xEvents

A lightweight, thread-safe event system for Python. Use it to wire up different parts of your application so they can communicate through events — like a message bus that lives inside your process.

Features

• Subscribe & Post — Register listeners for named events and fire them with data.
• Channels — Group related events under a namespace (e.g., orders, users) to keep things organized.
• Once Listeners — Listeners that automatically remove themselves after firing once.
• Wildcard Subscriptions — Listen to multiple events at once using patterns like orders:*.
• Event Filters — Attach a condition to a listener so it only fires when the data matches your criteria.
• Event History & Replay — Past events are recorded. Late subscribers can catch up by replaying history.
• Async Support — Async functions work as listeners alongside regular functions.
• Thread-Safe — All operations are protected by a reentrant lock, safe for multi-threaded apps.
• Zero Dependencies — Built entirely on the Python standard library.

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Requirements

• Python 3.12+

Installation

Install xEvents using pip

$ pip install xEvents

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from xevents import xEvents

Quick Start

from xevents import xEvents

# Create an event bus
bus = xEvents()

# Subscribe to an event
def on_user_login(data):
    print(f"Welcome, {data['name']}!")

bus.on("user:login", on_user_login)

# Fire the event
bus.post("user:login", {"name": "Alice"})
# Output: Welcome, Alice!

API Reference

Creating an Event Bus

bus = xEvents()

Parameters:

Parameter	Type	Default	Description
events	list[str] or None	None	Optional list of event names to pre-register. If not provided, events are created dynamically on first subscription.
debug	bool	False	When True, prints debug info for subscribe/post operations.
history_limit	int	100	Maximum number of events to keep in history.

# Pre-register events
bus = xEvents(events=["start", "stop", "error"])

# Enable debug logging
bus = xEvents(debug=True)

# Keep more history
bus = xEvents(history_limit=500)

Subscribing to Events

bus.on(event, listener) / bus.subscribe(event, listener)

Register a listener that fires every time the event is posted.

def handle_order(data):
    print(f"Order #{data['id']} received")

bus.on("order:created", handle_order)

bus.once(event, listener)

Register a listener that fires only once, then removes itself.

def on_ready(data):
    print("App is ready!")

bus.once("app:ready", on_ready)

bus.post("app:ready", {})   # Prints: App is ready!
bus.post("app:ready", {})   # Nothing happens — listener was removed

bus.on_any(pattern, listener)

Subscribe to all events matching a wildcard pattern. The listener receives both the event name and the data.

def log_all_orders(event, data):
    print(f"[{event}] {data}")

bus.on_any("orders:*", log_all_orders)

bus.post("orders:created", {"id": 1})   # Fires the listener
bus.post("orders:shipped", {"id": 1})   # Fires the listener
bus.post("users:created", {"id": 5})    # Does NOT fire — different namespace

Pattern examples:

• * — matches every event
• orders:* — matches orders:created, orders:deleted, etc.
• *.error — matches db.error, api.error, etc.

Subscribing with Filters

Add a condition so the listener only fires when the data matches.

def on_paid_order(data):
    print(f"Paid order: #{data['id']}")

bus.on("order", on_paid_order, filter_fn=lambda d: d.get("status") == "paid")

bus.post("order", {"id": 1, "status": "pending"})   # Skipped
bus.post("order", {"id": 2, "status": "paid"})       # Fires
bus.post("order", {"id": 3, "status": "refunded"})   # Skipped

Unsubscribing

bus.unsubscribe(event, listener)

Remove a specific listener from an event.

def handler(data):
    print(data)

bus.on("test", handler)
bus.unsubscribe("test", handler)
bus.post("test", {"msg": "hello"})  # Nothing happens

bus.off_any(pattern, listener)

Remove a wildcard listener.

bus.on_any("*", my_logger)
bus.off_any("*", my_logger)

Posting Events

bus.post(event, data)

Fire an event with a data dictionary. All matching listeners are called immediately.

bus.post("user:signup", {"email": "[email protected]", "plan": "pro"})

Channels

Channels let you group related events under a namespace. Instead of manually writing "orders:created", you work with a channel object that handles the prefixing for you.

orders = bus.channel("orders")
users = bus.channel("users")

# These are equivalent:
orders.on("created", handler)       # Subscribes to "orders:created"
bus.on("orders:created", handler)   # Same thing

# Post through the channel
orders.post("created", {"id": 100, "total": 59.99})

# Once listener on a channel
orders.once("cancelled", handle_cancellation)

# Unsubscribe from a channel event
orders.unsubscribe("created", handler)

Channel methods: on, once, subscribe, unsubscribe, post

Event History

Events are automatically recorded in a capped history (default: last 100 events).

bus.history(event=None, channel=None, limit=None)

Retrieve past event records.

