[FIXED] how to define a pydantic model with a field that is a list of a mix of several submodels

Issue

I am defining a large model with pydantic and I am stuck trying to define a list where construct multiple submodels, lets see the example:

from pydantic import BaseModel
from typing import Literal, Union

class Model_A(BaseModel):
   type: Literal["A"] = "A"
   name: str
   size: float


class Model_B(BaseModel):
   type: Literal["B"] = "B"
   name: str
   size: float
   color: str
   value: float

class GeneralModel(BaseModel):
   name: str
   objects_list: list[Union[Model_A, Model_B]]


if __name__ == "__main__":
  
  user_list = {
    "name": "John Doe",
    "objects_list": [
        {"name": "box 1", "size": 3},
        {"name": "box 2", "size": 4, "color": "red", "value": 5},
        {"name": "box 3", "size": 6},
    ],
}
  # I would like to obatain the constuction of each model, but my code only construct objects of the first type defined in the Union statement.
  order = GeneralModel(**user_list)
  assert isinstance(order.objects_list[0], Model_A)
  assert isinstance(order.objects_list[1], Model_B)   # Fails, construct Model_A instance

There is any way to solve this and let pydantic to chose the correct model type to use?

Thank you all in advance!

Solution

By default, pydantic allows extra fields when building an object. This means that all your objects can be interpreted as Model_A instances, some having extra fields such as color and value. You can prevent this behaviour by adding a configuration to your model:

class Model_A(BaseModel):
    model_config = ConfigDict(extra="forbid")

This means that your first and third objects cannot be interpreted as Model_A anymore, and will be treated as Model_B by default.

You can also use discriminated unions as ndclt suggested, but it is a bit trickier.

Edit: I'm assuming you're using pydantic v2.

Answered By - Ebig