Question

I know the issue of circular imports in python has come up many times before and I have read these discussions. The comment that is made repeatedly in these discussions is that a circular import is a sign of a bad design and the code should be reorganised to avoid the circular import.

Could someone tell me how to avoid a circular import in this situation?: I have two classes and I want each class to have a constructor (method) which takes an instance of the other class and returns an instance of the class.

More specifically, one class is mutable and one is immutable. The immutable class is needed
for hashing, comparing and so on. The mutable class is needed to do things too. This is similar to sets and frozensets or to lists and tuples.

I could put both class definitions in the same module. Are there any other suggestions?

A toy example would be class A which has an attribute which is a list and class B which has an attribute which is a tuple. Then class A has a method which takes an instance of class B and returns an instance of class A (by converting the tuple to a list) and similarly class B has a method which takes an instance of class A and returns an instance of class B (by converting the list to a tuple).

Answer 1

Only import the module, don’t import from the module:

Consider a.py:

import b

class A:
    def bar(self):
        return b.B()

and b.py:

import a

class B:
    def bar(self):
        return a.A()

This works perfectly fine.