6.12 import 语句 The import statement

import_stmt  ::=  "import" module ["as" name] ( "," module ["as" name] )*
    | "from" module "import" identifier ["as" name]
      ( "," identifier ["as" name] )*
    | "from" module "import" "*"
module  ::=  (identifier ".")* identifier
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Import statements are executed in two steps: (1) find a module, and initialize it if necessary; (2) define a name or names in the local namespace (of the scope where the import statement occurs). The first form (without from) repeats these steps for each identifier in the list. The form with from performs step (1) once, and then performs step (2) repeatedly.

import语句分两步执行:(1) 找到模块,如果需要则进行初始化;(2) 在(import语句所发生的范围内)局部名字空间中定义一个或者多个名字。(import语句的)第一形式(不带import那个)对列表中的每个标示符重复这些步骤。带import的形式只执行步骤(1)一次,然后重复地执行步骤(2)。

In this context, to ``initialize'' a built-in or extension module means to call an initialization function that the module must provide for the purpose (in the reference implementation, the function's name is obtained by prepending string ``init'' to the module's name); to ``initialize'' a Python-coded module means to execute the module's body.

The system maintains a table of modules that have been or are being initialized, indexed by module name. This table is accessible as sys.modules. When a module name is found in this table, step (1) is finished. If not, a search for a module definition is started. When a module is found, it is loaded. Details of the module searching and loading process are implementation and platform specific. It generally involves searching for a ``built-in'' module with the given name and then searching a list of locations given as sys.path.


If a built-in module is found, its built-in initialization code is executed and step (1) is finished. If no matching file is found, ImportError is raised. If a file is found, it is parsed, yielding an executable code block. If a syntax error occurs, SyntaxError is raised. Otherwise, an empty module of the given name is created and inserted in the module table, and then the code block is executed in the context of this module. Exceptions during this execution terminate step (1).


When step (1) finishes without raising an exception, step (2) can begin.


The first form of import statement binds the module name in the local namespace to the module object, and then goes on to import the next identifier, if any. If the module name is followed by as, the name following as is used as the local name for the module.

import语句的第一形式于当前局部名字空间中将模块名字约束到该模块对象上去,然后继续导入下个标识符,如果还有的话。如果模块名字后面带as,as后面跟着的名字就被用作该模块的局部名。为避免混淆,你不能把带点的模块名字导入为另外一个不同的名字。所以import module as m是合法的,但是import module.submod as s就不是。后者应写成from module import submod as s。参看下面所述。

The from form does not bind the module name: it goes through the list of identifiers, looks each one of them up in the module found in step (1), and binds the name in the local namespace to the object thus found. As with the first form of import, an alternate local name can be supplied by specifying "as localname". If a name is not found, ImportError is raised. If the list of identifiers is replaced by a star ("*"), all public names defined in the module are bound in the local namespace of the import statement..


The public names defined by a module are determined by checking the module's namespace for a variable named __all__; if defined, it must be a sequence of strings which are names defined or imported by that module. The names given in __all__ are all considered public and are required to exist. If __all__ is not defined, the set of public names includes all names found in the module's namespace which do not begin with an underscore character ("_"). __all__ should contain the entire public API. It is intended to avoid accidentally exporting items that are not part of the API (such as library modules which were imported and used within the module).


The from form with "*" may only occur in a module scope. If the wild card form of import -- "import *" -- is used in a function and the function contains or is a nested block with free variables, the compiler will raise a SyntaxError.


Hierarchical module names:when the module names contains one or more dots, the module search path is carried out differently. The sequence of identifiers up to the last dot is used to find a ``package''; the final identifier is then searched inside the package. A package is generally a subdirectory of a directory on sys.path that has a file __init__.py.[XXX Can't be bothered to spell this out right now; see the URL http://www.python.org/doc/essays/packages.html for more details, also about how the module search works from inside a package.]

有层次的模块名字:当模块名字包含一个或多个小数点时,模块的搜索路径执行起来有所不同。(从头一直)到最后一个小数点的标识符序列被用来找到一个“包”;然后在该包中搜索最末的那个标识符。包一般而言是在sys.path中的目录的一个有文件__init__.py的子目录。【XXX 现在在这里不能把这些都详细地写出来,更多的细节,以及关于包内模块搜索如何工作的详情,请见URLhttp://www.python.org/doc/essays/packages.html 】

The built-in function __import__() is provided to support applications that determine which modules need to be loaded dynamically; refer to Built-in Functions in the Python Library Reference for additional information.

提供了内置函数__import__()使得应用程序可动态地决定需要加载哪些模块,更多信息请参考Python 库参考 中的内建函数 。