Separating extra keys in an `extra` key (either implied by the validator or explicitly in the model)

extra_split_demo.py

from pydantic import BaseModel, model_validator

class Model(BaseModel, extra="allow"):
    a: int
    b: int

    @model_validator(mode="before")
    @classmethod
    def pop_off_extra(cls, values: dict):
        fields = list(cls.model_fields)
        known, extra = {}, {}
        for k,v in values.items():
            target = known if k in fields else extra
            target.update({k:v})
        assert "extra" not in known, "The key 'extra' is a reserved field"
        return dict(**known, extra=extra)

data = dict(a=1, b=2)
more = dict(a=1, b=2, c={"value": 3})

normal = Model.model_validate(data)
plus_c = Model.model_validate(more)

print(f"{normal=}")  # normal=Model(a=1, b=2, extra={})
print(f"{plus_c=}")  # plus_c=Model(a=1, b=2, extra={'c': {'value': 3}})

extra_split_demo_explicit_field.py

from pydantic import BaseModel, model_validator

class Model(BaseModel):
    a: int
    b: int
    extra: dict

    @model_validator(mode="before")
    @classmethod
    def pop_off_extra(cls, values: dict):
        fields = list(cls.model_fields)
        known, extra = {}, {}
        for k,v in values.items():
            target = known if k in fields else extra
            target.update({k:v})
        assert "extra" not in known, "The key 'extra' is a reserved field"
        return dict(**known, extra=extra)

data = dict(a=1, b=2)
more = dict(a=1, b=2, c={"value": 3})

normal = Model.model_validate(data)
plus_c = Model.model_validate(more)

print(f"{normal=}")  # normal=Model(a=1, b=2, extra={})
print(f"{plus_c=}")  # plus_c=Model(a=1, b=2, extra={'c': {'value': 3}})