How to resolve circular dependency when using composition?

Question

I came across some situation like shown below, where each class needs the other class and it creates the cyclic dependency. I came across this type of situation while wrapping some C code using ctypes. There already are lots of post on this topic but I didn't find them helpful, I need some example.

Module A:

from B import C2 class C1(object): def __init__(self): self.name = "C1" self.c2 = C2()

Module B

from A import C1 class C2(object): def __init__(self): self.name = "C2" self.c1 = C1()

Main

from A import C1 if __name__ == "__main__": o = C1() print(o.name)

This code contains an infinite recursion: The __init__() method of C1 unconditionally calls the __init__() of C2 and vice versa. — Sven Marnach
– Sven Marnach, Commented Jun 15, 2012 at 15:32
possible duplicate of Resolve circular import error in python. My example here is essentially the same as my answer there. — Martijn Pieters
– Martijn Pieters, Commented Jun 15, 2012 at 15:35

Sven Marnach · Accepted Answer · 2012-06-15 15:35:29Z

Solutions (assuming the inifinite recursion is resolved):

Avoid this kind of circular dependency.
Put the interdependent components into the same module.
Use import A and import B instead of the from ... form. (This might fail in certain situations if packages are involved.)
Use function-level imports.

Martijn Pieters · Accepted Answer · 2012-06-15 15:33:41Z

Change Module B to not import class C1 until the last possible moment, inside the __init__.py:

class C2(object): def __init__(self): from A import C1 self.name = "C2" self.c1 = C1()

Note that because C1 instantiates C2 and vice-versa, you'll end up with an infinite recursion problem, but I'll charitably assume that you don't have that problem outside of your illustration example.

Tony Hopkinson · Accepted Answer · 2012-06-15 15:42:26Z

That's a circular dependancy in any language, in fact it's an infinite recursion

Take c2 out of c1 and c1 out of c2

from B import C2 from A import C1 class C3(object): def __init__(self): self.c1 = C1() self.c2 = C2()

Harshdeep Khatke · Accepted Answer · 2019-02-15 12:08:46Z

As it is a Circular import error, remove the import statement from MODULE B -> "from A import C1".

import 'C1' where you need(inside the method).

Like this - from B import C2

class C1(object):

def __init__(self): self.name = "C1" self.c2 = C2()

class C2(object):

def __init__(self): import A.C1 self.name = "C2" self.c1 = C1()

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