[Python] Set initial value of a Pydantic 2 field using validator

Say I have the following Pydantic 2.10.6 model, where x is dynamically calculated from another field, y:

from pydantic import BaseModel, Field, field_validator, ValidationInfo

class Foo(BaseModel):
x: int = Field(init=False)
y: int

@field_validator("x", mode="before")
@staticmethod
def set_x(val: None, info: ValidationInfo):
return info.data["y"] + 1

Foo(y=1)


Running this as is fails validation:

pydantic_core._pydantic_core.ValidationError: 1 validation error for Foo
x
Field required [type=missing, input_value={'y': 1}, input_type=dict]


I can "fix" the runtime error by giving x a default:

x: int = Field(init=False, default=None)


But then this fails type checking in pyright with:

Type "None" is not assignable to declared type "int"


I can also fix it using a model_validator, but this is a tad less neat:

from pydantic import BaseModel, Field, model_validator

class Foo(BaseModel):
x: int = Field(init=False)
y: int

@model_validator(mode="before")
@staticmethod
def set_x(data: dict):
data["x"] = data["y"] + 1
return data

Foo(y=1)


How can I cleanly represent this model using only a field validator?

Continue reading...