Pydantic model does not validate on assignment with reproducable code

Question

When assigning an incorrect attribute to a Pydantic model field, no validation error occurs.

from pydantic import BaseModel

class pyUser(BaseModel):
    username: str

    class Config:
        validate_all = True
        validate_assignment = True

person = pyUser(username=1234)
person.username
>>>1234
try_again = pyUser()

pydantic.error_wrappers.ValidationError:
[ErrorWrapper(exc=MissingError(), loc=('username',))]
<class '__main__.pyUser'>

How can I get pydantic to validate on assignment?

Answer 1

It is expected behaviour according to the documentation :

str

strings are accepted as-is, int , float and Decimal are coerced using str(v)

You can use the StrictStr , StrictInt , StrictFloat , and StrictBool types to prevent coercion from compatible types.

from pydantic import BaseModel, StrictStr


class pyUser(BaseModel):
    username: StrictStr

    class Config:
        validate_all = True
        validate_assignment = True


person = pyUser(username=1234)  # ValidationError `str type expected`
print(person.username)