asyncio-concurrency-patterns

Asyncio Concurrency Patterns

A comprehensive skill for mastering Python's asyncio library and concurrent programming patterns. This skill covers event loops, coroutines, tasks, futures, synchronization primitives, async context managers, and production-ready patterns for building high-performance asynchronous applications.

When to Use This Skill

Use this skill when:

• Building I/O-bound applications that need to handle many concurrent operations
• Creating web servers, API clients, or websocket applications
• Implementing real-time systems with event-driven architecture
• Optimizing application performance with concurrent request handling
• Managing multiple async operations with proper coordination and error handling
• Building background task processors or job queues
• Implementing async database operations and connection pooling
• Creating chat applications, real-time dashboards, or notification systems
• Handling parallel HTTP requests efficiently
• Managing websocket connections with multiple event sources
• Building microservices with async communication patterns
• Optimizing resource utilization in network applications

Core Concepts

What is Asyncio?

Asyncio is Python's built-in library for writing concurrent code using the async/await syntax. It provides:

• Event Loop: The core of asyncio that schedules and runs asynchronous tasks
• Coroutines: Functions defined with async def that can be paused and resumed
• Tasks: Scheduled coroutines that run concurrently
• Futures: Low-level objects representing results of async operations
• Synchronization Primitives: Locks, semaphores, events for coordination

Event Loop Fundamentals

The event loop is the central execution mechanism in asyncio:

import asyncio

# Get or create an event loop
loop = asyncio.get_event_loop()

# Run a coroutine until complete
loop.run_until_complete(my_coroutine())

# Modern approach (Python 3.7+)
asyncio.run(my_coroutine())

Key Event Loop Concepts:

1. Single-threaded concurrency: One thread, many tasks
2. Cooperative multitasking: Tasks yield control voluntarily
3. I/O multiplexing: Efficient handling of many I/O operations
4. Non-blocking operations: Don't wait for I/O, do other work

Coroutines vs Functions

Regular Function:

def fetch_data():
    # Blocks until complete
    return requests.get('http://api.example.com')

Coroutine:

async def fetch_data():
    # Yields control while waiting
    async with aiohttp.ClientSession() as session:
        async with session.get('http://api.example.com') as resp:
            return await resp.text()

Tasks and Futures

Tasks wrap coroutines and schedule them on the event loop:

# Create a task
task = asyncio.create_task(my_coroutine())

# Task runs in background
# ... do other work ...

# Wait for result
result = await task

Futures represent eventual results:

# Low-level future (rarely used directly)
future = asyncio.Future()

# Set result
future.set_result(42)

# Get result
result = await future

Async Context Managers

Manage resources with async setup/teardown:

class AsyncResource:
    async def __aenter__(self):
        # Async setup
        await self.connect()
        return self

    async def __aexit__(self, exc_type, exc_val, exc_tb):
        # Async cleanup
        await self.disconnect()

# Usage
async with AsyncResource() as resource:
    await resource.do_work()

Concurrency Patterns

Pattern 1: Gather - Concurrent Execution

Run multiple coroutines concurrently and wait for all to complete:

import asyncio
import aiohttp

async def fetch(session, url):
    async with session.get(url) as response:
        return await response.text()

async def main():
    async with aiohttp.ClientSession() as session:
        # Run all fetches concurrently
        results = await asyncio.gather(
            fetch(session, 'http://python.org'),
            fetch(session, 'http://docs.python.org'),
            fetch(session, 'http://pypi.org')
        )
        return results

# Results is a list in the same order as inputs
results = asyncio.run(main())

When to use:

• Need all results
• Order matters
• Want to fail fast on first exception (default)
• Can handle partial results with return_exceptions=True

Pattern 2: Wait - Flexible Waiting

More control over how to wait for multiple tasks:

import asyncio

async def task_a():
    await asyncio.sleep(2)
    return 'A'

async def task_b():
    await asyncio.sleep(1)
    return 'B'

async def main():
    tasks = [
        asyncio.create_task(task_a()),
        asyncio.create_task(task_b())
    ]

