[Fixed] Pydantic V2 – field_validator() got an unexpected keyword argument 'pre' – Pydantic

@validator("x", pre=True)

@field_validator("x", mode="before")

The Problem:

During migration from Pydantic v1 to v2, the user is facing an issue while replacing @validator with @field_validator. Upon using the previously employed pre validator argument in @field_validator, an error is encountered: TypeError: field_validator() got an unexpected keyword argument 'pre'. Despite the documentation mentioning pre under the @field_validator section, it is unclear whether pre has been deprecated in v2.

The Solutions:

Solution 1: Use the `mode=”before”` argument

To replace the deprecated pre=True argument from Pydantic v1 in v2, use the mode="before" argument in @field_validator.

Example:

# Pydantic v1
class Foo(BaseModel):
    x: int

    @validator("x", pre=True)
    def do_stuff(cls, v: object) -> object:
        if v is None:
            return 0
        return v

# Pydantic v2
class Foo(BaseModel):
    x: int

    @field_validator("x", mode="before")
    def do_stuff(cls, v: object) -> object:
        if v is None:
            return 0
        return v