python runtime_error ai_generated true

PydanticUserError: @field_validator on 'email' requires a classmethod, but the decorator was applied to a regular method. Add @classmethod below @field_validator.

ID: python/pydantic-validator-not-classmethod

Also available as: JSON · Markdown · 中文
80%Fix Rate
90%Confidence
0Evidence
2024-01-22First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
2.x active — — —

Root Cause

In Pydantic v2, field_validator must decorate a classmethod; omitting @classmethod causes a PydanticUserError at class definition time.

generic

中文

在 Pydantic v2 中，field_validator 必须装饰 classmethod；省略 @classmethod 会在类定义时引发 PydanticUserError。

Workarounds

  1. 99% success 
    from pydantic import field_validator
class User(BaseModel):
    email: str
    @field_validator('email')
    @classmethod
    def check_email(cls, v):
        return v.lower()
  2. 94% success 
    from typing import Annotated
from pydantic import AfterValidator
Email = Annotated[str, AfterValidator(lambda v: v.lower())]

Dead Ends

Common approaches that don't work:

  1. 95% fail

    field_validator explicitly checks for classmethod; staticmethod is rejected.

  2. 90% fail

    Validators must be defined inside the model class to be registered.