    # Wait for first to complete
    done, pending = await asyncio.wait(
        tasks,
        return_when=asyncio.FIRST_COMPLETED
    )

    # Get first result
    first_result = done.pop().result()

    # Cancel remaining
    for task in pending:
        task.cancel()

    return first_result

result = asyncio.run(main())  # Returns 'B' after 1 second

Wait strategies:

• FIRST_COMPLETED: Return when first task finishes
• FIRST_EXCEPTION: Return when first task raises exception
• ALL_COMPLETED: Wait for all tasks (default)

Pattern 3: Semaphore - Limit Concurrency

Control maximum number of concurrent operations:

import asyncio
import aiohttp

async def fetch_with_limit(session, url, semaphore):
    async with semaphore:
        # Only N requests run concurrently
        async with session.get(url) as resp:
            return await resp.text()

async def main():
    # Limit to 5 concurrent requests
    semaphore = asyncio.Semaphore(5)

    urls = [f'http://api.example.com/item/{i}' for i in range(100)]

    async with aiohttp.ClientSession() as session:
        tasks = [
            fetch_with_limit(session, url, semaphore)
            for url in urls
        ]
        results = await asyncio.gather(*tasks)

    return results

asyncio.run(main())

When to use:

• Rate limiting API requests
• Controlling database connection usage
• Preventing resource exhaustion
• Respecting external service limits

Pattern 4: Lock - Mutual Exclusion

Ensure only one coroutine accesses a resource at a time:

import asyncio

class SharedCounter:
    def __init__(self):
        self.value = 0
        self.lock = asyncio.Lock()

    async def increment(self):
        async with self.lock:
            # Critical section - only one coroutine at a time
            current = self.value
            await asyncio.sleep(0)  # Simulate async work
            self.value = current + 1

async def worker(counter):
    for _ in range(100):
        await counter.increment()

async def main():
    counter = SharedCounter()

    # Run 10 workers concurrently
    await asyncio.gather(*[worker(counter) for _ in range(10)])

    print(f"Final count: {counter.value}")  # Always 1000

asyncio.run(main())

Pattern 5: Event - Signaling

Coordinate multiple coroutines with events:

import asyncio

async def waiter(event, name):
    print(f'{name} waiting for event')
    await event.wait()
    print(f'{name} received event')

async def setter(event):
    await asyncio.sleep(2)
    print('Setting event')
    event.set()

async def main():
    event = asyncio.Event()

    # Multiple waiters
    await asyncio.gather(
        waiter(event, 'Waiter 1'),
        waiter(event, 'Waiter 2'),
        waiter(event, 'Waiter 3'),
        setter(event)
    )

asyncio.run(main())

Pattern 6: Queue - Producer/Consumer

Coordinate work between producers and consumers:

import asyncio

async def producer(queue, n):
    for i in range(n):
        await asyncio.sleep(0.1)
        await queue.put(f'item-{i}')
        print(f'Produced item-{i}')

    # Signal completion
    await queue.put(None)

async def consumer(queue, name):
    while True:
        item = await queue.get()

        if item is None:
            # Propagate sentinel to other consumers
            await queue.put(None)
            break

        print(f'{name} processing {item}')
        await asyncio.sleep(0.2)
        queue.task_done()

async def main():
    queue = asyncio.Queue()

    # Start producer and consumers
    await asyncio.gather(
        producer(queue, 10),
        consumer(queue, 'Consumer-1'),
        consumer(queue, 'Consumer-2'),
        consumer(queue, 'Consumer-3')
    )

asyncio.run(main())

Task Management

Creating Tasks

Basic Task Creation:

import asyncio

async def background_task():
    await asyncio.sleep(10)
    return 'Done'

async def main():
    # Create task - starts running immediately
    task = asyncio.create_task(background_task())

    # Do other work while task runs
    await asyncio.sleep(1)

    # Wait for result
    result = await task
    return result

asyncio.run(main())

