Python asyncio internals

Python asyncio internals refer to the event loop and task scheduling that power async/await. The event loop repeatedly polls I/O operations and runs ready coroutines, allowing thousands of concurrent connections with minimal threads. Understanding the event loop helps debug async behavior and write efficient concurrent code.

Use asyncio for network I/O, concurrent file operations, or coordinating many independent tasks without threading overhead.

This example shows the event loop, tasks, and concurrent execution.

# run: python3 asyncio_internals.py
# description: asyncio event loop and task scheduling
 
import asyncio
import time
 
async def fetch(url, delay):
    """Simulated async fetch."""
    print(f"Starting fetch from {url}")
    await asyncio.sleep(delay)  # yield control to event loop
    print(f"Finished fetch from {url}")
    return f"Data from {url}"
 
async def main():
    # Create tasks (scheduled coroutines)
    start = time.time()
 
    # Sequential: slow
    # result1 = await fetch("url1", 1)
    # result2 = await fetch("url2", 1)
 
    # Concurrent: fast - both run at once
    task1 = asyncio.create_task(fetch("url1", 1))
    task2 = asyncio.create_task(fetch("url2", 1))
 
    result1 = await task1
    result2 = await task2
 
    elapsed = time.time() - start
    print(f"Completed in {elapsed:.2f}s (concurrent, not sequential)")
    print(f"Results: {result1}, {result2}")
 
# Get or create event loop and run
asyncio.run(main())
 
# Event loop internals
async def show_event_loop():
    loop = asyncio.get_event_loop()
    print(f"\nEvent loop type: {type(loop)}")
    print(f"Running: {loop.is_running()}")
 
    # Create multiple tasks
    tasks = [
        asyncio.create_task(asyncio.sleep(0.1 * i, result=f"Task {i}"))
        for i in range(1, 4)
    ]
 
    # Wait for all tasks
    results = await asyncio.gather(*tasks)
    print(f"All tasks done: {results}")
 
asyncio.run(show_event_loop())
 
# Timeouts and cancellation
async def with_timeout():
    try:
        await asyncio.wait_for(asyncio.sleep(5), timeout=1)
    except asyncio.TimeoutError:
        print("Task timed out")
 
    # Cancellation
    task = asyncio.create_task(asyncio.sleep(10))
    await asyncio.sleep(0.1)
    task.cancel()
    try:
        await task
    except asyncio.CancelledError:
        print("Task was cancelled")
 
asyncio.run(with_timeout())

Event loop:

• Single-threaded loop that alternates between:
  • Running ready coroutines
  • Polling I/O (waiting for network, disk, timers)
  • Running callbacks
• asyncio.run() creates, runs, and closes loop
• asyncio.get_event_loop() gets current loop (inside async function)

Tasks:

• asyncio.create_task(coro): schedule coroutine on event loop
• Returns immediately with Task object
• Task runs when event loop runs
• await task waits for completion

Concurrent execution:

• Multiple await asyncio.sleep() at same point are concurrent
• Event loop interleaves them
• No threads needed; single thread with efficient switching

Synchronization:

• await asyncio.gather(*tasks): wait for all
• await asyncio.wait(tasks): low-level, returns done/pending
• async with asyncio.Lock():: mutual exclusion
• async with asyncio.Semaphore(5):: limit concurrent access

Error handling:

• Exceptions in tasks stored; raise when awaited
• await asyncio.gather(..., return_exceptions=True): collect errors
• task.exception(): get exception without re-raising

Cancellation:

• task.cancel(): request cancellation
• Raises asyncio.CancelledError inside task
• Task should handle and clean up
• CancelledError is subclass of BaseException (Python 3.8+)

Timeouts:

• await asyncio.wait_for(coro, timeout=5): raises TimeoutError
• Cancels coroutine if it times out

Callbacks and futures (low-level):

• loop.call_soon(func): schedule function to run ASAP
• asyncio.Future(): low-level promise object
• Most code uses async/await, not callbacks/futures