Pydantic: Detect if a field value is missing or given as null

Question

I want to allow users to selectively update fields using PUT calls. On the pydantic model, I have made the fields Optional. In the FastAPI handler if the model attribute is None, then the field was not given and I do not update it.

The problem with this approach is that there is no way for the client to "blank out" a field that isn't required for certain types.

In particular, I have date fields that I want clients to be able to clear by sending in a null in the JSON. How can I detect the difference between the client sending null or the client not sending the field/value at all? The model attribute is just None in either case.

Answer 1

The pydantic documentation desccribes two options that can be used with the .dict() method of models.

• exclude_unset: whether fields which were not explicitly set when creating the model should be excluded from the returned dictionary; default False. Prior to v1.0, exclude_unset was known as skip_defaults; use of skip_defaults is now deprecated

• exclude_defaults: whether fields which are equal to their default values (whether set or otherwise) should be excluded from the returned dictionary; default False

So you can create a model class with optional fields:

from typing import Optional
from pydantic import BaseModel


class MyModel(BaseModel):
    foo: Optional[int] = None
    bar: Optional[int] = None

And still generate a dict with fields explicitely set to None, but without default values:

baz = MyModel(foo=None)
assert baz.dict(exclude_unset=True) == {"foo": None}

baz = MyModel(bar=None)
assert baz.dict(exclude_unset=True) == {"bar": None}

Answer 2

You can check obj.__fields_set__ to see whether the value was missing or not.

from typing import Optional
from pydantic import BaseModel

class Foo(BaseModel):
    first: Optional[int] = None
    second: Optional[int] = None

foo = Foo.parse_raw('{"first": null}')

assert foo.first is None and foo.second is None
assert foo.__fields_set__ == {"first"}