Named Tasks (Python 3.8+):

task = asyncio.create_task(
    background_task(),
    name='my-background-task'
)

print(task.get_name())  # 'my-background-task'

Task Cancellation

Graceful Cancellation:

import asyncio

async def long_running_task():
    try:
        while True:
            await asyncio.sleep(1)
            print('Working...')
    except asyncio.CancelledError:
        print('Task cancelled, cleaning up...')
        # Cleanup logic
        raise  # Re-raise to mark as cancelled

async def main():
    task = asyncio.create_task(long_running_task())

    # Let it run for 3 seconds
    await asyncio.sleep(3)

    # Request cancellation
    task.cancel()

    try:
        await task
    except asyncio.CancelledError:
        print('Task was cancelled')

asyncio.run(main())

Cancellation with Context Manager:

import asyncio
from contextlib import suppress

async def run_with_timeout():
    task = asyncio.create_task(long_running_task())

    try:
        # Wait with timeout
        await asyncio.wait_for(task, timeout=5.0)
    except asyncio.TimeoutError:
        task.cancel()
        with suppress(asyncio.CancelledError):
            await task

Exception Handling in Tasks

Gather with Exception Handling:

import asyncio

async def failing_task(n):
    await asyncio.sleep(n)
    raise ValueError(f'Task {n} failed')

async def successful_task(n):
    await asyncio.sleep(n)
    return f'Task {n} succeeded'

async def main():
    # return_exceptions=True: Returns exceptions instead of raising
    results = await asyncio.gather(
        successful_task(1),
        failing_task(2),
        successful_task(3),
        return_exceptions=True
    )

    for i, result in enumerate(results):
        if isinstance(result, Exception):
            print(f'Task {i} failed: {result}')
        else:
            print(f'Task {i} result: {result}')

asyncio.run(main())

Task Exception Retrieval:

import asyncio

async def main():
    task = asyncio.create_task(failing_task(1))

    # Wait for task
    await asyncio.sleep(2)

    # Check if task failed
    if task.done() and task.exception():
        print(f'Task failed with: {task.exception()}')

asyncio.run(main())

Event Loop Management

Event Loop Policies

Default Event Loop:

import asyncio

async def main():
    # Get running loop
    loop = asyncio.get_running_loop()
    print(f'Loop: {loop}')

asyncio.run(main())

Custom Event Loop:

import asyncio

async def main():
    pass

# Create new event loop
loop = asyncio.new_event_loop()
asyncio.set_event_loop(loop)

try:
    loop.run_until_complete(main())
finally:
    loop.close()

Event Loop Best Practices:

1. Use asyncio.run() for simple programs (Python 3.7+)
2. Avoid creating ClientSession outside event loop
3. Always close loops when done
4. Don't call blocking functions in event loop

Running Blocking Code

Using ThreadPoolExecutor:

import asyncio
import time
from concurrent.futures import ThreadPoolExecutor

def blocking_io():
    # Blocking operation
    time.sleep(2)
    return 'Done'

async def main():
    loop = asyncio.get_running_loop()

    # Run blocking code in thread pool
    result = await loop.run_in_executor(
        None,  # Use default executor
        blocking_io
    )

    return result

asyncio.run(main())

Custom Executor:

import asyncio
from concurrent.futures import ThreadPoolExecutor

async def main():
    loop = asyncio.get_running_loop()

    # Custom executor with 4 threads
    with ThreadPoolExecutor(max_workers=4) as executor:
        results = await asyncio.gather(*[
            loop.run_in_executor(executor, blocking_io)
            for _ in range(10)
        ])

    return results

asyncio.run(main())

Loop Callbacks

Schedule Callback:

import asyncio

def callback(arg):
    print(f'Callback called with {arg}')

async def main():
    loop = asyncio.get_running_loop()

    # Schedule callback
    loop.call_soon(callback, 'immediate')

    # Schedule with delay
    loop.call_later(2, callback, 'delayed')

    # Schedule at specific time
    loop.call_at(loop.time() + 3, callback, 'scheduled')

    await asyncio.sleep(4)

asyncio.run(main())

