Python 2.7基础教程之：输入输出

.. _tut-io:

*************************************

Input and Output 输入和输出

*************************************

There are several ways to present the output of a program; data can be printed

in a human-readable form, or written to a file for future use. This chapter will

discuss some of the possibilities.

有几个方法可以表达程序输出结果；数据可以打印为人类可读的形式，也可以写

入文件供以后使用。本章将讨论几种可选的方法。

.. _tut-formatting:

Fancier Output Formatting 玩转输出格式

=============================================

So far we've encountered two ways of writing values: *expression statements* and

the :keyword:`print` statement.  (A third way is using the :meth:`write` method

of file objects; the standard output file can be referenced as ``sys.stdout``.

See the Library Reference for more information on this.)

我们有两种大相径庭的输出值方法： *表达式语句* 和 :keyword:`print` 语句。（第三种访求

是使用文件对象的 :meth:`write` 方法，标准文件输出可以参考

``sys.stdout`` 。详细内容参见库参考手册。） 

.. index:: module: string

Often you'll want more control over the formatting of your output than simply

printing space-separated values.  There are two ways to format your output; the

first way is to do all the string handling yourself; using string slicing and

concatenation operations you can create any layout you can imagine.  The

standard module :mod:`string` contains some useful operations for padding

strings to a given column width; these will be discussed shortly.  The second

way is to use the :meth:`str.format` method.

可能你经常想要对输出格式做一些比简单的打印空格分隔符更为复杂的控制。有

两种方法可以格式化输出。第一种是由你来控制整个字符串，使用字符切割和联

接操作就可以创建出任何你想要的输出形式。标准模块 :mod:`string` 包括了一些操

作，将字符串填充入给定列时，这些操作很有用。随后我们会讨论这部分内容。

第二种方法是使用 :meth:`str.format` 方法。

One question remains, of course: how do you convert values to strings? Luckily,

Python has ways to convert any value to a string: pass it to the :func:`repr`

or :func:`str` functions.

当然，还有一个问题，如何将值转化为字符串？很幸运，Python 有办法将任意

值转为字符串：将它传入 :func:`repr` 或 :func:`str` 函数。

The :func:`str` function is meant to return representations of values which are

fairly human-readable, while :func:`repr` is meant to generate representations

which can be read by the interpreter (or will force a :exc:`SyntaxError` if

there is not equivalent syntax).  For objects which don't have a particular

representation for human consumption, :func:`str` will return the same value as

:func:`repr`.  Many values, such as numbers or structures like lists and

dictionaries, have the same representation using either function.  Strings and

floating point numbers, in particular, have two distinct representations.

函数 :func:`str` 用于将值转化为适于人阅读的形式，而 :func:`repr` 转化为供解释器读

取的形式（如果没有等价的语法，则会发生 :exc:`SyntaxError` 异常） 某对象没有适

于人阅读的解释形式的话， :func:`str` 会返回与 :func:`repr` 等同的值。很多类型，诸

如数值或链表、字典这样的结构，针对各函数都有着统一的解读方式。字符串和

浮点数，有着独特的解读方式。

Some examples:

下面有些例子 ::

   >>> s = 'Hello, world.'

   >>> str(s)

   'Hello, world.'

   >>> repr(s)

   "'Hello, world.'"

   >>> str(1.0/7.0)

   '0.142857142857'

   >>> repr(1.0/7.0)

   '0.14285714285714285'

   >>> x = 10 * 3.25

   >>> y = 200 * 200

   >>> s = 'The value of x is ' + repr(x) + ', and y is ' + repr(y) + '...'

   >>> print s

   The value of x is 32.5, and y is 40000...

   >>> # The repr() of a string adds string quotes and backslashes:

   ... hello = 'hello, world/n'

   >>> hellos = repr(hello)

   >>> print hellos

   'hello, world/n'

   >>> # The argument to repr() may be any Python object:

   ... repr((x, y, ('spam', 'eggs')))

   "(32.5, 40000, ('spam', 'eggs'))"

Here are two ways to write a table of squares and cubes:

有两种方式可以写平方和立方表 ::

   >>> for x in range(1, 11):

   ...     print repr(x).rjust(2), repr(x*x).rjust(3),

   ...     # Note trailing comma on previous line

   ...     print repr(x*x*x).rjust(4)

   ...

