fastevents 0.7.0

Lightweight asyncio event framework with layered propagation

FastEvents

Bring the programming model of great Python web frameworks to general event systems.

FastEvents is not just an event bus.

It is closer to an application model for events:

• organize behavior with declarative handlers
• build handler inputs through structured injection
• keep runtime complexity behind a clear runtime boundary
• compose higher-level protocols like RPC out of events
• leave room for streams, proxies, lifecycle events, and future distributed buses

If you like the feel of modern Python web frameworks - clear handlers, natural dependencies, explicit boundaries - FastEvents tries to bring that experience to more general event-driven systems.

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Install

If you want to use the repository directly:

uv add fastevents
# or using pip
# pip install fastevents

The current implementation targets Python 3.12+ and ships with an in-memory InMemoryBus.

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Quick Start

Start with the smallest possible hello world:

from fastevents import FastEvents

app = FastEvents()

@app.on("hello")
async def hello() -> None:
    print("hello world")

Those four lines already show the basic shape of FastEvents:

• import FastEvents
• declare an app
• register a handler with @app.on(...)
• express event behavior with a normal async function

As you go further, FastEvents keeps building on top of that minimal model:

• bus owns the runtime boundary
• app declares behavior
• handlers stay small and explicit
• RPC is not a separate core model, but a protocol built on top of events

Here is a slightly richer example that shows:

• dependency injection
• event.ctx.publish()
• EventModel payload validation

import asyncio

from fastevents import EventModel, FastEvents, InMemoryBus, RuntimeEvent, dependency

app = FastEvents()
bus = InMemoryBus()


class WorldPayload(EventModel):
    text: str


@dependency
def say_hello(event: RuntimeEvent) -> bool:
    return event.payload is not None


@app.on("hello")
async def hello(event: RuntimeEvent, ok: bool = say_hello()) -> None:
    if ok:
        await event.ctx.publish(tags="world", payload={"text": "world"})


@app.on("world")
async def world(data: WorldPayload) -> None:
    print(data.text)


async def main() -> None:
    await bus.astart(app)
    try:
        await app.publish(tags="hello", payload={"message": "hello"})
    finally:
        await bus.astop()


asyncio.run(main())

What happens here:

• the hello handler receives the result of say_hello() through dependency injection
• say_hello() depends on the current event and returns True when the payload is not empty
• the hello handler publishes a follow-up world event when the condition is met
• the world handler validates the payload with an EventModel and prints world

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Why It Is More Than Another Event Bus

Many event libraries stay close to transport or callback registries.

FastEvents is trying to solve a different problem:

How do you write event applications the way you write web applications?

In other words, the focus is not just "how do I send a message", but:

• how to clearly declare who handles which event
• how to inject payload, context, and dependencies naturally into handlers
• how to keep follow-up events, request/reply, and fallback inside one model
• how to keep runtime complexity in the bus instead of leaking it into the app layer

The goal is not to cram every feature into the core. The goal is to keep the core boundaries clear enough that higher-level protocols can grow naturally.

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Design Boundaries

The appeal of FastEvents is not only that it works, but that it stays disciplined about semantics.

1. publish() is a transport boundary, not a completion guarantee

Publishing an event means submitting it to the bus.

publish() does not guarantee that:

• any handler has already run
• any handler will succeed
• all processing has completed
• a distributed network has reached global consistency

It only guarantees this:

the event has been accepted by the bus and entered the bus send boundary.

This is intentional. It means the semantics of publish() do not need to change later, whether the runtime is an in-memory bus, a remote broker, or a distributed bus.

If you need replies, completion, aggregation, or acknowledgements, they should be built on top of events rather than pushed down into publish().

2. app is not a full host

FastEvents is a declaration and composition container.

It is not a full application host, it does not own every resource, and it does not try to become a framework-managed plugin platform. Runtime complexity belongs to the bus, not to the app.

3. extensions are composition, not framework-managed plugins

Extensions should be ordinary objects that compose higher-level behavior from the app's public capabilities.

• they should not depend on framework internals
• they should not have privileged status
• their resource ownership should stay explicit

The recommended direction is explicit construction:

from fastevents import FastEvents, RpcExtension

app = FastEvents()
rpc = RpcExtension(app)

This explicit composition style is clearer and works better with type inference.

