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 = "me@example.com")   # fails as expected
ms = MyStuff(name = "me")
ms.name = "me@example.com"
print(ms.name)   # prints me@example.com

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?

Answer 1

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