    1   1    1

    2   4    8

    3   9   27

    4  16   64

    5  25  125

    6  36  216

    7  49  343

    8  64  512

    9  81  729

   10 100 1000

   >>> for x in range(1,11):

   ...     print '{0:2d} {1:3d} {2:4d}'.format(x, x*x, x*x*x)

   ...

    1   1    1

    2   4    8

    3   9   27

    4  16   64

    5  25  125

    6  36  216

    7  49  343

    8  64  512

    9  81  729

   10 100 1000

(Note that in the first example, one space between each column was added by the

way :keyword:`print` works: it always adds spaces between its arguments.)

（注意第一个例子， :keyword:`print` 在每列之间加了一个空格，它总是在参

数间加入空格。）

This example demonstrates the :meth:`rjust` method of string objects, which

right-justifies a string in a field of a given width by padding it with spaces

on the left.  There are similar methods :meth:`ljust` and :meth:`center`.  These

methods do not write anything, they just return a new string.  If the input

string is too long, they don't truncate it, but return it unchanged; this will

mess up your column lay-out but that's usually better than the alternative,

which would be lying about a value.  (If you really want truncation you can

always add a slice operation, as in ``x.ljust(n)[:n]``.)

以上是一个 :meth:`rjust` 方法的演示，它把字符串输出到一列，并通过向左

侧填充空格来使其右对齐。类似的方法还有 :meth:`ljust` 和 :meth:`center`

。这些函数只是输出新的字符串，并不改变什么。如果输出的字符串太长，它们也不会截断

它，而是原样输出，这会使你的输出格式变得混乱，不过总强过另一种选择（截

断字符串），因为那样会产生错误的输出值。（如果你确实需要截断它，可以使

用切割操作，例如： ``x.ljust( n)[:n]`` 。）

There is another method, :meth:`zfill`, which pads a numeric string on the left

with zeros.  It understands about plus and minus signs:

还有另一个方法， :meth:`zfill` 它用于向数值的字符串表达左侧填充 0。该

函数可以正确理解正负号 ::

   >>> '12'.zfill(5)

   '00012'

   >>> '-3.14'.zfill(7)

   '-003.14'

   >>> '3.14159265359'.zfill(5)

   '3.14159265359'

Basic usage of the :meth:`str.format` method looks like this:

方法 :meth:`str.format` 的基本用法如下 ::

   >>> print 'We are the {} who say "{}!"'.format('knights', 'Ni')

   We are the knights who say "Ni!"

The brackets and characters within them (called format fields) are replaced with

the objects passed into the :meth:`~str.format` method.  A number in the

brackets refers to the position of the object passed into the

:meth:`~str.format` method. :

大括号和其中的字符会被替换成传入 :meth:`~str.format` 的参数。大括号中

的数值指明使用传入 :meth:`~str.format` 方法的对象中的哪一个 ::

   >>> print '{0} and {1}'.format('spam', 'eggs')

   spam and eggs

   >>> print '{1} and {0}'.format('spam', 'eggs')

   eggs and spam

If keyword arguments are used in the :meth:`~str.format` method, their values

are referred to by using the name of the argument. :

如果在 :meth:`~str.format` 调用时使用关键字参数，可以通过参数名来引用

值 ::

   >>> print 'This {food} is {adjective}.'.format(

   ...       food='spam', adjective='absolutely horrible')

   This spam is absolutely horrible.

Positional and keyword arguments can be arbitrarily combined:

定位和关键字参数可以组合使用 ::

   >>> print 'The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred',

   ...                other='Georg')

   The story of Bill, Manfred, and Georg.

``'!s'`` (apply :func:`str`) and ``'!r'`` (apply :func:`repr`) can be used to

convert the value before it is formatted. :

``'!s'`` （应用 :func:`str` ） 和 ``'!r'`` （应用 :func:`repr` ） 可以

在格式化之前转换值。 ::

   >>> import math

   >>> print 'The value of PI is approximately {}.'.format(math.pi)

   The value of PI is approximately 3.14159265359.

