.. _tut-modules:
************************
modules 模块
************************
if you quit from the python interpreter and enter it again, the definitions you
have made (functions and variables) are lost. therefore, if you want to write a
somewhat longer program, you are better off using a text editor to prepare the
input for the interpreter and running it with that file as input instead. this
is known as creating a *script*. as your program gets longer, you may want to
split it into several files for easier maintenance. you may also want to use a
handy function that you’ve written in several programs without copying its
definition into each program.
如果你退出 python 解释器重新进入，以前创建的一切定义（变量和函数）就全
部丢失了。因此，如果你想写一些长久保存的程序，最好使用一个文本编辑器来
编写程序，把保存好的文件输入解释器。我们称之为创建一个 *脚本* 。或者程
序变得更长了，你可能为了方便维护而把它分离成几个文件。你也可能想要在几
个程序中都使用一个常用的函数，但是不想把它的定义复制到每一个程序里。
to support this, python has a way to put definitions in a file and use them in a
script or in an interactive instance of the interpreter. such a file is called a
*module*; definitions from a module can be *imported* into other modules or into
the *main* module (the collection of variables that you have access to in a
script executed at the top level and in calculator mode).
为了满足这些需要，python提供了一个方法可以从文件中获取定义，在脚本或者
解释器的一个交互式实例中使用。这样的文件被称为 *模块* ；模块中的定义可
以 *导入*
到另一个模块或主模块中（在脚本执行时可以调用的变量集位于最高级，并且处
于计算器模式）。
a module is a file containing python definitions and statements. the file name
is the module name with the suffix :file:`.py` appended. within a module, the
module’s name (as a string) is available as the value of the global variable
“__name__“. for instance, use your favorite text editor to create a file
called :file:`fibo.py` in the current directory with the following contents:
模块是包括 python 定义和声明的文件。文件名就是模块名加上 :file:`.py` 后缀。模
块的模块名（做为一个字符串）可以由全局变量 “__name__“ 得到。例如，你可以
用自己惯用的文件编辑器在当前目录下创建一个叫 :file:`fibo.py` 的文件，录入如下
内容 ::
# fibonacci numbers module
def fib(n): # write fibonacci series up to n
a, b = 0, 1
while b < n:
print b,
a, b = b, a+b
def fib2(n): # return fibonacci series up to n
result = []
a, b = 0, 1
while b < n:
result.append(b)
a, b = b, a+b
return result
now enter the python interpreter and import this module with the following
command:
现在进入python解释器，用如下命令导入这个模块 ::
>>> import fibo
this does not enter the names of the functions defined in “fibo“ directly in
the current symbol table; it only enters the module name “fibo“ there. using
the module name you can access the functions:
这样做不会直接把 “fibo“ 中的函数导入当前的语义表；它只是引入了模块名
“fibo“ 。你可以通过模块名按如下方式访问这个函数 ::
>>> fibo.fib(1000)
1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
>>> fibo.fib2(100)
[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
>>> fibo.__name__
‘fibo’
if you intend to use a function often you can assign it to a local name:
如果你想要直接调用函数，通常可以给它赋一个本地名称 ::
>>> fib = fibo.fib
>>> fib(500)
1 1 2 3 5 8 13 21 34 55 89 144 233 377
.. _tut-moremodules:
more on modules 深入模块
==============================
a module can contain executable statements as well as function definitions.
these statements are intended to initialize the module. they are executed only
the *first* time the module is imported somewhere. [#]_
模块可以像函数定义一样包含执行语句。这些语句通常用于初始化模块。它们只
在模块 *第一次* 导入时执行一次。 [#]_
each module has its own private symbol table, which is used as the global symbol
table by all functions defined in the module. thus, the author of a module can
use global variables in the module without worrying about accidental clashes
with a user’s global variables. on the other hand, if you know what you are
doing you can touch a module’s global variables with the same notation used to
refer to its functions, “modname.itemname“.
对应于定义模块中所有函数的全局语义表，每一个模块有自己的私有语义表。因
此，模块作者可以在模块中使用一些全局变量，不会因为与用户的全局变量冲突
而引发错误。另一方面，如果你确定你需要这个，可以像引用模块中的函数一样获取模块中的全局变量，形如： “modname.itemname“ 。
modules can import other modules. it is customary but not required to place all
:keyword:`import` statements at the beginning of a module (or script, for that
matter). the imported module names are placed in the importing module’s global
symbol table.
