python module circular import

Cyclic imports terminate, but you need to be careful not to use the cyclically-imported modules during module initialization.

Consider the following files:

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

If you execute a.py, you‘ll get the following:

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

On the second import of b.py (in the second a in), the Python interpreter does not import b again, because it already exists in the module dict.

Edit: If you try to access b.x from a during module initialization, you will get an AttributeError.

Append the following line to a.py:

print b.x

Then, the output is:

$ 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‘

This is because modules are executed on import and at the time b.x is accessed, the line x = 3 has not be executed yet, which will only happen after b out.

python module circular import

原文：http://www.cnblogs.com/threef/p/3538990.html