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BlackSheep implements automatic binding of parameters for request handlers, a feature inspired by "Model Binding" in the ASP.NET web framework. This feature improves code quality and the developer experience since it provides a strategy to read values from request objects in a consistent way and removes the need to write parts that read values from the request object inside request handlers. It also enables a more accurate generation of OpenAPI Documentation, since the framework is aware of what kind of parameters are used by the request handlers (e.g. headers, cookies, query).

This page describes:

  • Implicit and explicit bindings.
  • Built-in binders.
  • How to define a custom binder.

It is recommended to read the following pages before this one:


Automatic binding of request query strings and route parameters has been described in several places in the previous pages, and explicit and implicit binding is introduced in the section about requests.

Binding is implicit when the source of a parameter is inferred by conventions, or explicit when the programmer specifies exact binders from blacksheep.server.bindings.

Implicit binding

An example of implicit binding is when a request handler parameter is read from the request URL's route parameters because its name matches the name of a route parameter:

async def get_cat(cat_id: str):

Another example of implicit binding is when a request handler parameter is annotated with a type that is configured in

class Foo:

async def do_something(foo: Foo):

In this case, Foo is obtained from application services since the type is registered in

Binding happens implicitly when parameters in the request handler's signature are not annotated with types, or are not annotated with types that inherit from BoundValue class, defined in blacksheep.server.bindings.


A parameter with the name "request" is always bound to the instance of the Request of the web request.

Explicit binding

Binders can be defined explicitly, using type annotations and classes from blacksheep.server.bindings (or just blacksheep).

from dataclasses import dataclass

from blacksheep import FromJSON, FromServices, post

from your_business_logic.handlers.cats import CreateCatHandler  # example

class CreateCatInput:
    name: str

async def create_cat(
    create_cat_handler: FromServices[CreateCatHandler],
    input: FromJSON[CreateCatInput],

In the example above, create_cat_handler is obtained from, an exception is thrown if the the service cannot be resolved. This happens if the service is not registered in application services, or any of the services on which it depends is not registered (see Service resolution for more information on services that depend on other services).

input is obtained by reading the request payload, parsing it as JSON, and creating an instance of CreateCatInput from it. If an exception occurs while trying to parse the request payload or when instantiating the CreateCatInput, the framework produces automatically a 400 Bad Request response for the client.

When mapping the request's payload to an instance of the desired type, the type is instantiated using cls(**data). If it necessary to parse dates or other complex types that are not handled by JSON deserialization, this must be done in the constructor of the class. To handle gracefully a JSON payload having extra unused properties, use *args in your class constructor: __init__(one, two, three, *args).

Optional parameters

Optional parameters can be defined in one of these ways:

  1. using typing.Optional annotation
  2. specifying a default value
async def example(
    page: int = 1,
    search: str = "",
    # page is read from the query string, if specified, otherwise defaults to 1
    # search is read from the query string, if specified, otherwise defaults to ""
from typing import Optional

async def example(
    page: Optional[int],
    search: Optional[str],
    # page is read from the query string, if specified, otherwise defaults to None
    # search is read from the query string, if specified, otherwise defaults to None
from blacksheep import FromQuery, get

async def example(
    page: FromQuery[int] = FromQuery(1),
    search: FromQuery[str] = FromQuery(""),
    # page.value defaults to 1
    # search.value defaults to ""
from typing import Optional

from blacksheep import FromQuery, get

async def example(
    page: FromQuery[Optional[int]],
    search: FromQuery[Optional[str]],
    # page.value defaults to None
    # search.value defaults to None
from typing import Optional

from blacksheep import FromQuery, get

async def example(
    page: Optional[FromQuery[int]],
    search: Optional[FromQuery[str]],
    # page defaults to None
    # search defaults to None

Built-in binders

Binder Description
FromHeader A parameter obtained from a header.
FromQuery A parameter obtained from URL query.
FromCookie A parameter obtained from a cookie.
FromServices Service from
FromJSON Request body read as JSON and optionally parsed.
FromForm A parameter obtained from Form request body: either application/x-www-form-urlencoded or multipart/form-data.
FromText Request payload read as text, using UTF-8 encoding.
FromBytes Request payload read as raw bytes.
FromFiles Request payload of file type.
ClientInfo Client IP and port information obtained from the request ASGI scope, as Tuple[str, int].
ServerInfo Server IP and port information obtained from the request scope.
RequestUser Request's identity.
RequestURL Request's URL.
RequestMethod Request's HTTP method.

FromHeader and FromCookie binders must be subclassed because they require a name class property:

from blacksheep import FromCookie, FromHeader, get

class FromAcceptHeader(FromHeader[str]):
    name = "Accept"

class FromFooCookie(FromCookie[Optional[str]]):
    name = "foo"

def home(accept: FromAcceptHeader, foo: FromFooCookie) -> Response:
    return text(
        Accept: {accept.value}
        Foo: {foo.value}

Defining a custom binder

To define a custom binder, define a BoundValue[T] class and a Binder class having handle class property referencing the custom BoundValue class. The following example demonstrates how to define a custom binder:

from typing import Optional

from blacksheep import Application, Request
from blacksheep.server.bindings import Binder, BoundValue

app = Application(show_error_details=True)
get = app.router.get

class FromCustomValue(BoundValue[str]):

class CustomBinder(Binder):

    handle = FromCustomValue

    async def get_value(self, request: Request) -> Optional[str]:
        # TODO: implement here the desired logic to read a value from
        # the request object
        return "example"

def home(something: FromCustomValue):
    assert something.value == "example"
    return f"OK {something.value}"

Last modified on: 2023-12-18 17:52:09