pythonvalidationconstraintspydanticpydantic-v2

How do conditionally apply field constraints based on value type in pydantic v2?

I have this pydantic model with a field, and this field could be either an int or a non numeric value like a str or list.

from pydantic import BaseModel, Field

class Foo(BaseModel):
    bar: str | list | int = Field('some string', ge=2)

I want it to be that the constraint ge=2 is applied only if the value given to bar happens to be an int.

foo_instance = Foo(bar='asdf') # constraint should not be applied
foo_instance = Foo(bar=4) # constraint should be applied

Trying the above throws an error: TypeError: Unable to apply constraint 'ge' to supplied value asdf

How do I ensure pydantic only tries to apply this constraint if the value is numerical (or if the operation can be performed on this value)?

Solution

  • You can apply the validator to just the int value like so:

    from pydantic import BaseModel, Field
from typing import Annotated


class Foo(BaseModel):
    bar: str | list | Annotated[int, Field(ge=2)]


foo_instance = Foo(bar="asdf")
foo_instance = Foo(bar=4)