   >>> print 'The value of PI is approximately {!r}.'.format(math.pi)

   The value of PI is approximately 3.141592653589793.

An optional ``':'`` and format specifier can follow the field name. This allows

greater control over how the value is formatted.  The following example

truncates Pi to three places after the decimal.

字段名后允许可选的 ``':'`` 和格式指令。这允许对值的格式化加以更深入的

控制。下例将 Pi 转为三位精度。 ::

   >>> import math

   >>> print 'The value of PI is approximately {0:.3f}.'.format(math.pi)

   The value of PI is approximately 3.142.

Passing an integer after the ``':'`` will cause that field to be a minimum

number of characters wide.  This is useful for making tables pretty. :

在字段后的 ``':'`` 后面加一个整数会限定该字段的最小宽度，这在美化表格时

很有用。 ::

   >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}

   >>> for name, phone in table.items():

   ...     print '{0:10} ==> {1:10d}'.format(name, phone)

   ...

   Jack       ==>       4098

   Dcab       ==>       7678

   Sjoerd     ==>       4127

If you have a really long format string that you don't want to split up, it

would be nice if you could reference the variables to be formatted by name

instead of by position.  This can be done by simply passing the dict and using

square brackets ``'[]'`` to access the keys :

如果你有个实在是很长的格式化字符串，不想分割它。如果你可以用命名来引用

被格式化的变量而不是位置就好了。有个简单的方法，可以传入一个字典，用中

括号访问它的键 ::

   >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}

   >>> print ('Jack: {0[Jack]:d}; Sjoerd: {0[Sjoerd]:d}; '

   ...        'Dcab: {0[Dcab]:d}'.format(table))

   Jack: 4098; Sjoerd: 4127; Dcab: 8637678

This could also be done by passing the table as keyword arguments with the '**'

notation. 

也可以用 ‘**’ 标志将这个字典以关键字参数的方式传入。 ::

   >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}

   >>> print 'Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table)

   Jack: 4098; Sjoerd: 4127; Dcab: 8637678

This is particularly useful in combination with the new built-in :func:`vars`

function, which returns a dictionary containing all local variables.

这种方式与新的内置函数 :func:`vars` 组合使用非常有效。该函数返回包含所有局

部变量的字典。

For a complete overview of string formatting with :meth:`str.format`, see

:ref:`formatstrings`.

要进一步了解字符串格式化方法 :meth:`str.format` ，参见

:ref:`formatstrings` 。

Old string formatting 旧式的字符串格式化

------------------------------------------

The ``%`` operator can also be used for string formatting. It interprets the

left argument much like a :cfunc:`sprintf`/ -style format string to be applied

to the right argument, and returns the string resulting from this formatting

operation. For example:

操作符 ``%`` 也可以用于字符串格式化。它以类似 :cfunc:`sprintf` 的方式

解析左参数，将右参数应用于此，得到格式化操作生成的字符串，例如 ::

   >>> import math

   >>> print 'The value of PI is approximately %5.3f.' % math.pi

   The value of PI is approximately 3.142.

Since :meth:`str.format` is quite new, a lot of Python code still uses the ``%``

operator. However, because this old style of formatting will eventually be

removed from the language, :meth:`str.format` should generally be used.

因为 :meth:`str.format` 还很新，大量 Python 代码还在使用 ``%`` 操作符。

然而，因为旧式的格式化方法最终将从语言中去掉，应该尽量使用

　:meth:`str.format` 。

More information can be found in the :ref:`string-formatting` section.

进一步的信息可以参见　:ref:`string-formatting` 一节。

.. _tut-files:

Reading and Writing Files 读写文件

==============================================

.. index::

   builtin: open

   object: file

:func:`open` returns a file object, and is most commonly used with two

arguments: ``open(filename, mode)``.

