Pydantic does not validate when assigning a number to a string

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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?

pydantic validation python
1 Answers

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)
user_137826690
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