Inheritance and inner classes in Python?

Question

In the following code class B has inherited yay attribute from class A, I expected this. I'd also expect that inner class B.Foo behaves the same way but it doesn't.

How to make B.Foo to inherit alice attribute from class A? I need that the inner subclass Foo in B has both the attributes alice and bob.

Thanks.

>>> class A:
...     yay = True
...     class Foo:
...             alice = True
...
>>> class B(A):
...     nay = False
...     class Foo:
...             bob = False
>>> B.yay
True
>>> B.Foo.alice
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: class Foo has no attribute 'alice'

Why do you think you need inner classes? I've never seen a single situation where they would have occured to me as solution. Also, what's wrong with explicitly inheriting (as in class Foo(A.Foo))? — user395760
– user395760, Commented Apr 7, 2011 at 21:07
You can always write equivalent solution without inner classes, but sometimes express code structure and class relations very well. Imagine a collection and an iterator - classical example of inner class usage (though more C++ oriented). — pajton
– pajton, Commented Apr 7, 2011 at 21:18

Pavel Repin · Accepted Answer · 2011-04-07 21:43:49Z

20

The reason why B.Foo.alice gave you an error is because there's no connection between Foo attribute of class A and Foo attribute of class B.

In B, attribute Foo has a class object value that completely replaces class object value inherited from A.

This should fix it:

class B(A):
    nay = False
    class Foo(A.Foo):
        bob = False

In general, it helps, at least for me, to think of a class body contents as a sequence of attributes with certain assigned values.

In case of class B, we have:

yay attribute that has value True inherited from A.
nay attribute that has value False.
Foo attribute that has class object.

Class methods are also attributes that have callable objects as values.

edited Apr 7, 2011 at 21:43

answered Apr 7, 2011 at 21:26

Pavel Repin

31k2 gold badges36 silver badges41 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

pajton · Accepted Answer · 2011-04-07 21:39:08Z

9

Inheritance is a per-class thing. In your code class B inherits from class A, but just because both of them have inner class Foo doesn't tell us anything about their inheritance.

If you want B.Foo to have the attributes from A.Foo, you need to make B.Foo inherit from A.Foo:

class B(A):
    class Foo(A.Foo):
        bob = False

edited Apr 7, 2011 at 21:39

answered Apr 7, 2011 at 21:11

pajton

16.4k8 gold badges60 silver badges65 bronze badges

3 Comments

Adriano Varoli Piazza Over a year ago

In fact, Python's last Zen Koan specifically says "Namespaces are one honking great idea -- let's do more of those!". B.Foo is then obviously different from A.Foo.

Pavel Repin Over a year ago

In Python 3, class Foo: pass is the correct syntax for new style (the only style) classes it has. See: docs.python.org/release/3.0.1/reference/…

pajton Over a year ago

@pavel You're right. So since the OP didn't say which version is using I will remove section about classes.

unholysampler · Accepted Answer · 2011-04-07 21:11:17Z

1

Foo is it's own class. It does not inherit from A. Because of this, it does not have any fields of A. The fact that is nested in a subclass of A does not change anything.

answered Apr 7, 2011 at 21:11

unholysampler

17.4k7 gold badges51 silver badges65 bronze badges

Collectives™ on Stack Overflow

Inheritance and inner classes in Python?

3 Answers 3

Comments

3 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related