函数 :func:`open` 返回文件对象，通常的用法需要两个参数：

``open(filename, mode)`` 。

::

   >>> f = open('/tmp/workfile', 'w')

   >>> print f

  

The first argument is a string containing the filename.  The second argument is

another string containing a few characters describing the way in which the file

will be used.  *mode* can be ``'r'`` when the file will only be read, ``'w'``

for only writing (an existing file with the same name will be erased), and

``'a'`` opens the file for appending; any data written to the file is

automatically added to the end.  ``'r+'`` opens the file for both reading and

writing. The *mode* argument is optional; ``'r'`` will be assumed if it's

omitted.

第一个参数是一个标识文件名的字符串。第二个参数是由有限的字母组成的字符

串，描述了文件将会被如何使用。可选的 *模式* 有： ``'r'`` ，此选项使文件只读；

``'w'`` ，此选项使文件只写（对于同名文件，该操作使原有文件被覆盖）； ``'a'`` ，

此选项以追加方式打开文件； ``'r+'`` ，此选项以读写方式打开文件； 

*模式* 参数是可选的。如果没有指定，默认为 ``'r'`` 模式。

On Windows, ``'b'`` appended to the mode opens the file in binary mode, so there

are also modes like ``'rb'``, ``'wb'``, and ``'r+b'``.  Python on Windows makes

a distinction between text and binary files; the end-of-line characters in text

files are automatically altered slightly when data is read or written.  This

behind-the-scenes modification to file data is fine for ASCII text files, but

it'll corrupt binary data like that in :file:`JPEG` or :file:`EXE` files.  Be

very careful to use binary mode when reading and writing such files.  On Unix,

it doesn't hurt to append a ``'b'`` to the mode, so you can use it

platform-independently for all binary files.

在Windows 平台上， ``'b'`` 模式以二进制方式打开文件，所以可能会

有类似于 ``'rb'`` ， ``'wb'`` ， ``'r+b'`` 等等模式组合。Windows 平台上文本文件与二

进制文件是有区别的，读写文本文件时，行尾会自动添加行结束符。这种后台操

作方式对 ASCII 文本文件没有什么问题，但是操作 JPEG 或 .EXE这样的二进制

文件时就会产生破坏。在操作这些文件时一定要记得以二进制模式打开。在

Unix 上，加一个 ``'b'`` 模式也一样是无害的，所以你可以一切二进制文件处

理中平台无关的使用它。

.. _tut-filemethods:

Methods of File Objects 文件对象方法

-----------------------------------------

The rest of the examples in this section will assume that a file object called

``f`` has already been created.

本节中的示例都默认文件对象 ``f`` 已经创建。

To read a file's contents, call ``f.read(size)``, which reads some quantity of

data and returns it as a string.  *size* is an optional numeric argument.  When

*size* is omitted or negative, the entire contents of the file will be read and

returned; it's your problem if the file is twice as large as your machine's

memory. Otherwise, at most *size* bytes are read and returned.  If the end of

the file has been reached, ``f.read()`` will return an empty string (``""``).

要读取文件内容，需要调用 ``f.read(size)`` ，该方法读取若干数量的数据并以字

符串形式返回其内容， *size* 是可选的数值，指定字符串长度。如果没有指定

size或者指定为负数，就会读取并返回整个文件。当文件大小为当前机器内存两

倍时，就会产生问题。反之，会尽可能按比较大的 size 读取和返回数据。

如果到了文件末尾，f.read()会返回一个空字符串（""）。

::

   >>> f.read()

   'This is the entire file./n'

   >>> f.read()

   ''

``f.readline()`` reads a single line from the file; a newline character (``/n``)

is left at the end of the string, and is only omitted on the last line of the

file if the file doesn't end in a newline.  This makes the return value

unambiguous; if ``f.readline()`` returns an empty string, the end of the file

has been reached, while a blank line is represented by ``'/n'``, a string

containing only a single newline. 

