## Custom Validation { #custom-validation }

There could be cases where you need to do some **custom validation** that can't be done with the parameters shown above.

In those cases, you can use a **custom validator function** that is applied after the normal validation (e.g. after validating that the value is a `str`).

@app.get("/model-required-list-validation-alias")
async def read_model_required_list_validation_alias(
    p: Annotated[QueryModelRequiredListValidationAlias, Query()],
):
    return {"p": p.p}


@pytest.mark.parametrize(
    "path",
    ["/required-list-validation-alias", "/model-required-list-validation-alias"],
)
def test_required_list_validation_alias_schema(path: str):

@app.post(
    "/model-optional-list-validation-alias",
    operation_id="model_optional_list_validation_alias",
)
def read_model_optional_list_validation_alias(
    p: Annotated[FormModelOptionalListValidationAlias, Form()],
):
    return {"p": p.p}


@pytest.mark.parametrize(
    "path",
    ["/optional-list-validation-alias", "/model-optional-list-validation-alias"],
)

    dependentRequired: Optional[dict[str, set[str]]] = None
    # Ref: JSON Schema Validation 2020-12: https://json-schema.org/draft/2020-12/json-schema-validation.html#name-vocabularies-for-semantic-c
    # Vocabularies for Semantic Content With "format"
    format: Optional[str] = None
    # Ref: JSON Schema Validation 2020-12: https://json-schema.org/draft/2020-12/json-schema-validation.html#name-a-vocabulary-for-the-conten

    field_mapping: dict[
        tuple[ModelField, Literal["validation", "serialization"]], JsonSchemaValue
    ],
    separate_input_output_schemas: bool = True,
) -> dict[str, Any]:
    override_mode: Union[Literal["validation"], None] = (
        None
        if (separate_input_output_schemas or _has_computed_fields(field))
        else "validation"
    )
    field_alias = (

                            "content": {"application/json": {"schema": {}}},
                        },
                        "422": {
                            "description": "Validation Error",
                            "content": {
                                "application/json": {
                                    "schema": {

@app.post(
    "/model-optional-validation-alias", operation_id="model_optional_validation_alias"
)
def read_model_optional_validation_alias(
    p: BodyModelOptionalValidationAlias,
):
    return {"p": p.p}


@pytest.mark.parametrize(
    "path",
    ["/optional-validation-alias", "/model-optional-validation-alias"],
)
def test_optional_validation_alias_schema(path: str):

# Validation alias


@app.get("/required-validation-alias")
def read_required_validation_alias(
    p: Annotated[str, Cookie(validation_alias="p_val_alias")],
):
    return {"p": p}


class CookieModelRequiredValidationAlias(BaseModel):
    p: str = Field(validation_alias="p_val_alias")


@app.get("/model-required-validation-alias")

# Validation alias


@app.post(
    "/required-bytes-validation-alias", operation_id="required_bytes_validation_alias"
)
def read_required_bytes_validation_alias(
    p: Annotated[bytes, File(validation_alias="p_val_alias")],
):
    return {"file_size": len(p)}


@app.post(
    "/required-uploadfile-validation-alias",

# Validation alias


@app.get("/required-validation-alias")
def read_required_validation_alias(
    p: Annotated[str, Query(validation_alias="p_val_alias")],
):
    return {"p": p}


class QueryModelRequiredValidationAlias(BaseModel):
    p: str = Field(validation_alias="p_val_alias")


@app.get("/model-required-validation-alias")