pydantic validator example

test.py

import json

from pydantic import BaseModel, validator


class FirstLevel(BaseModel):
    bar: "SecondLevel"
    foo: int

    @validator("foo")
    def check_conditional_foo_values(cls, foo, values):
        if values["bar"].baz == 2 and foo == 1:
            raise ValueError(f"Can't have {foo=} if bar.baz={values['bar'].baz}.")
        return foo  # return new value for foo, in case you want to update it


class SecondLevel(BaseModel):
    baz: int


FirstLevel.update_forward_refs()  # necessary, because we are nesting models

a = json.loads("""{"foo": 2, "bar": {"baz": 2}}""")
b = json.loads("""{"foo": 1, "bar": {"baz": 2}}""")

FirstLevel(**a)  # works
FirstLevel(**b)  # crashes with a Pydantic error wrapping the ValueError