Python class inheritance dynamic class variable

Question

This is what I want to do:

class Base:
 _type = None
 name: str = _type.name
class a(Base):
 _type = UnityType
a_instance = a()
a_instance.name # Expecting UnityType.name to be some string.

While trying this, I get 'NoneType' object has no attribute 'name'. The reason is clear and understandable. But how can I pass the responsibility of implementing a variable class to a subclass?

• Samwise: I don't understand why you're saying _type is a str in the first place if you want it to be something that has a name attribute...
• fsimoes: Just fixed the typing error. @Samwise. _type is a class.
• Mark: You could define __init__() and put the name on the instance when the instance is initialized? Although it's not clear what should happen when someone creates Base().
• kindall: As long as you don't need a._type.name to work ,only a_instance._type.name, initialize the attributes in a's __init__ method. If you need it to work on a itself, put it in the metaclass.

Answer 1

I think you need Base.name to be a property, so that it's evaluated when it's accessed rather than when Base is defined:

from typing import Optional, Protocol, Type
class Named(Protocol):
 name: str
class Base:
 _type: Optional[Type[Named]] = None
 @classmethod
 @property
 def name(cls) -> str:
 assert cls._type is not None
 return cls._type.name
class UnityType:
 name = "Unity"
class a(Base):
 _type = UnityType
a_instance = a()
print(a_instance.name) # prints "Unity"

I'm assuming from your example that UnityType is a type, not an instance, and that you want Base to work with either that type or other types with a name class attribute, so I defined a Protocol that requires that and used that for the type annotation of Base (the above passes mypy without complaint).

• fsimoes: Great answer. You didn't have all the information to give me a specific answer, but you helped me a lot and I solved the problem with your help. Thank you.