Async Context Managers

Creating Async Context Managers

Class-Based:

import asyncio

class AsyncDatabaseConnection:
    def __init__(self, host):
        self.host = host
        self.connection = None

    async def __aenter__(self):
        print(f'Connecting to {self.host}')
        await asyncio.sleep(0.1)  # Simulate connection
        self.connection = f'Connection to {self.host}'
        return self

    async def __aexit__(self, exc_type, exc_val, exc_tb):
        print(f'Closing connection to {self.host}')
        await asyncio.sleep(0.1)  # Simulate cleanup
        self.connection = None

    async def query(self, sql):
        if not self.connection:
            raise RuntimeError('Not connected')
        await asyncio.sleep(0.05)
        return f'Results for: {sql}'

async def main():
    async with AsyncDatabaseConnection('localhost') as db:
        result = await db.query('SELECT * FROM users')
        print(result)

asyncio.run(main())

Decorator-Based:

import asyncio
from contextlib import asynccontextmanager

@asynccontextmanager
async def async_resource(name):
    # Setup
    print(f'Acquiring {name}')
    await asyncio.sleep(0.1)

    try:
        yield name
    finally:
        # Cleanup
        print(f'Releasing {name}')
        await asyncio.sleep(0.1)

async def main():
    async with async_resource('database') as db:
        print(f'Using {db}')

asyncio.run(main())

Real-World Example: aiohttp ClientSession

import aiohttp
import asyncio

async def fetch(session, url):
    async with session.get(url) as response:
        return await response.text()

async def main():
    # ClientSession as async context manager
    async with aiohttp.ClientSession() as session:
        html = await fetch(session, 'http://python.org')
        print(f'Body: {html[:100]}...')

asyncio.run(main())

Why use async context manager for ClientSession?

1. Ensures proper cleanup of connections
2. Prevents resource leaks
3. Manages SSL connections correctly
4. Handles graceful shutdown

Performance Optimization

Profiling Async Code

Basic Timing:

import asyncio
import time

async def slow_operation():
    await asyncio.sleep(1)

async def main():
    start = time.perf_counter()

    await slow_operation()

    elapsed = time.perf_counter() - start
    print(f'Took {elapsed:.2f} seconds')

asyncio.run(main())

Profiling Multiple Operations:

import asyncio
import time

async def timed_task(name, duration):
    start = time.perf_counter()
    await asyncio.sleep(duration)
    elapsed = time.perf_counter() - start
    print(f'{name} took {elapsed:.2f}s')
    return name

async def main():
    await asyncio.gather(
        timed_task('Task 1', 1),
        timed_task('Task 2', 2),
        timed_task('Task 3', 0.5)
    )

asyncio.run(main())

Optimizing Concurrency

Bad - Sequential Execution:

async def slow_approach():
    results = []
    for i in range(10):
        result = await fetch_data(i)
        results.append(result)
    return results
# Takes 10 * fetch_time

Good - Concurrent Execution:

async def fast_approach():
    tasks = [fetch_data(i) for i in range(10)]
    results = await asyncio.gather(*tasks)
    return results
# Takes ~fetch_time

Better - Controlled Concurrency:

async def controlled_approach():
    semaphore = asyncio.Semaphore(5)  # Max 5 concurrent

    async def fetch_with_limit(i):
        async with semaphore:
            return await fetch_data(i)

    tasks = [fetch_with_limit(i) for i in range(10)]
    results = await asyncio.gather(*tasks)
    return results
# Takes ~2 * fetch_time, but respects limits

Avoiding Common Performance Pitfalls

1. Don't create sessions per request:

# BAD - Creates new session each time
async def bad_fetch(url):
    async with aiohttp.ClientSession() as session:
        async with session.get(url) as resp:
            return await resp.text()

# GOOD - Reuse session
async def good_fetch():
    async with aiohttp.ClientSession() as session:
        results = await asyncio.gather(
            session.get('http://example.com/1'),
            session.get('http://example.com/2'),
            session.get('http://example.com/3')
        )
        return results

