Why doesn't Pydantic validate field assignments?

Question

I want to use Pydantic to validate fields in my object, but it seems like validation only happens when I create an instance, but not when I later modify fields.

from pydantic import BaseModel, validator

class MyStuff(BaseModel):
    name: str

    @validator("name")
    def ascii(cls, v):
        assert v.isalpha() and v.isascii(), "must be ASCII letters only"
        return v

# ms = MyStuff(name = "[email protected]")   # fails as expected
ms = MyStuff(name = "me")
ms.name = "[email protected]"
print(ms.name)   # prints [email protected]

In the above example, Pydantic complains when I try to pass an invalid value when creating MyStuff, as expected.

But when I modify the field afterwards, Pydantic does not complain. Is this expected, or how can I have Pydantic also run the validator when assigning a field?

jwodder · Accepted Answer · 2021-04-09 17:54:37Z

16

This is the default behavior. To enable validation on field assignment, set validate_assignment in the model config to true:

class MyStuff(BaseModel):
    name: str

    @validator("name")
    def ascii(cls, v):
        assert v.isalpha() and v.isascii(), "must be ASCII letters only"
        return v

    class Config:
        validate_assignment = True

answered Apr 9, 2021 at 17:54

jwodder

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2 Comments

Antonio Romero Oca Over a year ago

Is there any disadventage setting validate_assignment = True? I wonder why it is default False.

winwin Over a year ago

@AntonioRomeroOca i think it's due to the overhead of running the validation every time there is a change.

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