## 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):

# 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")

@app.post(
    "/model-required-validation-alias", operation_id="model_required_validation_alias"
)
def read_model_required_validation_alias(
    p: Annotated[FormModelRequiredValidationAlias, Form()],
):
    return {"p": p.p}


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

@app.get("/model-required-list-validation-alias")
async def read_model_required_list_validation_alias(
    p: Annotated[HeaderModelRequiredListValidationAlias, Header()],
):
    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):

# Validation alias


@app.post("/list-bytes-validation-alias", operation_id="list_bytes_validation_alias")
def read_list_bytes_validation_alias(
    p: Annotated[list[bytes], File(validation_alias="p_val_alias")],
):
    return {"file_size": [len(file) for file in p]}


@app.post(
    "/list-uploadfile-validation-alias",
    operation_id="list_uploadfile_validation_alias",

                                                 data-validation="custom"
                                                 data-validation-regexp="(\+|\-)?\d+(\.\d+)?((e|E)(\+|\-)?\d+)?"
                                                 data-validation-help="number (Float)"/>
                                    </div>
                                </div>

# Validation alias


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


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


@app.get("/model-required-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

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


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