从其他文件导入函数时的Python NameError

Question

例如，我有以下源文件： 档案a.py

from b import *

def A():
    print 'function A'
    B()
    return

档案b.py

from a import *

def B():
    print 'function B'
    A()
    return

def main():
    A()
    B()
    return

if __name__ == '__main__':
    main()

然后我运行python b.py，我遇到了以下错误：

NameError: global name 'A' is not defined

完整的信息和印刷品如下：

function A
function B
Traceback (most recent call last):
  File "b.py", line 14, in <module>
    main()
  File "b.py", line 9, in main
    A()
  File "/home/mypath/a.py", line 5, in A
    B()
  File "/home/mypath/b.py", line 5, in B
    A()
NameError: global name 'A' is not defined

它有什么问题？ 谢谢大家的帮助！