模块可以导入（ :keyword:`import` ）其它模块。习惯上所有的 import 语句都放在模块（或
脚本，等等）的开头，但这并不是必须的。被导入的模块名入在本模块的全局语
义表中。
there is a variant of the :keyword:`import` statement that imports names from a
module directly into the importing module’s symbol table. for example:
:keyword:`import` 语句的一个变体直接从被导入的模块中导入命名到本模块的语义表中。
例如 ::
>>> from fibo import fib, fib2
>>> fib(500)
1 1 2 3 5 8 13 21 34 55 89 144 233 377
this does not introduce the module name from which the imports are taken in the
local symbol table (so in the example, “fibo“ is not defined).
这样不会从局域语义表中导入模块名（如上所示， “fibo“ 没有定义）。
there is even a variant to import all names that a module defines:
甚至有种方式可以导入模块中的所有定义 ::
>>> from fibo import *
>>> fib(500)
1 1 2 3 5 8 13 21 34 55 89 144 233 377
this imports all names except those beginning with an underscore (“_“).
这样可以导入所有除了以下划线( “_“ )开头的命名。
note that in general the practice of importing “*“ from a module or package is
frowned upon, since it often causes poorly readable code. however, it is okay to
use it to save typing in interactive sessions.
需要注意的是在实践中往往不鼓励从一个模块或包中使用 “*“ 导入所有，因
为这样会让代码变得很难读。不过，在交互式会话中这样用很方便省力。
.. note::
.. [#] for efficiency reasons, each module is only imported once per interpreter
session. therefore, if you change your modules, you must restart the
interpreter — or, if it’s just one module you want to test interactively,
use :func:`reload`, e.g. “reload(modulename)“.
.. [#] 出于性能考虑，每个模块在每个解释器会话中只导入一遍。因此，如果你修
改了你的模块，需要重启解释器——或者，如果你就是想交互式的测试这么一
个模块，可以用 :func:`reload` 重新加载，例如 “reload(modulename)“ 。
.. _tut-modulesasscripts:
executing modules as scripts 作为脚本来执行模块
————————————————–
when you run a python module with :
使用如下方式执行一个 python 模块 ::
python fibo.py
the code in the module will be executed, just as if you imported it, but with
the “__name__“ set to ““__main__”“. that means that by adding this code at
the end of your module:
模块中的代码会被执行，就像导入它一样，不过此时 “__name__“ 被设置为
““__main__”“ 。这相当于，如果你在模块后加入如下代码 ::
if __name__ == “__main__”:
import sys
fib(int(sys.argv[1]))
you can make the file usable as a script as well as an importable module,
because the code that parses the command line only runs if the module is
executed as the “main” file:
就可以让此文件像作为模块导入时一样作为脚本执行。此代码只有在模块作为
“main” 文件执行时才被调用 ::
$ python fibo.py 50
1 1 2 3 5 8 13 21 34
if the module is imported, the code is not run:
如果模块被导入，不会执行这段代码 ::
>>> import fibo
>>>
this is often used either to provide a convenient user interface to a module, or
for testing purposes (running the module as a script executes a test suite).
这通常用来为模块提供一个便于测试的用户接口（将模块作为脚本执行测试需求）。
.. _tut-searchpath:
the module search path 模块搜索路径
———————————————
.. index:: triple: module; search; path
when a module named :mod:`spam` is imported, the interpreter searches for a file
named :file:`spam.py` in the current directory, and then in the list of
directories specified by the environment variable :envvar:`pythonpath`. this
has the same syntax as the shell variable :envvar:`path`, that is, a list of
directory names. when :envvar:`pythonpath` is not set, or when the file is not
found there, the search continues in an installation-dependent default path; on
unix, this is usually :file:`.:/usr/local/lib/python`.
导入一个叫 :mod:`spam` 的模块时，解释器先在当前目录中搜索名为
:file:`spam.py` 的文件，然后在环境变量 :envvar:`pythonpath` 表示的目录
列表中搜索，然后是环境变量 :envvar:`path` 中的路径列表。如果
:envvar:`pythonpath` 没有设置，或者文件没有找到，接下来搜索安装目录，
在 unix 中，通常是 :file:`.:/usr/local/lib/python` 。
actually, modules are searched in the list of directories given by the variable
“sys.path“ which is initialized from the directory containing the input script
(or the current directory), :envvar:`pythonpath` and the installation- dependent
default. this allows python programs that know what they’re doing to modify or
replace the module search path. note that because the directory containing the
script being run is on the search path, it is important that the script not have
the same name as a standard module, or python will attempt to load the script as
a module when that module is imported. this will generally be an error. see
section :ref:`tut-standardmodules` for more information.
