The functionality of isinstance(...); but without an instance?

Question

At runtime, I want to check whether a specified child class is derived from a specified parent class.

With an object instance, it's easy:

def is_related(child_instance, parent_type):
    return isinstance(child_instance, parent_type)

Is there some way to do this without having (or creating) an instance of the child but, instead, having a reference to the child's type?

Something like...

def is_related(child_type, parent_type):
    return is_child_class(child_type, parent_type)

Provide an implementation for is_child_class will answer this question.

(By comparison, types in C# know about their supertypes. I don't know whether this is also true in Python.)

Does this answer your question? [How do I check (at runtime) if one class is a subclass of another?](https://stackoverflow.com/questions/4912972/how-do-i-check-at-runtime-if-one-class-is-a-subclass-of-another) — Riccardo Bucco, Dec 05 '19 at 11:38

score 2 · Accepted Answer · edited Dec 05 '19 at 11:48

2

Let's say that ChildClass is a subclass of ParentClass. Then

issubclass(ChildClass, ParentClass)

would return True

edited Dec 05 '19 at 11:48

Riccardo Bucco

answered Dec 05 '19 at 11:37

mcsoini

1

Your answer came in seconds before @RiccardoBucco's. Thanks to you both! – Eric McLachlan Dec 05 '19 at 11:50

score 2 · Answer 2 · answered Dec 05 '19 at 11:38

2

Here is a possible solution:

class A:
    pass

class B(A):
    pass

issubclass(B, A) # True

answered Dec 05 '19 at 11:38

Riccardo Bucco

1

Thanks, @RiccardoBucco. Much appreciated. – Eric McLachlan Dec 05 '19 at 11:50

score 2 · Answer 3 · answered Dec 05 '19 at 11:45

2

This is what you need to define the is_child_class method as:

def is_child_class(child_type, parent_type):
  return issubclass(child_type, parent_type)

answered Dec 05 '19 at 11:45

Eb J

3 Answers3