Creating handlers

Handlers to handle the things that need handling.


A request is a command, event, or query represented as a self contained object. An object that you dispatch to a handler to decouple invocation from implementation.

Each has different processing expectations, but all are examples of the command pattern. To avoid confusion over the word “command”, banshee refers to these objects as requests.

Request handlers

Handlers are function or classes that can process a request. They must be callable, and they must take a request as the single positional argument. The Handler protocol defines this contract.

If we want to handle a GreetCommand request.

class GreetCommand:
    name: str

The handler can be a function, taking the command as its first argument.

async def do_greeting(command: GreetCommand) -> None:
    print(f"Hello {}!")

Command, Query, or Event

  • An event is a notification that something happened in the past. An occurance that has already happened. Events form a historic record of how and when state changed in an application.

    Events can have many handlers, or none at all.

  • A command is an imperative instruction to do something, a desire to take some form of action. Commands express a wish for a change in the applications state to take place.

    Command should have exactly one handler.

  • A query is a real-time instruction to look something up, a desire to have some data fetched from a store. Queries express a wish to both retrieve and return some part of the applications state.

    Queries should have exactly one handler.


class banshee.Handler(*args, **kwargs)

Bases: Protocol[T_contra]

Handler protocol.

Callable to process a request.

abstract async __call__(request, /)


Perform any business logic applicable to the request.


request (T_contra) – request object


result processing request

Return type: