python中import的相关知识总结

 

原文地址：http://jianpx.iteye.com/blog/486466

1. import 实际上是python虚拟机把当前的globals()和locals()传进__builtins__.__import__内置函数了，所以实际上干活的是那个__import__函数！

2. import对命名空间的影响

1）如果是python的内置模块，例如os模块。这些模块是随着python虚拟机启动而加载进来的，但是并没有暴露出来。我们可以通过dir()命令查看当前命名空间

>>> dir()
['__builtins__', '__doc__', '__name__']

>>> dir()
['__builtins__', '__doc__', '__name__']

>>> dir()
['__builtins__', '__doc__', '__name__']

可以看到，并没有看到像os，sys等模块。

但是我们如果执行import sys后，那就有了。

>>> import sys
>>> dir()
['__builtins__', '__doc__', '__name__', 'sys']

>>> import sys
>>> dir()
['__builtins__', '__doc__', '__name__', 'sys']

>>> import sys
>>> dir()
['__builtins__', '__doc__', '__name__', 'sys']

通过sys.modules这个字典（key: 模块名；value: 模块的路径），我们可以查看模块的信息。再通过id函数，我们可以知道两个sys模块是否为同一个模块，如下：

>>> id(sys)
135708788
>>> id(sys.modules['sys'])
135708788

>>> id(sys)
135708788
>>> id(sys.modules['sys'])
135708788

>>> id(sys)
135708788
>>> id(sys.modules['sys'])
135708788

2）import只影响当前模块的命名空间

例如a.py里面有一句import bb, 而bb.py里面有一句import os，那么可以这样查看

>>> import a
>>> dir()
['__builtins__', '__doc__', '__name__', 'a']
>>> dir(a)
['__builtins__', '__doc__', '__file__', '__name__', 'bb']
>>> dir(a.bb)
['__builtins__', '__doc__', '__file__', '__name__', 'os']

>>> import a
>>> dir()
['__builtins__', '__doc__', '__name__', 'a']
>>> dir(a)
['__builtins__', '__doc__', '__file__', '__name__', 'bb']
>>> dir(a.bb)
['__builtins__', '__doc__', '__file__', '__name__', 'os']

>>> import a
>>> dir()
['__builtins__', '__doc__', '__name__', 'a']
>>> dir(a)
['__builtins__', '__doc__', '__file__', '__name__', 'bb']
>>> dir(a.bb)
['__builtins__', '__doc__', '__file__', '__name__', 'os']

3. import package

首先合法的package必须含有一个__init__.py文件，package可以包含0个或多个module（py文件）。假设aa文件夹中有test.py文件和bb文件夹，bb文件夹里面有c.py。那么当import aa.bb.c的时候，究竟对命名空间产生什么影响呢？

>>> import aa.bb.c
>>> dir()
['__builtins__', '__doc__', '__name__', 'aa']
>>> import sys
>>> for k,v in sys.modules.items():
... print k,'\t',v
...
<SPAN style="COLOR: #ff0000">aa <module 'aa' from 'aa/__init__.pyc'></SPAN>



copy_reg <module 'copy_reg' from '/usr/local/lib/python2.5/copy_reg.pyc'>
__main__ <module '__main__' (built-in)>
site <module 'site' from '/usr/local/lib/python2.5/site.pyc'>
__builtin__ <module '__builtin__' (built-in)>
encodings <module 'encodings' from '/usr/local/lib/python2.5/encodings/__init__.pyc'>
encodings.encodings None
<SPAN style="COLOR: #ff0000">aa.bb.c <module 'aa.bb.c' from 'aa/bb/c.pyc'></SPAN>