``f.readline()`` 从文件中读取单独一行，字符串结尾会自动加上一个换行符

（ ``/n`` ），只有当文件最后一行没有以换行符结尾时，这一操作才会被忽略。

这样返回值就不会有混淆，如果如果 ``f.readline()`` 返回一个空字符串，那就表

示到达了文件末尾，如果是一个空行，就会描述为 ``'/n`` ，一个只包含换行符的字符串。 ::

   >>> f.readline()

   'This is the first line of the file./n'

   >>> f.readline()

   'Second line of the file/n'

   >>> f.readline()

   ''

``f.readlines()`` returns a list containing all the lines of data in the file.

If given an optional parameter *sizehint*, it reads that many bytes from the

file and enough more to complete a line, and returns the lines from that.  This

is often used to allow efficient reading of a large file by lines, but without

having to load the entire file in memory.  Only complete lines will be returned.

f.readlines()返回一个列表，其中包含了文件中所有的数据行。如果给定了

*sizehint* 参数，就会读入多于一行的比特数，从中返回多行文本。这个功能

通常用于高效读取大型行文件，避免了将整个文件读入内存。这种操作只返回完

整的行。 ::

   >>> f.readlines()

   ['This is the first line of the file./n', 'Second line of the file/n']

An alternative approach to reading lines is to loop over the file object. This is

memory efficient, fast, and leads to simpler code

有个按行读取的好办法是在文件对象上循环。这样容易记忆，高速而且代码更简单 ::

   >>> for line in f:

           print line,

   This is the first line of the file.

   Second line of the file

The alternative approach is simpler but does not provide as fine-grained

control.  Since the two approaches manage line buffering differently, they

should not be mixed.

这个办法很简单，但不能完整的控制操作。因为两个方法用不同的方式管理行缓

冲区，它们不能混用。

``f.write(string)`` writes the contents of *string* to the file, returning

``None``.  

``f.write(string)`` 将 *string* 的内容写入文件，返回 ``None`` 。 ::

   >>> f.write('This is a test/n')

To write something other than a string, it needs to be converted to a string

first:

如果需要写入字符串以外的数据，就要先把这些数据转换为字符串。 ::

   >>> value = ('the answer', 42)

   >>> s = str(value)

   >>> f.write(s)

``f.tell()`` returns an integer giving the file object's current position in the

file, measured in bytes from the beginning of the file.  To change the file

object's position, use ``f.seek(offset, from_what)``.  The position is computed

from adding *offset* to a reference point; the reference point is selected by

the *from_what* argument.  A *from_what* value of 0 measures from the beginning

of the file, 1 uses the current file position, and 2 uses the end of the file as

the reference point.  *from_what* can be omitted and defaults to 0, using the

beginning of the file as the reference point. 

``f.tell()`` 返回一个整数，代表文件对象在文件中的指针位置，该数值计量了自文

件开头到指针处的比特数。需要改变文件对象指针话话，使用

``f.seek(offset,from_what)`` 。指针在该操作中从指定的引用位置移动 *offset*

比特，引用位置由 *from_what* 参数指定。 *from_what* 值为 0 表示自文件

起始处开始，1 表示自当前文件指针位置开始，2 表示自文件末尾开始。 *from_what* 可以

忽略，其默认值为零，此时从文件头开始。 ::

   >>> f = open('/tmp/workfile', 'r+')

   >>> f.write('0123456789abcdef')

   >>> f.seek(5)     # Go to the 6th byte in the file

   >>> f.read(1)

   '5'

   >>> f.seek(-3, 2) # Go to the 3rd byte before the end

   >>> f.read(1)

   'd'

When you're done with a file, call ``f.close()`` to close it and free up any

system resources taken up by the open file.  After calling ``f.close()``,

attempts to use the file object will automatically fail. 

文件使用完后，调用 ``f.close()`` 可以关闭文件，释放打开文件后占用的系统资源。

调用 ``f.close()`` 之后，再调用文件对象会自动引发错误。 ::

   >>> f.close()

   >>> f.read()

   Traceback (most recent call last):

     File "", line 1, in ?

