Class BeforeValidator

pydantic/functional_validators.py:90–153

!!! abstract "Usage Documentation" [field *before* validators](../concepts/validators.md#field-before-validator) A metadata class that indicates that a validation should be applied **before** the inner validation logic. Attributes: func: The validator function. json

@dataclasses.dataclass(frozen=True, **_internal_dataclass.slots_true)
class BeforeValidator:
    """!!! abstract "Usage Documentation"
        [field *before* validators](../concepts/validators.md#field-before-validator)

    A metadata class that indicates that a validation should be applied **before** the inner validation logic.

    Attributes:
        func: The validator function.
        json_schema_input_type: The input type used to generate the appropriate
            JSON Schema (in validation mode). The actual input type is `Any`.

    Example:
        ```python
        from typing import Annotated

        from pydantic import BaseModel, BeforeValidator

        MyInt = Annotated[int, BeforeValidator(lambda v: v + 1)]

        class Model(BaseModel):
            a: MyInt

        print(Model(a=1).a)
        #> 2

        try:
            Model(a='a')
        except TypeError as e:
            print(e)
            #> can only concatenate str (not "int") to str
        ```
    """

    func: core_schema.NoInfoValidatorFunction | core_schema.WithInfoValidatorFunction
    json_schema_input_type: Any = PydanticUndefined

    def __get_pydantic_core_schema__(self, source_type: Any, handler: GetCoreSchemaHandler) -> core_schema.CoreSchema:
        schema = handler(source_type)
        input_schema = (
            None
            if self.json_schema_input_type is PydanticUndefined
            else handler.generate_schema(self.json_schema_input_type)
        )

        info_arg = _inspect_validator(self.func, mode='before', type='field')
        if info_arg:
            func = cast(core_schema.WithInfoValidatorFunction, self.func)
            return core_schema.with_info_before_validator_function(
                func,
                schema=schema,
                json_schema_input_schema=input_schema,
            )
        else:
            func = cast(core_schema.NoInfoValidatorFunction, self.func)
            return core_schema.no_info_before_validator_function(
                func, schema=schema, json_schema_input_schema=input_schema
            )