How to split a Python module into multiple files?

Question

I have a single Python module which contains 3 classes: A, A1 and A2. A1 and A2 derive from A. A contains functions which operate on A1 and A2.

This all works fine when it's in one .py file. But that file has grown quite long and I would like to split A1 and A2 off into their own files. How can I split this file despite a circular dependency?

Answer 1

modA.py:

class A(...):
   ...

modA1.py:

import modA
class A1(modA.A):
   ...

modA2.py:

import modA
class A2(modA.A):
   ...

modfull:

from modA import A
from modA1 import A1
from modA2 import A2

Even if A "processes" A1s and A2s you should be fine because thanks to duck typing you don't need to import the actual names.

Answer 2

How can I split this file with despite a circular dependency?

Option 1: break the cycles: Put the base class in its own module, the derived classes in additional modules, and functions operating on those derived classes in yet another module.

---

Option 2: Ignore the cycles, import only modules/packages into the global namespace, IE:

foo.py

class Bar:
    "Frobs Quuxen"

Should never be imported as from foo import Bar, just use import foo and refer to foo.Bar in functions as needed.