实际上，解释器由 “sys.path“ 变量指定的路径目录搜索模块，该变量初始化
时默认包含了输入脚本（或者当前目录）， :envvar:`pythonpath` 和安装目录。
这样就允许 python 程序了解如何修改或替换模块搜索目录。需要注意的是由于
这些目录中包含有搜索路径中运行的脚本，所以这些脚本不应该和标准模块重
名，否则在导入模块时 python 会尝试把这些脚本当作模块来加载。这通常会引
发错误。请参见 :ref:`tut-standardmodules` 以了解更多的信息。
“compiled” python files “编译的” python 文件
——————————————————
as an important speed-up of the start-up time for short programs that use a lot
of standard modules, if a file called :file:`spam.pyc` exists in the directory
where :file:`spam.py` is found, this is assumed to contain an
already-“byte-compiled” version of the module :mod:`spam`. the modification time
of the version of :file:`spam.py` used to create :file:`spam.pyc` is recorded in
:file:`spam.pyc`, and the :file:`.pyc` file is ignored if these don’t match.
对于引用了大量标准模块的短程序，有一个提高启动速度的重要方法，如果在
:file:`spam.py` 所在的目录下存在一个名为 :file:`spam.pyc` 的文件，它会
被视为 :mod:`spam` 模块的预“编译”（“byte-compiled” ，二进制编译）
版本。用于创建 :file:`spam.pyc` 的这一版 :file:`spam.py` 的修改时间记
录在 :file:`spam.pyc` 文件中，如果两者不匹配，:file:`.pyc` 文件就被忽
略。
normally, you don’t need to do anything to create the :file:`spam.pyc` file.
whenever :file:`spam.py` is successfully compiled, an attempt is made to write
the compiled version to :file:`spam.pyc`. it is not an error if this attempt
fails; if for any reason the file is not written completely, the resulting
:file:`spam.pyc` file will be recognized as invalid and thus ignored later. the
contents of the :file:`spam.pyc` file are platform independent, so a python
module directory can be shared by machines of different architectures.
通常你不需要为创建 :file:`spam.pyc` 文件做任何工作。一旦 :file:`spam.py` 成功编译，就
会尝试生成对应版本的 :file:`spam.pyc` 。如果有任何原因导致写入不成功，
生成的 :file:`spam.pyc` 文件就会视为无效，随后即被忽略。 :file:`spam.pyc` 文件的内容是平台独
立的，所以python模块目录可以在不同架构的机器之间共享。
some tips for experts:
部分高级技巧：
* when the python interpreter is invoked with the :option:`-o` flag, optimized
code is generated and stored in :file:`.pyo` files. the optimizer currently
doesn’t help much; it only removes :keyword:`assert` statements. when
:option:`-o` is used, *all* :term:`bytecode` is optimized; “.pyc“ files are
ignored and “.py“ files are compiled to optimized bytecode.
以 :option:`-o` 参数调用python解释器时，会生成优化代码并保存在 :file:`.pyo` 文件中。现
在的优化器没有太多帮助；它只是删除了断言（ :keyword:`assert` ）语句。使用 :option:`-o` 参
参数， *所有* 的字节码（ :term:`bytecode` ）都会被优化； “.pyc“ 文
件被忽略， “.py“ 文件被编译为优化代码。
* passing two :option:`-o` flags to the python interpreter (:option:`-oo`) will
cause the bytecode compiler to perform optimizations that could in some rare
cases result in malfunctioning programs. currently only “__doc__“ strings are
removed from the bytecode, resulting in more compact :file:`.pyo` files. since
some programs may rely on having these available, you should only use this
option if you know what you’re doing.
向python解释器传递两个 :option:`-o` 参数（ :option:`-oo` ）会执行完全
优化的二进制优化编译，这偶尔会生成错误的程序。现在的优化器，只是从字
节码中删除了 “__doc__“ 符串，生成更为紧凑的 :file:`.pyo` 文件。因为
某些程序依赖于这些变量的可用性，你应该只在确定无误的场合使用这一选项。
* a program doesn’t run any faster when it is read from a :file:`.pyc` or
:file:`.pyo` file than when it is read from a :file:`.py` file; the only thing
that’s faster about :file:`.pyc` or :file:`.pyo` files is the speed with which
they are loaded.
