python __import__簡介

下面先看個小例子

[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()

覺得__import__最大的好處就是可以使程式在runtime時動態載入一些模組，而不是在py檔案的開頭使用import來載入

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官方文檔如下

__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.eggssaus = _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.