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, Jun 15 '12 at 15:32
possible duplicate of [Resolve circular import error in python](http://stackoverflow.com/questions/10027011/resolve-circular-import-error-in-python). My example here is essentially the same as my answer there. — Martijn Pieters, Jun 15 '12 at 15:35

score 5 · Answer 1 · answered Jun 15 '12 at 15:35

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.

score 2 · Answer 2 · answered Jun 15 '12 at 15:33

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.

score 1 · Answer 3 · answered Jun 15 '12 at 15:42

1

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()

answered Jun 15 '12 at 15:42

Tony Hopkinson

20,172
3
31
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Harshdeep Khatke · Answer 4 · 2019-02-15T12:08:46.720

0

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()

edited Feb 15 '19 at 12:08

answered Mar 02 '18 at 11:48

Harshdeep Khatke

131
1
8

How to resolve circular dependency when using composition?

4 Answers4