system resources taken up by the open file. After calling ``f.close()``,
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	350 attempts to use the file object will automatically fail. ::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	351
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	352 >>> f.close()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	353 >>> f.read()
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	354 Traceback (most recent call last):
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	355 File "<stdin>", line 1, in ?
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	356 ValueError: I/O operation on closed file
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	357
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	358 File objects have some additional methods, such as :meth:`isatty` and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	359 :meth:`truncate` which are less frequently used; consult the Library Reference
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	360 for a complete guide to file objects.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	361
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	362
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	363 .. _tut-pickle:
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	364
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	365 The :mod:`pickle` Module
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	366 ------------------------
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	367
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	368 .. index:: module: pickle
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	369
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	370 Strings can easily be written to and read from a file. Numbers take a bit more
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	371 effort, since the :meth:`read` method only returns strings, which will have to
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	372 be passed to a function like :func:`int`, which takes a string like ``'123'``
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	373 and returns its numeric value 123. However, when you want to save more complex
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	374 data types like lists, dictionaries, or class instances, things get a lot more
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	375 complicated.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	376
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	377 Rather than have users be constantly writing and debugging code to save
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	378 complicated data types, Python provides a standard module called :mod:`pickle`.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	379 This is an amazing module that can take almost any Python object (even some
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	380 forms of Python code!), and convert it to a string representation; this process
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	381 is called :dfn:`pickling`. Reconstructing the object from the string
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	382 representation is called :dfn:`unpickling`. Between pickling and unpickling,
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	383 the string representing the object may have been stored in a file or data, or
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	384 sent over a network connection to some distant machine.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	385
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	386 If you have an object ``x``, and a file object ``f`` that's been opened for
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	387 writing, the simplest way to pickle the object takes only one line of code::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	388
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	389 pickle.dump(x, f)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	390
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	391 To unpickle the object again, if ``f`` is a file object which has been opened
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	392 for reading::
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	393
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	394 x = pickle.load(f)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	395
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	396 (There are other variants of this, used when pickling many objects or when you
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	397 don't want to write the pickled data to a file; consult the complete
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	398 documentation for :mod:`pickle` in the Python Library Reference.)
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	399
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	400 :mod:`pickle` is the standard way to make Python objects which can be stored and
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	401 reused by other programs or by a future invocation of the same program; the
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	402 technical term for this is a :dfn:`persistent` object. Because :mod:`pickle` is
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	403 so widely used, many authors who write Python extensions take care to ensure
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	404 that new data types such as matrices can be properly pickled and unpickled.
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	405
9e1529bf0442 Move the 3k reST doc tree in place. Georg Brandl <georg@python.org> parents: diff changeset	406

Mercurial > cpython-lru_cache-weakref

annotate Doc/tutorial/inputoutput.rst @ 47426:4dedef4c0dfd