import b
def f():
return b.x
print f()

import a
x = 1
def g():
print a.f()

> import a
1

> import b
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   File "b.py", line 1, in <module>
  import a
   File "a.py", line 6, in <module>
  print f()
   File "a.py", line 4, in f
  return b.x
AttributeError: 'module' object has no attribute 'x'

x = 1
def g():
  import a  
# 只有当g()被调用的时候才加载
  print a.f()

> import b
> b.g()
1  
# 第一次输出，因为模块a在最后调用了‘print f()'
1  
# 第二次输出，这是我们调用g()

stringmodule = __import__('string')

import imp
stringmodule = imp.load_module('string',*imp.find_module('string'))

import_string = "import string as stringmodule"
exec import_string