来自 :file:`.pyc` 文件或 :file:`.pyo` 文件中的程序不会比来自
:file:`.py` 文件的运行更快； :file:`.pyc` 或 :file:`.pyo` 文件只是在
它们加载的时候更快一些。
* when a script is run by giving its name on the command line, the bytecode for
the script is never written to a :file:`.pyc` or :file:`.pyo` file. thus, the
startup time of a script may be reduced by moving most of its code to a module
and having a small bootstrap script that imports that module. it is also
possible to name a :file:`.pyc` or :file:`.pyo` file directly on the command
line.
通过脚本名在命令行运行脚本时，不会将为该脚本创建的二进制代码写入
:file:`.pyc` 或 :file:`.pyo` 文件。当然，把脚本的主要代码移进一个模
块里，然后用一个小的启动脚本导入这个模块，就可以提高脚本的启动速度。
也可以直接在命令行中指定一个 :file:`.pyc` 或 :file:`.pyo` 文件。
* it is possible to have a file called :file:`spam.pyc` (or :file:`spam.pyo`
when :option:`-o` is used) without a file :file:`spam.py` for the same module.
this can be used to distribute a library of python code in a form that is
moderately hard to reverse engineer.
对于同一个模块（这里指例程 :file:`spam.py` －－译者），可以只
有 :file:`spam.pyc` 文件（或者 :file:`spam.pyc` ，在使用
:option:`-o` 参数时）而没有 :file:`spam.py` 文件。这样可以打包发布比
较难于逆向工程的 python 代码库。
.. index:: module: compileall
* the module :mod:`compileall` can create :file:`.pyc` files (or :file:`.pyo`
files when :option:`-o` is used) for all modules in a directory.
:mod:`compileall` 模块 可以为指定目录中的所有模块创建 :file:`.pyc` 文
件（或者使用 :file:`.pyo` 参数创建 :file:`.pyo` 文件）。
.. _tut-standardmodules:
standard modules 标准模块
================================
.. index:: module: sys
python comes with a library of standard modules, described in a separate
document, the python library reference (“library reference” hereafter). some
modules are built into the interpreter; these provide access to operations that
are not part of the core of the language but are nevertheless built in, either
for efficiency or to provide access to operating system primitives such as
system calls. the set of such modules is a configuration option which also
depends on the underlying platform for example, the :mod:`winreg` module is only
provided on windows systems. one particular module deserves some attention:
:mod:`sys`, which is built into every python interpreter. the variables
“sys.ps1“ and “sys.ps2“ define the strings used as primary and secondary
prompts:
python带有一个标准模块库，并发布有独立的文档，名为 python 库参考手册
（此后称其为“库参考手册”）。有一些模块内置于解释器之中，这些操作的访
问接口不是语言内核的一部分，但是已经内置于解释器了。这既是为了提高效
率，也是为了给系统调用等操作系统原生访问提供接口。这类模块集合是一个依
赖于底层平台的配置选项。例如，:mod:`winreg` 模块只提供在 windows 系统
上才有。有一个具体的模块值得注意： :mod:`sys` ，这个模块内置于所有的
python 解释器。变量 sys.ps1 和 sys.ps2定义了主提示符和副助提示符字符串 ::
>>> import sys
>>> sys.ps1
‘>>> ‘
>>> sys.ps2
‘… ‘
>>> sys.ps1 = ‘c> ‘
c> print ‘yuck!’
yuck!
c>
these two variables are only defined if the interpreter is in interactive mode.