4. runtime complexity belongs in the bus

The bus is responsible for:

• accepting published events
• binding to or creating an event loop
• managing runtime lifecycle
• transporting events into the application core
• absorbing future remote transport or distributed delivery complexity

That complexity belongs to the runtime layer. It should not leak into the application model.

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A Simple Mental Model

You can think about FastEvents as three layers.

Runtime layer: bus

The bus is the lowest runtime boundary.

It decides how events are accepted, how runtime starts, who owns the loop, and how events enter the application core.

Application core: dispatcher + injector

The application core turns an event into a concrete application call:

• dispatcher selects matching subscribers
• injector builds handler inputs from payload, context, and dependencies
• then the actual call runs

Top layer: handlers + extensions

• handler expresses business behavior
• extension composes higher-level protocols, such as RPC, from public capabilities

Both layers are built on the same application core rather than inventing separate models.

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Propagation Model

FastEvents uses level-based propagation.

Intuitively:

• lower levels run first
• subscribers in the same level run concurrently
• level < 0 is for observation and never consumes the event
• level >= 0 is for handling and fallback

More concretely:

• the dispatcher finds all matching subscribers
• subscribers are grouped by level
• each group runs concurrently
• if a non-negative level consumes the event, higher levels stop
• if that level does not consume the event, propagation continues upward

Here, consumed is a propagation concept, not a broad business-success signal.

A common convention is:

• -1: audit, metrics, tracing, passive observers
• 0: primary business handler
• 1+: fallback

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Handler Injection

FastEvents supports structured parameter injection. You declare a handler signature, and the framework builds the current inputs for you.

Common forms:

• RuntimeEvent: get the current event
• pydantic.BaseModel: get validated structured payload in compatibility mode
• EventModel: recommended validated structured payload base

Example:

from pydantic import BaseModel

from fastevents import EventModel, RuntimeEvent


class OrderCreated(EventModel):
    order_id: int


@app.on("order.created")
async def event_only(event: RuntimeEvent) -> None:
    ...


@app.on("order.created")
async def typed_payload(event: RuntimeEvent, data: OrderCreated) -> None:
    ...


class LegacyOrderCreated(BaseModel):
    order_id: int


@app.on("order.created")
async def compatible_typed_payload(data: LegacyOrderCreated) -> None:
    ...

EventModel is the recommended payload base. Plain pydantic.BaseModel remains supported for compatibility.

If payload validation or any injection-stage dependency build fails, the current handler is treated as declining the event, and propagation may continue to a higher level.

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Dependency Injection

Besides payload and event context, FastEvents also supports lightweight function-style dependencies:

from fastevents import EventContext, RuntimeEvent, dependency


@dependency
def order_ctx(event: RuntimeEvent, ctx: EventContext) -> tuple[str, bool]:
    return (event.id, ctx is event.ctx)


@app.on("order.created")
async def handle(info=order_ctx()) -> None:
    print(info)

Dependencies can themselves depend on:

• the current event
• ctx
• payload models
• other dependencies

Resolution is cached for one event dispatch, so multiple handlers handling the same event can reuse the same dependency result.

For custom typed injection, define a static _provider() on your type and return a provider declared with dependency(). FastEvents only keeps privileged typed injection for RuntimeEvent and pydantic-based payload models.

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SessionNotConsumed and Fallback

If a primary handler wants to explicitly decline handling, it can raise SessionNotConsumed.

This allows propagation to continue to a higher level, which makes fallback straightforward:

from fastevents import SessionNotConsumed


@app.on("user.lookup", level=0)
async def primary(data: LegacyOrderCreated) -> None:
    if data.order_id == -1:
        raise SessionNotConsumed()


@app.on("user.lookup", level=1)
async def fallback(event: RuntimeEvent, data: LegacyOrderCreated) -> None:
    await event.ctx.publish(tags="user.lookup.fallback", payload={"path": "fallback", "order_id": data.order_id})

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RuntimeEvent and ctx

If a handler needs to keep talking to the bus - for example by publishing a follow-up event - it does so through event.ctx.

