async-py-bus

The library is designed for asynchronous event-driven and cqrs Python projects, has no third-party dependencies, and is useful for handling domain events, queries and commands.

See more examples on GitHub

Basic usage

The core of the library is the Dispatcher class. It handles three types of messages: events, commands and queries.

Dispatcher initializing

You can use the default object from pybus

from pybus import dispatcher as dp

Note that the event, command and query engines are not enabled by default. Each of them would be instantiated with default routers (if not passed specific class) with the first handler registered to this router.

You can also override default engines, routers and dispatcher and pass your classes to the Dispatcher constructor:

from pybus import Dispatcher, RequestRouter

class CustomRequestRouter(RequestRouter):
  def bind(
        self,
        message: MessageType,
        handler: HandlerType,
        argname: t.Optional[str] = EMPTY,
        **initkwargs,
    ) -> PyBusWrappedHandler:
        # your implementation here

dp = Dispatcher(
    queries_router_cls=CustomRequestRouter,
)

Handlers` signature

A basic handler is an asynchronous function that takes either one or zero arguments.

async def handler_with_arg(event: EventType):
    # do something with event...

async def handler_without_arg():
    # do something

Additionally, a handler can accept any number of keyword arguments (how to pass them to the handler is explained below). However, if the parameter expecting the message is strictly positional, you must specify the argname parameter when registering the handler.

A handler can also be a class implementing the HandlerProtocol protocol or an asynchronous __call__ method. The signature rules for the handle or __call__ methods are the same as for a regular function.

class HandlerProtocol(Protocol):

    async def handle(self, message: MessageType, **kwargs) -> Any:
        """Handle message."""

    async def add_event(self, event: EventType) -> None:
        """Add event to emit later."""

    async def dump_events(self) -> list[EventType]:
        """Return list of collected events."""

If your class needs some initialization parameters, you can specify them during handler registration as named argument pairs. These arguments will be passed directly to the class’s __init__ method (make sure there are no strictly positional arguments).

class CustomHandler:
    def __init__(self, repo: RepoType):
        self._repo = repo

    async def __call__(self, cmd: CreateUserCommand):
        user = await self._repo.create(cmd.data)
        return UserCreated(data=user)

The same behavior applies if the handler is a simple function but takes more than one parameter without default values.

async def create_user_handler(cmd: CreateUserCommand, repo: RepoType):
    user = await repo.create(cmd.data)
    return UserCreated(data=user)


# main.py
user_repo = UserRepoImpl()
dp.commands.bind(CreateUserCommand, handler=create_user_handler, repo=user_repo)

You may notice that both the create_user_handler function and the __call__ method of the CustomHandler class return an event. This allows you to pass new outgoing events to the dispatcher, which will forward them to the appropriate handler. In the case of a class implementing HandlerProtocol, additionally events can be added during processing inside the handle method.

Handlers binding

We’ve talked a lot about how to declare handlers; now let’s register them.

Currently, there are several ways to do this:

• Pass the handler to the dispatcher’s register_<message>_handler method, where <message> is one of event, command, or query.

  dp.register_event_handler(UserCreated, create_user_handler)
  

• Use the bind method of one of the dispatcher’s routers (dp.events, dp.commands, dp.queries).

  dp.events.bind(UserCreated, create_user_handler)
  

• Use the register decorator from one of the dispatcher’s routers (dp.events, dp.commands, dp.queries).

  @dp.events.register(UserCreated)
  async def create_user_handler(event): ...
  

Dispatcher start

After registering the handlers, you just need to start the dispatcher:

dp.start()

During this operation, the handler map will be finalized, and you won’t be able to register new handlers. Please keep this in mind.

If no handlers registered dispatcher will drop setup.

Utils

For simple dependency injection, the library provides two classes: Singleton and Factory. They use the slice-syntax: the name of the resource and the callable that retrieves it (can be either synchronous or asynchronous).

In general, there are more convenient and optimized libraries for this task.

def get_user_repo() -> UserRepo: ...


@dp.queries.register(UserQuery, repo=Singleton["repo": get_user_repo])
async def book_query_handler(query: UserQuery, repo: UserRepo) -> BookQueryResult: ...

The docs are being updated...