这两个变量只在解释器的交互模式下有意义。
the variable “sys.path“ is a list of strings that determines the interpreter’s
search path for modules. it is initialized to a default path taken from the
environment variable :envvar:`pythonpath`, or from a built-in default if
:envvar:`pythonpath` is not set. you can modify it using standard list
operations:
变量 “sys.path“ 是解释器模块搜索路径的字符串列表。它由环境变
量:envvar:`pythonpath` 初始化，如果没有设定 :envvar:`pythonpath` ，就由
内置的默认值初始化。你可以用标准的字符串操作修改它 ::
>>> import sys
>>> sys.path.append(‘/ufs/guido/lib/python’)
.. _tut-dir:
the :func:`dir` function :func:`dir` 函数
======================================================
the built-in function :func:`dir` is used to find out which names a module
defines. it returns a sorted list of strings:
内置函数 :func:`dir` 用于按模块名搜索模块定义，它返回一个字符串类型的存储列
表 ::
>>> import fibo, sys
>>> dir(fibo)
[‘__name__’, ‘fib’, ‘fib2’]
>>> dir(sys)
[‘__displayhook__’, ‘__doc__’, ‘__excepthook__’, ‘__name__’, ‘__stderr__’,
‘__stdin__’, ‘__stdout__’, ‘_getframe’, ‘api_version’, ‘argv’,
‘builtin_module_names’, ‘byteorder’, ‘callstats’, ‘copyright’,
‘displayhook’, ‘exc_clear’, ‘exc_info’, ‘exc_type’, ‘excepthook’,
‘exec_prefix’, ‘executable’, ‘exit’, ‘getdefaultencoding’, ‘getdlopenflags’,
‘getrecursionlimit’, ‘getrefcount’, ‘hexversion’, ‘maxint’, ‘maxunicode’,
‘meta_path’, ‘modules’, ‘path’, ‘path_hooks’, ‘path_importer_cache’,
‘platform’, ‘prefix’, ‘ps1’, ‘ps2’, ‘setcheckinterval’, ‘setdlopenflags’,
‘setprofile’, ‘setrecursionlimit’, ‘settrace’, ‘stderr’, ‘stdin’, ‘stdout’,
‘version’, ‘version_info’, ‘warnoptions’]
without arguments, :func:`dir` lists the names you have defined currently:
无参数调用时， :func:`dir` 函数返回当前定义的命名 ::
>>> a = [1, 2, 3, 4, 5]
>>> import fibo
>>> fib = fibo.fib
>>> dir()
[‘__builtins__’, ‘__doc__’, ‘__file__’, ‘__name__’, ‘a’, ‘fib’, ‘fibo’, ‘sys’]
note that it lists all types of names: variables, modules, functions, etc.
注意该列表列出了所有类型的名称：变量，模块，函数，等等。
.. index:: module: __builtin__
:func:`dir` does not list the names of built-in functions and variables. if you
want a list of those, they are defined in the standard module
:mod:`__builtin__`:
:func:`dir` 不会列出内置函数和变量名。如果你想列出这些内容，它们在标准模块
:mod:`__builtin__` 中定义 ::
>>> import __builtin__
>>> dir(__builtin__)
[‘arithmeticerror’, ‘assertionerror’, ‘attributeerror’, ‘deprecationwarning’,
‘eoferror’, ‘ellipsis’, ‘environmenterror’, ‘exception’, ‘false’,
‘floatingpointerror’, ‘futurewarning’, ‘ioerror’, ‘importerror’,
‘indentationerror’, ‘indexerror’, ‘keyerror’, ‘keyboardinterrupt’,
‘lookuperror’, ‘memoryerror’, ‘nameerror’, ‘none’, ‘notimplemented’,
‘notimplementederror’, ‘oserror’, ‘overflowerror’,
‘pendingdeprecationwarning’, ‘referenceerror’, ‘runtimeerror’,
‘runtimewarning’, ‘standarderror’, ‘stopiteration’, ‘syntaxerror’,
‘syntaxwarning’, ‘systemerror’, ‘systemexit’, ‘taberror’, ‘true’,
‘typeerror’, ‘unboundlocalerror’, ‘unicodedecodeerror’,
‘unicodeencodeerror’, ‘unicodeerror’, ‘unicodetranslateerror’,
‘userwarning’, ‘valueerror’, ‘warning’, ‘windowserror’,
‘zeropisionerror’, ‘_’, ‘__debug__’, ‘__doc__’, ‘__import__’,
‘__name__’, ‘abs’, ‘apply’, ‘basestring’, ‘bool’, ‘buffer’,
‘callable’, ‘chr’, ‘classmethod’, ‘cmp’, ‘coerce’, ‘compile’,
‘complex’, ‘copyright’, ‘credits’, ‘delattr’, ‘dict’, ‘dir’, ‘pmod’,
‘enumerate’, ‘eval’, ‘execfile’, ‘exit’, ‘file’, ‘filter’, ‘float’,
‘frozenset’, ‘getattr’, ‘globals’, ‘hasattr’, ‘hash’, ‘help’, ‘hex’,
‘id’, ‘input’, ‘int’, ‘intern’, ‘isinstance’, ‘issubclass’, ‘iter’,
‘len’, ‘license’, ‘list’, ‘locals’, ‘long’, ‘map’, ‘max’, ‘memoryview’,
‘min’, ‘object’, ‘oct’, ‘open’, ‘ord’, ‘pow’, ‘property’, ‘quit’, ‘range’,
‘raw_input’, ‘reduce’, ‘reload’, ‘repr’, ‘reversed’, ’round’, ‘set’,
‘setattr’, ‘slice’, ‘sorted’, ‘staticmethod’, ‘str’, ‘sum’, ‘super’,
‘tuple’, ‘type’, ‘unichr’, ‘unicode’, ‘vars’, ‘xrange’, ‘zip’]
.. _tut-packages:
packages 包
==============
packages are a way of structuring python’s module namespace by using “dotted
module names”. for example, the module name :mod:`a.b` designates a submodule
named “b“ in a package named “a“. just like the use of modules saves the
authors of different modules from having to worry about each other’s global
variable names, the use of dotted module names saves the authors of multi-module
packages like numpy or the python imaging library from having to worry about
each other’s module names.
