When inheriting from a Pydantic model and attempting to override a parameter with a list of a subclass, mypy throws a type error. The base class defines the parameter as a list of the parent class, but when trying to override it with a list of the subclass, mypy sees it as incompatible.

Example that fails:

from pydantic import BaseModel

class Name(BaseModel):
data: str

class EnglishName(Name):
data: str

class Animal(BaseModel):
name: list[Name]

class AmericaAnimal(Animal):
name: list[EnglishName]  # Incompatible types in assignment (expression has type "list[EnglishName]", base class "Animal" defined the type as "list[Name]")

In contrast, the following example passes:

from pydantic import BaseModel

class Name(BaseModel):
data: str

class EnglishName(Name):
data: str

class Animal(BaseModel):
name: Name

class AmericaAnimal(Animal):
name: EnglishName

Given that EnglishName inherits from Name, the type should be compatible when using a list, but this is not the case with mypy when overriding with a list of the subclass.

I tried defining a base Pydantic model Name with a data field of type str. Then, I created a subclass EnglishName inheriting from Name. Next, I defined another base Pydantic model Animal with a field name as a list of Name objects. However, when I tried to override this field in a subclass AmericaAnimal to be a list of EnglishName objects, I expected the type to be compatible since EnglishName is a subclass of Name. Instead, I encountered a type incompatibility error from mypy.

Expected Behavior:
I expected AmericaAnimal to accept a list of EnglishName objects as its name field due to the inheritance relationship between Name and EnglishName.

Actual Behavior:
Mypy raised an “Incompatible types in assignment” error, indicating a type mismatch between the name field of Animal and the overridden name field of AmericaAnimal.