# Get all history
all_records = bus.history()

# Filter by event name
login_records = bus.history(event="user:login")

# Filter by channel
order_records = bus.history(channel="orders")

# Get only the last 5
recent = bus.history(limit=5)

# Combine filters
recent_orders = bus.history(channel="orders", limit=10)

Each record is an EventRecord with these fields:

Field	Type	Description
event	str	The event name
data	dict	The data that was posted
timestamp	float	Unix timestamp when the event was posted
channel	str or None	The channel name, if the event used channel:event format

bus.clear_history()

Remove all event history.

bus.clear_history()

Replay

Deliver past events to a listener that subscribed late — useful for catching up on what already happened.

bus.replay(event, listener, limit=None)

Replay past occurrences of a specific event.

# Post some events
bus.post("price:update", {"symbol": "BTC", "price": 50000})
bus.post("price:update", {"symbol": "BTC", "price": 51000})
bus.post("price:update", {"symbol": "BTC", "price": 52000})

# A new listener joins late and wants to catch up
def show_price(data):
    print(f"{data['symbol']}: ${data['price']}")

count = bus.replay("price:update", show_price, limit=2)
# Prints the last 2 price updates
# Returns: 2 (number of events replayed)

bus.replay_channel(channel_name, listener, limit=None)

Replay all past events from a specific channel.

count = bus.replay_channel("orders", process_order, limit=10)

Utility Methods

bus.events()

List all event names that have listeners registered.

bus.on("a", handler)
bus.on("b", handler)
print(bus.events())  # ["a", "b"]

bus.channels()

List all channel names that have been created.

bus.channel("orders")
bus.channel("users")
print(bus.channels())  # ["orders", "users"]

bus.listener_count(event)

Get the number of listeners for a specific event.

bus.on("test", handler_a)
bus.on("test", handler_b)
print(bus.listener_count("test"))  # 2

bus.enabled

Disable or enable the entire event system. When disabled, post() silently does nothing.

bus.enabled = False
bus.post("test", {"a": 1})  # Nothing happens

bus.enabled = True
bus.post("test", {"a": 1})  # Listeners fire normally

bus.reset()

Clear everything — all listeners, channels, history, and filters.

bus.reset()

Async Listeners

Async functions work as listeners. When called from within a running event loop, they're scheduled as tasks. Otherwise, they're run to completion.

import asyncio

bus = xEvents()

async def async_handler(data):
    await asyncio.sleep(0.1)
    print(f"Processed: {data}")

bus.on("task:complete", async_handler)

# Inside an async context
async def main():
    bus.post("task:complete", {"task_id": 42})
    await asyncio.sleep(0.2)  # Give the task time to complete

asyncio.run(main())

Error Handling

If a listener throws an error, it's caught and logged — other listeners still get called normally.

def bad_listener(data):
    raise RuntimeError("Something broke")

def good_listener(data):
    print(f"Got: {data}")

bus.on("test", bad_listener)
bus.on("test", good_listener)
bus.post("test", {"msg": "hello"})
# bad_listener error is logged
# good_listener still prints: Got: {'msg': 'hello'}

Full Example: Task Queue

from xevents import xEvents

bus = xEvents(debug=False)

# Set up channels
tasks = bus.channel("tasks")
notifications = bus.channel("notifications")

# Worker that processes tasks
def process_task(data):
    print(f"Processing task: {data['name']}")
    tasks.post("completed", {"name": data["name"], "result": "success"})

# Notification system
def send_notification(data):
    print(f"Notification: Task '{data['name']}' finished with result: {data['result']}")

# Logger that watches everything
def log_everything(event, data):
    print(f"  [LOG] {event} -> {data}")

# Wire it up
tasks.on("new", process_task)
tasks.on("completed", send_notification)
bus.on_any("tasks:*", log_everything)

# Submit a task
tasks.post("new", {"name": "Generate Report"})

Output:

  [LOG] tasks:new -> {'name': 'Generate Report'}
Processing task: Generate Report
  [LOG] tasks:completed -> {'name': 'Generate Report', 'result': 'success'}
Notification: Task 'Generate Report' finished with result: success

Running Tests

python -m unittest discover -s tests -v

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📚 ・ xDev Utilities

This library is part of xDev Utilities. As set of power tool to streamline your workflow.

• xAPI: A lightweight, flexible asynchronous API client for Python built on Pydantic and httpx
• xEvents: A lightweight, thread-safe event system for Python

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License

See LICENSE.md for details.