python __import__简介

下面先看个小例子

[wanghai01@tc-crm-rd03 test]$ cat a.py

def func1():
    print 'in a.func1'

[wanghai01@tc-crm-rd03 test]$ cat c.py
package = 'test'
module = 'a'

def func2():
    p = __import__("%s.%s"%(package,module))
    m = getattr(p, 'a')
    f = getattr(m, 'func1')
    f()
    print 'in c.func2()'

func2()

[wanghai01@tc-crm-rd03 test]$ python c.py
in a.func1

in c.func2()

觉得__import__最大的好处就是可以使程序在runtime时动态加载一些模块，而不是在py文件的开头使用import来加载

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官方文档如下

__import__ ( name [, globals [, locals [, fromlist [, level ] ] ] ] )

Note

This is an advanced function that is not needed in everyday Python programming.

This function is invoked by the import statement. It can be replaced (by importing the__builtin__ module and assigning to__builtin__.__import__) in order to change semantics of theimport statement, but nowadays it is usually simpler to use import hooks (seePEP 302). Direct use of__import__() is rare, except in cases where you want to import a module whose name is only known at runtime.

The function imports the module name, potentially using the given globals and locals to determine how to interpret the name in a package context. Thefromlist gives the names of objects or submodules that should be imported from the module given byname. The standard implementation does not use its locals argument at all, and uses itsglobals only to determine the package context of the import statement.

level specifies whether to use absolute or relative imports. The default is-1 which indicates both absolute and relative imports will be attempted.0 means only perform absolute imports. Positive values forlevel indicate the number of parent directories to search relative to the directory of the module calling__import__().

When the name variable is of the form package.module, normally, the top-level package (the name up till the first dot) is returned,not the module named by name. However, when a non-empty fromlist argument is given, the module named byname is returned.

For example, the statement importspam results in bytecode resembling the following code:

spam = __import__('spam', globals(), locals(), [], -1)

The statement import spam.ham results in this call:

spam = __import__('spam.ham', globals(), locals(), [], -1)

Note how __import__() returns the toplevel module here because this is the object that is bound to a name by theimport statement.

On the other hand, the statement fromspam.ham import eggs, sausage as saus results in

_temp = __import__('spam.ham', globals(), locals(), ['eggs', 'sausage'], -1)
eggs = _temp.eggs
saus = _temp.sausage

Here, the spam.ham module is returned from__import__(). From this object, the names to import are retrieved and assigned to their respective names.

If you simply want to import a module (potentially within a package) by name, you can call__import__() and then look it up insys.modules:

>>> import sys
>>> name = 'foo.bar.baz'
>>> __import__(name)
<module 'foo' from ...>
>>> baz = sys.modules[name]
>>> baz
<module 'foo.bar.baz' from ...>

Changed in version 2.5: The level parameter was added.

Changed in version 2.5: Keyword support for parameters was added.