foo.py

class Foo(object):
    foo_str = 'foo_str'

    def __call__(self):
        print('call', Foo.foo_str)

    @staticmethod
    def config(arg=None):
        if arg:
            Foo.foo_str = arg
        import sys
        foo = Foo()
        foo.Foo = Foo
        foo.config = Foo.config
        sys.modules['foo'] = foo
        globals()['Foo'] = foo.Foo      #1
        #setattr(foo, 'Foo', foo.Foo)  #2 

Foo.config()

bar.py

import foo

foo()
foo.config('new str')
foo()

为什么bar.py的运行情况是(python 2.7.3):

• 只注释掉1报错

Traceback (most recent call last):                           
  File "bar.py", line 3, in <module>                         
    foo()                                                    
  File "/tmp/foo.py", line 5, in __call__            
    print('call', Foo.foo_str)                               
AttributeError: 'NoneType' object has no attribute 'foo_str' 

• 只注释掉2正常

('call', 'foo_str') 
('call', 'new str')