PyMsgbus

pymsgbus is a lightweight, extensible Python message bus framework designed to simplify event-driven and message-driven architectures using services, producers, consumers, publishers, and subscribers with dependency injection support. It offers synchronous and asynchronous support for handling messages/events with automatic dependency resolution.

Features

• Event-driven communication via Producers and Consumers
• Topic-based messaging with Publishers and Subscribers
• Dependency injection via Depends decorator and provider overrides
• Support for sync and async message handling
• Flexible message routing with automatic handler registration
• Uses ubiquitous language concepts for clear event modeling
• Supports generic and union event types
• Designed for domain-driven design (DDD) and microservices architecture

Installation

pip install pymsgbus

Quickstart

User Registration with Notifications and Persistence example

Let's create a simple event-driven registration system for users. First, we define a basic User entity and some related events.

from dataclasses import dataclass
from pymsgbus import event 
 
@dataclass
class User:
    id: int
    name: str

@event
class Created[T]:  # Generics are supported.
    entity: T

@event
class UpdatedUser:
    entity: User

@event
class RegistrationDone:
    text: str

Declaring Dependencies and Writing Handlers

Next, declare dependencies and write handlers for these events. You can implement them inline or use dependency injection to provide the actual implementations later.

A Consumer handles events, while a Subscriber handles messages. Consumers are smart—they determine which events to handle based on type hints. You can group multiple event types for a single consumer using unions, allowing one handler to process several event types. Multiple consumers can handle the same event type as well.

Messages passed to a Subscriber are routed via topics. Subscribers can listen to multiple topics, and multiple subscribers can subscribe to the same topic.

from pymsgbus import Depends
from pymsgbus import Consumer, Subscriber

consumer = Consumer()
subscriber = Subscriber() 
 
def getdb() -> dict:  # Database dependency, to be overridden later
    raise NotImplementedError

def getnfs() -> list:  # Notifications dependency, to be overridden later
    raise NotImplementedError

@consumer.handler
def handle_put(event: Created[User] | UpdatedUser, db: dict = Depends(getdb)):
    db[event.entity.id] = event.entity.name

@consumer.handler
def handle_registered(event: Created[User]):
    consumer.consume(RegistrationDone(f"User {event.entity.id} registered."))

@subscriber.subscribe('notifications')
def handle_notification(msg: RegistrationDone, nfs: list = Depends(getnfs)):
    nfs.append(msg.text)

@consumer.handler
def forward_to_subscriber(event: RegistrationDone):
    subscriber.receive(event, 'notifications')

Producing events.

Now, write code that produces the events you defined. These can be simple functions or part of a Service. Services also support dependency injection and can be invoked directly or by name.

from pymsgbus import Service
from pymsgbus import Depends

def register(name: str):
    user = User(id=1, name=name)
    consumer.consume(Created(user))

service = Service()

def somedep():
    return "I'm a dependency"

@service.handler
def update(id: int, name: str, dep: str = Depends(somedep)):
    user = User(id=id, name=name)
    consumer.consume(UpdatedUser(user))
    print(dep) #"I'm a dependency"

Overriding Dependencies and Running the Logic

Finally, override the dependencies with actual infrastructure—in this case, simple Python objects. Then execute your logic; all events and messages will be dispatched and handled by the registered components.

db = {}
nfs = []

def getrealdb():
    return db

def getrealnfs():
    yield nfs #Generators with cleanup supported. 

consumer.override(getdb, getrealdb)
subscriber.override(getnfs, getrealnfs)

register('Alice')
update(1, 'Alice Updated') #or service.execute('update', 1, 'Alice Updated')

assert db[1] == 'Alice Updated'
print(db)
assert nfs == ["User 1 registered."]
print(nfs)

Late Binding with Producers and Publishers

Consumers and subscribers can also be late-bound using a Producer (for consumers) and a Publisher (for subscribers). Define these alongside your services.

from pymsgbus import Service
from pymsgbus import Depends
from pymsgbus import Producer, Publisher 

service = Service()
producer = Producer()
publisher = Publisher()


def register(name: str):
    user = User(id=1, name=name)
    producer.dispatch(Created(user))

def somedep():
    return "I'm a dependency"

@service.handler
def update(id: int, name: str, dep: str = Depends(somedep)):
    user = User(id=id, name=name)
    producer.dispatch(UpdatedUser(user))
    publisher.publish("Update performed", 'some-topic')

Then, bind them at runtime like this:

producer.register(consumer)
publisher.register(subscriber)

...

register('Alice')
update(1, 'Alice Updated')  # Or: service.execute('update', 1, 'Alice Updated')

Async support

All core components (e.g., Consumer, Service, Producer, etc.) have asynchronous counterparts available in the pymsgbus.asyncio subpackage. Use async and await to integrate them into an asynchronous workflow.

from pymsgbus.asyncio import Service
from pymsgbus.asyncio import Depends
from pymsgbus.asyncio import Producer, Publisher

service = Service()
producer = Producer()
publisher = Publisher()

async def register(name: str):
    user = User(id=1, name=name)
    await producer.dispatch(Created(user))

async def somedep():
    return "I'm a dependency" 

@service.handler
async def update(id: int, name: str, dep: str = Depends(somedep)):
    user = User(id=id, name=name)
    await producer.dispatch(UpdatedUser(user))
    await publisher.publish("Update performed", 'some-topic')

...

async def main():
    await register('Alice')
    await update(1, 'Alice Updated')  # Or: await service.execute('update', 1, 'Alice Updated')

API Summary

Producer

• register(*consumers): Register consumers to send events to.
• dispatch(event): Dispatch an event to registered consumers.

Consumer

• handler(func): Decorator to register an event handler.
• consume(event): Method to consume an event.
• override(dep, impl): Override a dependency.

Publisher

• register(*subscribers): Register subscribers.
• publish(message, topic): Method to publish a message to subscribers.

Subscriber

• subscribe(*topics): Decorator to register handlers for topics.
• receive(message, topic): Method to receive messages for a topic.
• override(dep, impl): Override dependencies.

License

This project is licensed under the Apache License, Version 2.0 — see the LICENSE file for details.

© 2025 Eric M. Cardozo (Eric Hermosis). All rights reserved.

Contributing

Contributions are welcome! Please open issues or pull requests for bugs, features, or enhancements.