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
<open file '/tmp/workfile', mode 'w' at 80a0960>
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 "<stdin>", 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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