如果你导入foo(在bar.py内部)和导入bar(在foo.py内部)，它会工作得很好。在实际运行任何东西时，两个模块都将完全加载，并将相互引用。

问题是当你做from foo import abc(在bar.py内)和from bar import xyz(在foo.py内)时。因为现在每个模块都需要另一个模块已经被导入(以便导入的名称存在)，然后才能导入它。

Ok, I think I have a pretty cool solution. Let's say you have file a and file b. You have a def or a class in file b that you want to use in module a, but you have something else, either a def, class, or variable from file a that you need in your definition or class in file b. What you can do is, at the bottom of file a, after calling the function or class in file a that is needed in file b, but before calling the function or class from file b that you need for file a, say import b Then, and here is the key part, in all of the definitions or classes in file b that need the def or class from file a (let's call it CLASS), you say from a import CLASS

这是可行的，因为您可以导入文件b，而无需Python执行文件b中的任何导入语句，因此您可以避免任何循环导入。

例如:

文件:

class A(object):

     def __init__(self, name):

         self.name = name

CLASS = A("me")

import b

go = B(6)

go.dostuff

文件b:

class B(object):

     def __init__(self, number):

         self.number = number

     def dostuff(self):

         from a import CLASS

         print "Hello " + CLASS.name + ", " + str(number) + " is an interesting number."

喉咙痛。

去年在comp.lang.python上对此进行了很好的讨论。它很彻底地回答了你的问题。

Imports are pretty straightforward really. Just remember the following: 'import' and 'from xxx import yyy' are executable statements. They execute when the running program reaches that line. If a module is not in sys.modules, then an import creates the new module entry in sys.modules and then executes the code in the module. It does not return control to the calling module until the execution has completed. If a module does exist in sys.modules then an import simply returns that module whether or not it has completed executing. That is the reason why cyclic imports may return modules which appear to be partly empty. Finally, the executing script runs in a module named __main__, importing the script under its own name will create a new module unrelated to __main__. Take that lot together and you shouldn't get any surprises when importing modules.

循环导入可能令人困惑，因为导入做了两件事:

它执行导入的模块代码 将导入的模块添加到导入模块全局符号表中

前者只执行一次，而后者则在每个import语句中执行。循环导入会在导入模块使用已导入的模块和部分执行的代码时产生这种情况。因此，它将看不到import语句后创建的对象。下面的代码示例演示了它。

循环进口并不是要不惜一切代价避免的终极祸害。在一些框架中，比如Flask，它们是很自然的，调整你的代码来消除它们并不会让代码变得更好。

main.py

print 'import b'
import b
print 'a in globals() {}'.format('a' in globals())
print 'import a'
import a
print 'a in globals() {}'.format('a' in globals())
if __name__ == '__main__':
    print 'imports done'
    print 'b has y {}, a is b.a {}'.format(hasattr(b, 'y'), a is b.a)

b.by

print "b in, __name__ = {}".format(__name__)
x = 3
print 'b imports a'
import a
y = 5
print "b out"

a.py

print 'a in, __name__ = {}'.format(__name__)
print 'a imports b'
import b
print 'b has x {}'.format(hasattr(b, 'x'))
print 'b has y {}'.format(hasattr(b, 'y'))
print "a out"

Python main.py输出带有注释

import b
b in, __name__ = b    # b code execution started
b imports a
a in, __name__ = a    # a code execution started
a imports b           # b code execution is already in progress
b has x True
b has y False         # b defines y after a import,
a out
b out
a in globals() False  # import only adds a to main global symbol table 
import a
a in globals() True
imports done
b has y True, a is b.a True # all b objects are available

循环导入会终止，但在模块初始化期间，您需要注意不要使用循环导入的模块。

考虑以下文件:

a.py:

print "a in"
import sys
print "b imported: %s" % ("b" in sys.modules, )
import b
print "a out"

b.py:

print "b in"
import a
print "b out"
x = 3

如果你执行a.py，你会得到以下结果:

$ python a.py
a in
b imported: False
b in
a in
b imported: True
a out
b out
a out

在第二次导入b.py时(在第二个ain中)，Python解释器不会再次导入b，因为它已经存在于模块dict中。

如果你试图在模块初始化期间从a访问b.x，你会得到一个AttributeError。

将下面的行追加到a.py:

print b.x

则输出为:

$ python a.py
a in                    
b imported: False
b in
a in
b imported: True
a out
Traceback (most recent call last):
  File "a.py", line 4, in <module>
    import b
  File "/home/shlomme/tmp/x/b.py", line 2, in <module>
    import a
 File "/home/shlomme/tmp/x/a.py", line 7, in <module>
    print b.x
AttributeError: 'module' object has no attribute 'x'

这是因为模块是在导入时执行的，在访问b.x时，x = 3行还没有执行，这只会在b退出后发生。

模块a.py:

import b
print("This is from module a")

模块b.py

import a
print("This is from module b")

运行“Module a”将输出:

>>> 
'This is from module a'
'This is from module b'
'This is from module a'
>>> 

它输出了这3行，而由于循环导入，它应该输出不定式。 这里列出了运行“模块a”时逐行发生的事情:

The first line is import b. so it will visit module b The first line at module b is import a. so it will visit module a The first line at module a is import b but note that this line won't be executed again anymore, because every file in python execute an import line just for once, it does not matter where or when it is executed. so it will pass to the next line and print "This is from module a". After finish visiting whole module a from module b, we are still at module b. so the next line will print "This is from module b" Module b lines are executed completely. so we will go back to module a where we started module b. import b line have been executed already and won't be executed again. the next line will print "This is from module a" and program will be finished.