包通常是使用用“圆点模块名”的结构化模块命名空间。例如，名为
:mod:`a.b` 的模块表示了名为 “b“ 的包中名为 “a“ 的子模块。正如同用
模块来保存不同的模块架构可以避免全局变量之间的相互冲突，使用圆点模块名
保存像 numpy 或 python imaging library 之类的不同类库架构可以避免模块
之间的命名冲突。
suppose you want to design a collection of modules (a “package”) for the uniform
handling of sound files and sound data. there are many different sound file
formats (usually recognized by their extension, for example: :file:`.wav`,
:file:`.aiff`, :file:`.au`), so you may need to create and maintain a growing
collection of modules for the conversion between the various file formats.
there are also many different operations you might want to perform on sound data
(such as mixing, adding echo, applying an equalizer function, creating an
artificial stereo effect), so in addition you will be writing a never-ending
stream of modules to perform these operations. here’s a possible structure for
your package (expressed in terms of a hierarchical filesystem):
假设你现在想要设计一个模块集（一个“包”）来统一处理声音文件和声音数据。
存在几种不同的声音格式（通常由它们的扩展名来标识，例如： :file:`.wav`
， :file:`.aiff` ， :file:`.au` ），于是，为了在不同类型的文件格式之间
转换，你需要维护一个不断增长的包集合。可能你还想要对声音数据做很多不同
的操作（例如混音，添加回声，应用平衡功能，创建一个人造效果），所以你要
加入一个无限流模块来执行这些操作。你的包可能会是这个样子（通过分级的文
件体系来进行分组） ::
sound/ top-level package
__init__.py initialize the sound package
formats/ subpackage for file format conversions
__init__.py
wavread.py
wavwrite.py
aiffread.py
aiffwrite.py
auread.py
auwrite.py
…
effects/ subpackage for sound effects
__init__.py
echo.py
surround.py
reverse.py
…
filters/ subpackage for filters
__init__.py
equalizer.py
vocoder.py
karaoke.py
…
when importing the package, python searches through the directories on
“sys.path“ looking for the package subdirectory.
导入模块时，python通过 “sys.path“ 中的目录列表来搜索存放包的子目录。
the :file:`__init__.py` files are required to make python treat the directories
as containing packages; this is done to prevent directories with a common name,
such as “string“, from unintentionally hiding valid modules that occur later
on the module search path. in the simplest case, :file:`__init__.py` can just be
an empty file, but it can also execute initialization code for the package or
set the “__all__“ variable, described later.
必须要有一个 :file:`__init__.py` 文件的存在，才能使 python 视该目录为
一个包；这是为了防止某些目录使用了 “string“ 这样的通用名而无意中在随
后的模块搜索路径中覆盖了正确的模块。最简单的情况下，
:file:`__init__.py` 可以只是一个空文件，不过它也可能包含了包的初始化代码，或者设置了 “__all__“ 变量，后面会有相关介绍。
users of the package can import inpidual modules from the package, for
example:
包用户可以从包中导入合法的模块，例如 ::
import sound.effects.echo
this loads the submodule :mod:`sound.effects.echo`. it must be referenced with
its full name. :
这样就导入了 :mod:`sound.effects.echo` 子模块。它必需通过完整的名称来引用。 ::
sound.effects.echo.echofilter(input, output, delay=0.7, atten=4)
an alternative way of importing the submodule is:
导入包时有一个可以选择的方式 ::
from sound.effects import echo
this also loads the submodule :mod:`echo`, and makes it available without its
package prefix, so it can be used as follows:
这样就加载了 :mod:`echo` 子模块，并且使得它在没有包前缀的情况下也可以
使用，所以它可以如下方式调用 ::
echo.echofilter(input, output, delay=0.7, atten=4)
yet another variation is to import the desired function or variable directly:
还有另一种变体用于直接导入函数或变量 ::
from sound.effects.echo import echofilter
again, this loads the submodule :mod:`echo`, but this makes its function
:func:`echofilter` directly available:
这样就又一次加载了 :mod:`echo` 子模块，但这样就可以直接调用它的
:func:`echofilter` 函数 ::
echofilter(input, output, delay=0.7, atten=4)
note that when using “from package import item“, the item can be either a
submodule (or subpackage) of the package, or some other name defined in the
package, like a function, class or variable. the “import“ statement first
tests whether the item is defined in the package; if not, it assumes it is a
module and attempts to load it. if it fails to find it, an :exc:`importerror`
exception is raised.
