A custom asyncio event loop gives you full control over task scheduling, I/O integration, and performance tuning—ideal for web servers, game engines, or hardware projects. In this guide, you’ll:

  • Understand core concepts of an event loop
  • Implement a minimal loop with call_soon and call_later
  • Add error handling, coroutines, and I/O hooks
  • Explore advanced schedulers (priority, rate limiting)
  • Package your loop as a drop‑in asyncio backend

What Is an Event Loop?

An event loop is the heart of asyncio, repeating these steps until stopped:

  1. Run tasks queued for immediate execution (call_soon).
  2. Execute delayed tasks when their timer expires (call_later).
  3. Poll I/O (files, sockets) and dispatch readiness callbacks.

Keeping logic simple and non-blocking makes your application performant and scalable.

Why Build Your Own Loop?

Most developers use the built-in Python loop. A custom loop lets you:

  • Optimize for specific workloads (low-latency, high-throughput)
  • Integrate with non-standard I/O (game engine tick, hardware interrupts)
  • Customize scheduling policies (priorities, rate limits, time slices)

Implementing a Minimal Loop

Below is a minimal MiniLoop supporting call_soon, call_later, and stop. 

import time
import heapq

class MiniLoop:
    def __init__(self):
        self._ready = []          # immediate callbacks
        self._scheduled = []      # heap of (timestamp, callback, args)
        self._running = False

    def run_forever(self):
        self._running = True
        while self._running:
            now = time.time()

            # Run ready callbacks
            while self._ready:
                cb, args = self._ready.pop(0)
                try:
                    cb(*args)
                except Exception as e:
                    print("Callback error:", e)

            # Run delayed callbacks
            while self._scheduled and self._scheduled[0][0] <= now:
                _, cb, args = heapq.heappop(self._scheduled)
                try:
                    cb(*args)
                except Exception as e:
                    print("Scheduled error:", e)

            # Sleep until next timer or briefly
            if self._scheduled:
                sleep_time = max(0, self._scheduled[0][0] - now)
                time.sleep(sleep_time)
            else:
                time.sleep(0.01)

    def stop(self):
        self._running = False

    def call_soon(self, callback, *args):
        self._ready.append((callback, args))

    def call_later(self, delay, callback, *args):
        heapq.heappush(self._scheduled, (time.time() + delay, callback, args))

Code Explanation:

  • Uses a list for immediate callbacks and a heapq for timers.
  • Catches exceptions so one failing callback won’t halt the loop.

Error Handling Best Practices

Always wrap your callback calls in try/except: 

try:
    cb(*args)
except Exception as e:
    print("Loop callback error:", e)

This prevents any single error from stopping your entire event loop.

Adding Tasks & Coroutines

To support async/await, integrate with asyncio.Task: 

import asyncio

def create_task(loop, coro):
    task = asyncio.Task(coro, loop=loop)
    loop.call_soon(task._step)
    return task

Code Explanation:

  • Wraps a coroutine in a Task, then schedules its first step.

Timers & Priority Scheduling

Implement priorities so critical tasks run first: 

import heapq

class PriorityLoop(MiniLoop):
    def __init__(self):
        super().__init__()
        self._priority = []  # heap of (prio, callback, args)

    def call_soon_priority(self, prio, callback, *args):
        heapq.heappush(self._priority, (prio, callback, args))

    def run_forever(self):
        self._running = True
        while self._running:
            # High priority
            while self._priority:
                _, cb, args = heapq.heappop(self._priority)
                cb(*args)
            # Normal via parent
            super().run_forever()

I/O Integration

Use selectors for efficient I/O polling: 

import selectors
class IOLoop(MiniLoop):
    def __init__(self):
        super().__init__()
        self._sel = selectors.DefaultSelector()

    def add_reader(self, fd, cb, *args):
        self._sel.register(fd, selectors.EVENT_READ, (cb, args))

    def run_forever(self):
        while self._running:
            super().run_forever_iteration()
            events = self._sel.select(timeout=0)
            for key, _ in events:
                cb, args = key.data
                cb(*args)

Advanced Schedulers

  • Real-Time Deadlines: schedule at exact timestamps
  • Rate Limiting: throttle callback frequency
  • Cooperative Slices: give each task a CPU time slice

Packaging & Publishing

Provide an EventLoopPolicy for easy adoption: 

import asyncio
class CustomPolicy(asyncio.DefaultEventLoopPolicy):
    def new_event_loop(self):
        return FullLoop()

asyncio.set_event_loop_policy(CustomPolicy())

Next Steps & Resources

  • Benchmark against SelectorEventLoop with timeit
  • Experiment with custom I/O sources and schedulers