   ValueError: I/O operation on closed file

It is good practice to use the :keyword:`with` keyword when dealing with file

objects.  This has the advantage that the file is properly closed after its

suite finishes, even if an exception is raised on the way.  It is also much

shorter than writing equivalent :keyword:`try`/ -/ :keyword:`finally` blocks

用关键字 :keyword:`with` 处理文件对象是个好习惯。它的先进之处在于文件

用完后会自动关闭，就算发生异常也没关系。它是 :keyword:`try`/ -/

:keyword:`finally` 块的简写。 ::

    >>> with open('/tmp/workfile', 'r') as f:

    ...     read_data = f.read()

    >>> f.closed

    True

File objects have some additional methods, such as :meth:`~file.isatty` and

:meth:`~file.truncate` which are less frequently used; consult the Library

Reference for a complete guide to file objects.

文件对象还有一些不太常用的附加方法，比如 :meth:`~file.isatty` 和 :meth:`~file.truncate` 在库参

考手册中有文件对象的完整指南。

.. _tut-pickle:

The :mod:`pickle` Module   :mod:`pickle` 模块

------------------------------------------------

.. index:: module: pickle

Strings can easily be written to and read from a file. Numbers take a bit more

effort, since the :meth:`read` method only returns strings, which will have to

be passed to a function like :func:`int`, which takes a string like ``'123'``

and returns its numeric value 123.  However, when you want to save more complex

data types like lists, dictionaries, or class instances, things get a lot more

complicated.

我们可以很容易的读写文件中的字符串。数值就要多费点儿周折，因为 :meth:`read`

方法只会返回字符串，应该将其传入 :func:`int` 这样的方法中，就可以将

``'123'`` 这样的字符转为对应的数值 123 。不过，当你需要保存更为复杂的

数据类型，例如列表、字典，类的实例，事情就会变得更复杂了。

Rather than have users be constantly writing and debugging code to save

complicated data types, Python provides a standard module called :mod:`pickle`.

This is an amazing module that can take almost any Python object (even some

forms of Python code!), and convert it to a string representation; this process

is called :dfn:`pickling`.  Reconstructing the object from the string

representation is called :dfn:`unpickling`.  Between pickling and unpickling,

the string representing the object may have been stored in a file or data, or

sent over a network connection to some distant machine.

好在用户不必要非得自己编写和调试保存复杂数据类型的代码。 Python提供了

一个名为 :mod:`pickle` 的标准模块。这是一个令人赞叹的模块，几乎可以把任何

Python 对象 （甚至是一些 Python 代码段！）表达为为字符串，这一过程称之

为封装 （ :dfn:`pickling` ）。从字符串表达出重新构造对象称之为拆封

（ :dfn:`unpickling` ）。封装状态中的对象可以存储在文件或对象中，也可以通过网

络在远程的机器之间传输。

If you have an object ``x``, and a file object ``f`` that's been opened for

writing, the simplest way to pickle the object takes only one line of code

如果你有一个对象 ``x`` ，一个以写模式打开的文件对象 ``f`` ，封装对象的最简单的

方法只需要一行代码 ::

   pickle.dump(x, f)

To unpickle the object again, if ``f`` is a file object which has been opened

for reading

如果 ``f`` 是一个以读模式打开的文件对象，就可以重装拆封这个对象 ::

   x = pickle.load(f)

(There are other variants of this, used when pickling many objects or when you

don't want to write the pickled data to a file; consult the complete

documentation for :mod:`pickle` in the Python Library Reference.)

（如果不想把封装的数据写入文件，这里还有一些其它的变化可用。完整的

:mod:`pickle` 文档请见Python 库参考手册）。

:mod:`pickle` is the standard way to make Python objects which can be stored and

reused by other programs or by a future invocation of the same program; the

technical term for this is a :dfn:`persistent` object.  Because :mod:`pickle` is

so widely used, many authors who write Python extensions take care to ensure

that new data types such as matrices can be properly pickled and unpickled.

:mod:`pickle` 是存储 Python 对象以供其它程序或其本身以后调用的标准方法。提供

这一组技术的是一个 :dfn:`持久化` 对象（ :dfn:`persistent` object ）。因为 :mod:`pickle` 的用

途很广泛，很多 Python 扩展的作者都非常注意类似矩阵这样的新数据类型是否适合封装和拆封。

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