The most common capability is:

• await event.ctx.publish(...)

Example:

@app.on("order.created")
async def handle(event: RuntimeEvent, data: OrderCreated) -> None:
    await event.ctx.publish(tags="order.validated", payload={"order_id": data.order_id})

This means handlers do not need to carry a direct bus reference, and runtime interaction stays behind a clear boundary.

RuntimeEvent is the only runtime-side privileged typed injection surface. Higher-level capability objects should be expressed through dependencies or typed providers rather than new framework special cases.

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Debug Mode

FastEvents accepts debug=True:

from fastevents import FastEvents

app = FastEvents(debug=True)

Debug mode does not change propagation semantics. It only prints diagnostics for:

• events flowing into the app
• events published out of the app
• subscriber matching and level propagation
• absorbed injection-stage errors

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Serialization and Send Boundary

FastEvents now treats the application boundary and the bus boundary differently.

At the app boundary, FastEvents.publish() and EventContext.publish() accept richer Python values and normalize them before they enter the bus.

At the bus boundary, StandardEvent stores only scalar send values:

• None
• bool
• int
• float
• str

Application-facing values are encoded through encode_app_value(). The current supported inputs are:

• scalar values
• list
• tuple
• dict[str, ...]
• dataclass instances
• pydantic.BaseModel
• EventModel

Normalization rules:

• tuple is encoded as {"tuple": [...]} so the immutable intent survives transport
• dict keys must be strings
• dataclasses are converted with asdict(...)
• pydantic models are converted with model_dump(mode="json")
• non-scalar structured values are serialized to JSON strings before entering the bus

When events are handled inside the app, RuntimeEventView.payload and RuntimeEventView.meta decode these values back into structured runtime forms.

This keeps the bus contract minimal while preserving a convenient application programming model.

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Bus Lifecycle

In async environments:

await bus.astart(app)
try:
    ...
finally:
    await bus.astop()

In synchronous environments:

bus.start(app)
try:
    ...
finally:
    bus.stop()

You can also use bus.run(app) for a blocking runtime loop.

Before the bus starts, calling publish(), or calling app.publish() / app.listen() before the app is bound to a running bus, raises BusNotStartedError.

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Listen

listen() lives on app and creates a temporary stream subscriber:

async with app.listen("notification.sent", level=-1) as stream:
    async for event in stream:
        print(event.payload)

This is useful when you want to observe a class of events at runtime without turning it into a permanent handler.

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RPC

RPC is not the core model of FastEvents.

It is a higher-level protocol built on top of events.

The current RPC extension provides:

• request_stream()
• request_one()
• request()
• rpc_context()

You can import the first-party RPC surface directly from fastevents/__init__.py.

These helpers build request/reply behavior on top of ordinary events without changing the core event semantics.

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Minimal Example

If you only want the smallest publish example:

import asyncio

from pydantic import BaseModel

from fastevents import FastEvents, InMemoryBus

app = FastEvents()
bus = InMemoryBus()


class Message(BaseModel):
    text: str


@app.on("message")
async def handle_message(payload: Message) -> None:
    print(f"got: {payload.text}")


async def main() -> None:
    await bus.astart(app)
    try:
        await app.publish(tags="message", payload=Message(text="hello"))
    finally:
        await bus.astop()


asyncio.run(main())

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A More Realistic Example

demo.py is closer to the real direction of the project. It combines:

• dependency injection
• streamed token events
• ctx.publish()
• RPC request/reply
• interactive terminal behavior

If you want to understand how FastEvents feels in a more realistic application, run it directly:

uv run python demo.py

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Current Status

FastEvents is evolving toward a clearer event-application model with stricter boundaries.

If you are evaluating it, the most important things to understand are:

• publish() intentionally has weak guarantees
• the bus owns runtime complexity
• the app is for declaration and composition
• extensions are explicit composition objects, not managed plugins
• higher-level protocols should be built on top of events rather than pushed into the core

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Philosophy in One Sentence

FastEvents takes the application model already proven in Python web frameworks, generalizes it from special events like HTTP to general event systems, and keeps the boundaries explicit enough to stay composable, extensible, and evolvable.