Python - reference inner class from other inner class

Question

I am trying to reference an inner class from another inner class. I have tried both :

class Foo(object):

  class A(object):
    pass

  class B(object):
    other = A

and

class Foo(object):

  class A(object):
    pass

  class B(object):
    other = Foo.A

with respective results:

Traceback (most recent call last):
  File "python", line 1, in <module>
  File "python", line 6, in Foo
  File "python", line 7, in B
NameError: name 'A' is not defined

and

Traceback (most recent call last):
  File "python", line 1, in <module>
  File "python", line 6, in Foo
  File "python", line 7, in B
NameError: name 'Foo' is not defined

Is this possible?

Well clearly from your attempts the answer is no, but what are you trying to do here eventually? What problem are you trying to solve? — Burhan Khalid
Why do you have nested classes at all? They are very rarely useful in Python. — Daniel Roseman
Using classes is not always a good fit for the problem you are trying to solve. — Roland Smith
Part 1) I knew this would inevitably lead to "you're not using pythonic design." I'm aware of that, but that argument is largely subjective. My application has a few different models, e.g. User and Transaction, located in models/user.py and models/transaction.py, respectively. Both model classes have protorpc message classes, "Msg", e.g. User.Msg and Transaction.Msg. For the sake of import brevity and to maintain the namespace between the two message classes, I made them inner classes. — crunk1
Part 2) So, instead of "import user" and using "user.User" and "user.Msg", I could do "from user import User" and use "User" and "User.Msg". The example with "Transaction" is longer. The problem is that I also had a "Transaction.Status" that "Transaction.Msg" needed to reference. — crunk1

Israel Unterman Israel Unterman · Accepted Answer · 2017-02-12T09:18:46

This is not possible, since everything you define in a class becomes a valid member only in an instance of that class, unless you define a method with @staticmethod, but there is no such property for a class.

So, this won't work either:

class Foo(object):
    x = 10

    class A(object):
        pass

    class B(object):
        other = x

This will work, but it is not what you intended:

class Foo(object):
  x = 10

  class A(object):
    pass

  class B(object):
    def __init__(self):
        self.other = Foo.A

f = Foo()
print(f.B().other)

The output is:

<class '__main__.Foo.A'>

The reason this works is that the methods (in this case __init__) are evaluated when the object is created, while assignment before the __init__ are evaluated while the class is read and interpreted.

You can get about the same thing you want by simply define all the classes inside a module of their own. The importing the module, makes it an object whose fields are the classes you define in it.

Python - reference inner class from other inner class

1 Answers