需要注意的是使用 “from package import item“ 方式导入包时，这个子项（item）既可以是包中的一个子模块（或一个子包），也可以是包中定义的其它命名，像函数、类或变量。import 语句首先核对是否包中有这个子项，如果没有，它假定这是一个模块，并尝试加载它。如果没有找到它，会引发一个 importerror 异常。
contrarily, when using syntax like “import item.subitem.subsubitem“, each item
except for the last must be a package; the last item can be a module or a
package but can’t be a class or function or variable defined in the previous
item.
相反，使用类似 “import item.subitem.subsubitem“ 这样的语法时，这些子项必
须是包，最后的子项可以是包或模块，但不能是前面子项中定义的类、函数或变
量。
.. _tut-pkg-import-star:
importing /* from a package
—————————
.. index:: single: __all__
now what happens when the user writes “from sound.effects import *“? ideally,
one would hope that this somehow goes out to the filesystem, finds which
submodules are present in the package, and imports them all. this could take a
long time and importing sub-modules might have unwanted side-effects that should
only happen when the sub-module is explicitly imported.
那么当用户写下 “from sound.effects import *“ 时会发生什么事？理想中，总
是希望在文件系统中找出包中所有的子模块，然后导入它们。这可能会花掉委有
长时间，并且出现期待之外的边界效应，导出了希望只能显式导入的包。
the only solution is for the package author to provide an explicit index of the
package. the :keyword:`import` statement uses the following convention: if a package’s
:file:`__init__.py` code defines a list named “__all__“, it is taken to be the
list of module names that should be imported when “from package import *“ is
encountered. it is up to the package author to keep this list up-to-date when a
new version of the package is released. package authors may also decide not to
support it, if they don’t see a use for importing /* from their package. for
example, the file :file:`sounds/effects/__init__.py` could contain the following
code:
对于包的作者来说唯一的解决方案就是给提供一个明确的包索引。
:keyword:`import` 语句按如下条件进行转换：执行 “from package import
*“ 时，如果包中的 :file:`__init__.py` 代码定义了一个名为 “__all__“
的列表，就会按照列表中给出的模块名进行导入。新版本的包发布时作者可以任
意更新这个列表。如果包作者不想 import * 的时候导入他们的包中所有模块，
那么也可能会决定不支持它（import /*）。例如，
:file:`sounds/effects/__init__.py` 这个文件可能包括如下代码 ::
__all__ = [“echo”, “surround”, “reverse”]
this would mean that “from sound.effects import *“ would import the three
named submodules of the :mod:`sound` package.
这意味着 “from sound.effects import *“ 语句会从 :mod:`sound` 包中导入以上三个已命名的子模块。
if “__all__“ is not defined, the statement “from sound.effects import *“
does *not* import all submodules from the package :mod:`sound.effects` into the
current namespace; it only ensures that the package :mod:`sound.effects` has
been imported (possibly running any initialization code in :file:`__init__.py`)
and then imports whatever names are defined in the package. this includes any
names defined (and submodules explicitly loaded) by :file:`__init__.py`. it
also includes any submodules of the package that were explicitly loaded by
previous :keyword:`import` statements. consider this code:
如果没有定义 “__all__“ ， “from sound.effects import *“ 语句不会从
:mod:`sound.effects` 包中导入所有的子模块。无论包中定义多少命名，只能
确定的是导入了 :mod:`sound.effects` 包（可能会运行 __init__.py 中的初
始化代码）以及包中定义的所有命名会随之导入。这样就从 “__init__.py“
中导入了每一个命名（以及明确导入的子模块）。同样也包括了前述的
:keyword:`import` 语句从包中明确导入的子模块，考虑以下代码 ::
import sound.effects.echo
import sound.effects.surround
from sound.effects import *
in this example, the :mod:`echo` and :mod:`surround` modules are imported in the
current namespace because they are defined in the :mod:`sound.effects` package
when the “from…import“ statement is executed. (this also works when
“__all__“ is defined.)
