How to make Pydantic recognize attributes from non-model parent class?

Question

I have a normal Python class:

class NormalClass:
  a: str
  b: bool

I don't want NormalClass to inherit from pydantic.BaseModel class, but I still want another class with the same attributes as NormalClass and it being a Pydantic model. So here's what I try:

class ModelClass(BaseModel, NormalClass):
  pass

Unfortunately, when I try to use this ModelClass to validate server response in FastAPI, I always get {}. What happens here?

Answer 1

I believe that you cannot expect to inherit the features of a pydantic model (including fields) from a class that is not a pydantic model.

a and b in NormalClass are class attributes. Although the fields of a pydantic model are usually defined as class attributes, that does not mean that any class attribute is automatically a field. NormalClass is not a pydantic model because it does not inherit from BaseModel. ModelClass has no fields because it does not define any fields by itself and has not inherited any fields from a pydantic model.