Optional field missing value error in custom Pydantic model

David Y.

—January 15, 2024

The ProblemThe Problem

I’ve built a custom Pydantic schema for my FastAPI project with a couple of optional fields of custom types:

class PetOwner:
    cat: Optional['Cat']

However, when Pydantic validates instances of this class that do not have a cat specified, it returns the following error message for every field in Cat:

field required (type=value_error.missing)

How do I make cat an optional field?

The SolutionThe Solution

The line cat: Optional['Cat'] declares a class attribute named cat with the type hint Optional['Cat']. Optional is a class from the Python typing module, used to indicate that a value can be of the specified type or it can be None. It can also be written as Cat | None.

While we’ve assigned a type hint to cat, we have not assigned it a default value. In older versions of Pydantic, attributes with a type hint including None were implicitly assigned a default value of None, but this was changed for version 2. Therefore, it is necessary to assign cat a default value of None to produce the expected behavior.

class PetOwner:
    cat: Optional['Cat'] = None