posixpath <module 'posixpath' from '/usr/local/lib/python2.5/posixpath.pyc'>
errno <module 'errno' (built-in)>
encodings.codecs None
encodings.latin_1 <module 'encodings.latin_1' from '/usr/local/lib/python2.5/encodings/latin_1.pyc'>
os.path <module 'posixpath' from '/usr/local/lib/python2.5/posixpath.pyc'>
_codecs <module '_codecs' (built-in)>
stat <module 'stat' from '/usr/local/lib/python2.5/stat.pyc'>
zipimport <module 'zipimport' (built-in)>
warnings <module 'warnings' from '/usr/local/lib/python2.5/warnings.pyc'>
encodings.types None
UserDict <module 'UserDict' from '/usr/local/lib/python2.5/UserDict.pyc'>
sys <module 'sys' (built-in)>
codecs <module 'codecs' from '/usr/local/lib/python2.5/codecs.pyc'>
readline <module 'readline' from '/usr/local/lib/python2.5/lib-dynload/readline.so'>
types <module 'types' from '/usr/local/lib/python2.5/types.pyc'>
_types <module '_types' (built-in)>
signal <module 'signal' (built-in)>
linecache <module 'linecache' from '/usr/local/lib/python2.5/linecache.pyc'>
posix <module 'posix' (built-in)>
encodings.aliases <module 'encodings.aliases' from '/usr/local/lib/python2.5/encodings/aliases.pyc'>
<SPAN style="COLOR: #ff0000">aa.bb <module 'aa.bb' from 'aa/bb/__init__.pyc'></SPAN>



exceptions <module 'exceptions' (built-in)>
os <module 'os' from '/usr/local/lib/python2.5/os.pyc'>

>>> import aa.bb.c
>>> dir()
['__builtins__', '__doc__', '__name__', 'aa']
>>> import sys
>>> for k,v in sys.modules.items():
... print k,'\t',v
...
<SPAN style="COLOR: #ff0000">aa <module 'aa' from 'aa/__init__.pyc'></SPAN>



copy_reg <module 'copy_reg' from '/usr/local/lib/python2.5/copy_reg.pyc'>
__main__ <module '__main__' (built-in)>
site <module 'site' from '/usr/local/lib/python2.5/site.pyc'>
__builtin__ <module '__builtin__' (built-in)>
encodings <module 'encodings' from '/usr/local/lib/python2.5/encodings/__init__.pyc'>
encodings.encodings None
<SPAN style="COLOR: #ff0000">aa.bb.c <module 'aa.bb.c' from 'aa/bb/c.pyc'></SPAN>



posixpath <module 'posixpath' from '/usr/local/lib/python2.5/posixpath.pyc'>
errno <module 'errno' (built-in)>
encodings.codecs None
encodings.latin_1 <module 'encodings.latin_1' from '/usr/local/lib/python2.5/encodings/latin_1.pyc'>
os.path <module 'posixpath' from '/usr/local/lib/python2.5/posixpath.pyc'>
_codecs <module '_codecs' (built-in)>
stat <module 'stat' from '/usr/local/lib/python2.5/stat.pyc'>
zipimport <module 'zipimport' (built-in)>
warnings <module 'warnings' from '/usr/local/lib/python2.5/warnings.pyc'>
encodings.types None
UserDict <module 'UserDict' from '/usr/local/lib/python2.5/UserDict.pyc'>
sys <module 'sys' (built-in)>
codecs <module 'codecs' from '/usr/local/lib/python2.5/codecs.pyc'>
readline <module 'readline' from '/usr/local/lib/python2.5/lib-dynload/readline.so'>
types <module 'types' from '/usr/local/lib/python2.5/types.pyc'>
_types <module '_types' (built-in)>
signal <module 'signal' (built-in)>
linecache <module 'linecache' from '/usr/local/lib/python2.5/linecache.pyc'>
posix <module 'posix' (built-in)>
encodings.aliases <module 'encodings.aliases' from '/usr/local/lib/python2.5/encodings/aliases.pyc'>
<SPAN style="COLOR: #ff0000">aa.bb <module 'aa.bb' from 'aa/bb/__init__.pyc'></SPAN>



exceptions <module 'exceptions' (built-in)>
os <module 'os' from '/usr/local/lib/python2.5/os.pyc'>

>>> import aa.bb.c
>>> dir()
['__builtins__', '__doc__', '__name__', 'aa']
>>> import sys 
>>> for k,v in sys.modules.items():
...     print k,'\t',v
... 
aa 	<module 'aa' from 'aa/__init__.pyc'>



copy_reg 	<module 'copy_reg' from '/usr/local/lib/python2.5/copy_reg.pyc'>
__main__ 	<module '__main__' (built-in)>
site 	<module 'site' from '/usr/local/lib/python2.5/site.pyc'>
__builtin__ 	<module '__builtin__' (built-in)>
encodings 	<module 'encodings' from '/usr/local/lib/python2.5/encodings/__init__.pyc'>
encodings.encodings 	None
aa.bb.c 	<module 'aa.bb.c' from 'aa/bb/c.pyc'>



