Possible Duplicate:
Circular (or cyclic) imports in Python
I have class B that imports and creates instances of class A. Class A needs reference to B in its contructor and so includes B.
from a import A class B: def __init__(self): self.a = A() from b import B class A: def __init__(self, ref): assert isinstance(ref, B) self.ref = ref This doesn't work. The main file imports B and uses it... not. Something with the imports is wrong.
Error from file a ImportError: cannot import name B
Apart from "don't do that, you are painting yourself into a corner", you could also postpone the import of B until you need it. File a.py:
class A: def __init__(self, ref): from b import B assert isinstance(ref, B) self.ref = ref Class B won't be imported until you instantiate class A, by which time the module already has been imported fully by module b.
You can also use a common base class and test for that.
Just import classes in __init__ method
class A: def __init__(self, ref): from b import B assert isinstance(ref, B) self.ref = ref The __init__ method executes when you create an instance of the class. In this case, you should get it to work by simply changing the import statements like this:
import a class B: def __init__(self): self.a = a.A(self) import b class A: def __init__(self, ref): assert isinstance(ref, b.B) self.ref = ref 