在这个例子中， :mod:`echo` 和 :mod:`surround` 模块导入了当前的命名空
间，这是因为执行 “from…import“ 语句时它们已经定义在
:mod:`sound.effects` 包中了（定义了 “__all__“ 时也会同样工作）。
although certain modules are designed to export only names that follow certain
patterns when you use “import *“, it is still considered bad practise in
production code.
尽管某些模块设计为使用 “import *“ 时它只导出符全某种模式的命名，仍然
不建议在生产代码中使用这种写法。
remember, there is nothing wrong with using “from package import
specific_submodule“! in fact, this is the recommended notation unless the
importing module needs to use submodules with the same name from different
packages.
记住， “from package import specific_submodule“ 没有错误！事实上，除
非导入的模块需要使用其它包中的同名子模块，否则这是推荐的写法。
intra-package references 包内引用
————————————-
the submodules often need to refer to each other. for example, the
:mod:`surround` module might use the :mod:`echo` module. in fact, such
references are so common that the :keyword:`import` statement first looks in the
containing package before looking in the standard module search path. thus, the
:mod:`surround` module can simply use “import echo“ or “from echo import
echofilter“. if the imported module is not found in the current package (the
package of which the current module is a submodule), the :keyword:`import`
statement looks for a top-level module with the given name.
子模块之间经常需要互相引用。例如，:mod:`surround` 模块可能会引用
:mod:`echo` 模块。事实上，这样的引用如此普遍，以致于 :keyword:`import`
语句会先搜索包内部，然后才是标准模块搜索路径。因此 :mod:`surround` 模
块可以简单的调用 “import echo“ 或者 “from echo import echofilter“
。如果没有在当前的包中发现要导入的模块，:keyword:`import` 语句会依据指
定名寻找一个顶级模块。
when packages are structured into subpackages (as with the :mod:`sound` package
in the example), you can use absolute imports to refer to submodules of siblings
packages. for example, if the module :mod:`sound.filters.vocoder` needs to use
the :mod:`echo` module in the :mod:`sound.effects` package, it can use “from
sound.effects import echo“.
如果包中使用了子包结构（就像示例中的 :mod:`sound` 包），可以按绝对位置
从相邻的包中引入子模块。例如，如果 :mod:`sound.filters.vocoder` 包需要
使用 :mod:`sound.effects` 包中的 :mod:`echo` 模块，它可以
“from sound.effects import echo“ 。
starting with python 2.5, in addition to the implicit relative imports described
above, you can write explicit relative imports with the “from module import
name“ form of import statement. these explicit relative imports use leading
dots to indicate the current and parent packages involved in the relative
import. from the :mod:`surround` module for example, you might use:
从 python 2.5 开始，前述的这种内部的显式相对位置导入得到改进，你可以用这样
的形式 “from module import name“ 来写显式的相对位置导入。那些显式相
对导入用点号标明关联导入当前和上级包。以 :mod:`surround` 模块为例，你可以
这样用 ::
from . import echo
from .. import formats
from ..filters import equalizer
note that both explicit and implicit relative imports are based on the name of
the current module. since the name of the main module is always ““__main__”“,
modules intended for use as the main module of a python application should
always use absolute imports.
需要注意的是显式或隐式相对位置导入都基于当前模块的命名。因为主模块的名
字总是 ““__main__”“ ，python 应用程序的主模块应该总是用绝对导入。
packages in multiple directories 多重目录中的包
——————————————————
packages support one more special attribute, :attr:`__path__`. this is
initialized to be a list containing the name of the directory holding the
package’s :file:`__init__.py` before the code in that file is executed. this
variable can be modified; doing so affects future searches for modules and
subpackages contained in the package.
包支持一个更为特殊的特性， :attr:`__path__` 。 在包的
:file:`__init__.py`
文件代码执行之前，该变量初始化一个目录名列表。该变量可以修改，它作用于
包中的子包和模块的搜索功能。
while this feature is not often needed, it can be used to extend the set of
modules found in a package.
这个功能可以用于扩展包中的模块集，不过它不常用。
.. rubric:: footnotes
.. [#] in fact function definitions are also ‘statements’ that are ‘executed’; the
execution of a module-level function enters the function name in the module’s
global symbol table.
.. [#] 事实上函数定义既是“声明”又是“可执行体”；执行体由函数在模块全局语义表中的命名导入。
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