posixpath 	<module 'posixpath' from '/usr/local/lib/python2.5/posixpath.pyc'>
errno 	<module 'errno' (built-in)>
encodings.codecs 	None
encodings.latin_1 	<module 'encodings.latin_1' from '/usr/local/lib/python2.5/encodings/latin_1.pyc'>
os.path 	<module 'posixpath' from '/usr/local/lib/python2.5/posixpath.pyc'>
_codecs 	<module '_codecs' (built-in)>
stat 	<module 'stat' from '/usr/local/lib/python2.5/stat.pyc'>
zipimport 	<module 'zipimport' (built-in)>
warnings 	<module 'warnings' from '/usr/local/lib/python2.5/warnings.pyc'>
encodings.types 	None
UserDict 	<module 'UserDict' from '/usr/local/lib/python2.5/UserDict.pyc'>
sys 	<module 'sys' (built-in)>
codecs 	<module 'codecs' from '/usr/local/lib/python2.5/codecs.pyc'>
readline 	<module 'readline' from '/usr/local/lib/python2.5/lib-dynload/readline.so'>
types 	<module 'types' from '/usr/local/lib/python2.5/types.pyc'>
_types 	<module '_types' (built-in)>
signal 	<module 'signal' (built-in)>
linecache 	<module 'linecache' from '/usr/local/lib/python2.5/linecache.pyc'>
posix 	<module 'posix' (built-in)>
encodings.aliases 	<module 'encodings.aliases' from '/usr/local/lib/python2.5/encodings/aliases.pyc'>
aa.bb 	<module 'aa.bb' from 'aa/bb/__init__.pyc'>



exceptions 	<module 'exceptions' (built-in)>
os 	<module 'os' from '/usr/local/lib/python2.5/os.pyc'>

可以看到只是一句import aa.bb.c，却把aa，aa.bb，aabb.c全部加入了当前命名空间。这种做法是为了防止重名的吧，因为如果不加前缀的话，其他包里面有重名的模块就不能识别了！原理是python现在当前命名空间查找符号‘aa’对应的object，然后再在它的命名空间查找符号'bb',然后在bb的属性（命名空间）里面寻找c，所以，需要把aa，bb都加载进来。不过这些都是只加载一次的，不信的话，大家可以试一下再import aa.bb.d这样的模块，同时预先在aa文件夹中的

__init__.py文件中输入print ‘hello’，hello只在第一次import aa.bb.c的时候输出！

4.from与import

1）例如：from aa import bb

>>> from aa import bb
>>> dir()
['__builtins__', '__doc__', '__name__', 'bb']
>>> import sys
>>> sys.modules['bb']
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'bb'
>>> sys.modules['aa.bb']
<module 'aa.bb' from 'aa/bb/__init__.pyc'>
>>> bb
<module 'aa.bb' from 'aa/bb/__init__.pyc'>
>>> aa.bb
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'aa' is not defined

>>> from aa import bb
>>> dir()
['__builtins__', '__doc__', '__name__', 'bb']
>>> import sys
>>> sys.modules['bb']
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'bb'
>>> sys.modules['aa.bb']
<module 'aa.bb' from 'aa/bb/__init__.pyc'>
>>> bb
<module 'aa.bb' from 'aa/bb/__init__.pyc'>
>>> aa.bb
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'aa' is not defined

>>> from aa import bb
>>> dir()
['__builtins__', '__doc__', '__name__', 'bb']
>>> import sys 
>>> sys.modules['bb']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'bb'
>>> sys.modules['aa.bb']
<module 'aa.bb' from 'aa/bb/__init__.pyc'>
>>> bb
<module 'aa.bb' from 'aa/bb/__init__.pyc'>
>>> aa.bb
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'aa' is not defined

from import只是把import后面的名字引入了命名空间，让我们可以更方便的 使用而已，但是内部实际上还是带有模块名的。import 。。。as。。。也是同样的道理了。

2）from 。。。import *

这种形式的import是不推荐的，因为很容易污染命名空间。但是也可以了解一下。

首先如果package中的__init__.py文件并没有声明__all__ 的值的话，from package import * 是没有用的。因为python源码中的import_from_all这个函数估计会去读__all__的值，然后再import。

而这种形式的import对命名空间的影响又是怎样的呢？

答案是和import aa.bb.c那种一样的。