2. Don't use blocking operations:

import asyncio
import requests  # Blocking library

# BAD - Blocks event loop
async def bad_request():
    response = requests.get('http://example.com')  # BLOCKS!
    return response.text

# GOOD - Use async library
async def good_request():
    async with aiohttp.ClientSession() as session:
        async with session.get('http://example.com') as resp:
            return await resp.text()

# ACCEPTABLE - If must use blocking, use executor
async def acceptable_request():
    loop = asyncio.get_running_loop()
    result = await loop.run_in_executor(
        None,
        lambda: requests.get('http://example.com').text
    )
    return result

3. Proper cleanup with zero-sleep:

async def proper_cleanup():
    async with aiohttp.ClientSession() as session:
        async with session.get('http://example.org/') as resp:
            await resp.read()

    # Zero-sleep to allow underlying connections to close
    await asyncio.sleep(0)

Common Pitfalls

Pitfall 1: Creating ClientSession Outside Event Loop

Problem:

import aiohttp

# BAD - Session created outside event loop
session = aiohttp.ClientSession()

async def fetch(url):
    async with session.get(url) as resp:
        return await resp.text()

Why it's bad:

• Session binds to event loop at creation time
• If loop changes (e.g., uvloop), session becomes invalid
• Can cause program to hang

Solution:

import aiohttp
import asyncio

async def main():
    # Create session inside async function
    async with aiohttp.ClientSession() as session:
        async with session.get('http://python.org') as resp:
            print(await resp.text())

asyncio.run(main())

Pitfall 2: Session as Class Variable

Problem:

class API:
    session = aiohttp.ClientSession()  # BAD - global instance

    async def fetch(self, url):
        async with self.session.get(url) as resp:
            return await resp.text()

Solution:

class API:
    def __init__(self):
        self.session = None

    async def __aenter__(self):
        self.session = aiohttp.ClientSession()
        return self

    async def __aexit__(self, *args):
        await self.session.close()

    async def fetch(self, url):
        async with self.session.get(url) as resp:
            return await resp.text()

# Usage
async def main():
    async with API() as api:
        result = await api.fetch('http://example.com')

Pitfall 3: Forgetting await

Problem:

async def process_data():
    # Forgot await - returns coroutine, doesn't execute!
    result = fetch_data()  # Missing await
    return result

Solution:

async def process_data():
    result = await fetch_data()  # Proper await
    return result

Pitfall 4: Blocking the Event Loop

Problem:

import asyncio
import time

async def bad_sleep():
    time.sleep(5)  # BAD - Blocks entire event loop!

async def main():
    await asyncio.gather(
        bad_sleep(),
        another_task()  # Blocked for 5 seconds
    )

Solution:

import asyncio

async def good_sleep():
    await asyncio.sleep(5)  # GOOD - Yields control

async def main():
    await asyncio.gather(
        good_sleep(),
        another_task()  # Runs concurrently
    )

Pitfall 5: Not Handling Task Cancellation

Problem:

async def bad_task():
    while True:
        await asyncio.sleep(1)
        process_data()
        # No cleanup on cancellation!

Solution:

async def good_task():
    try:
        while True:
            await asyncio.sleep(1)
            process_data()
    except asyncio.CancelledError:
        # Cleanup resources
        cleanup()
        raise  # Re-raise to mark as cancelled

Pitfall 6: Deadlocks with Locks

Problem:

import asyncio

lock1 = asyncio.Lock()
lock2 = asyncio.Lock()

async def task_a():
    async with lock1:
        await asyncio.sleep(0.1)
        async with lock2:  # Deadlock potential
            pass

async def task_b():
    async with lock2:
        await asyncio.sleep(0.1)
        async with lock1:  # Deadlock potential
            pass

Solution:

# Always acquire locks in same order
async def safe_task_a():
    async with lock1:
        async with lock2:
            pass

async def safe_task_b():
    async with lock1:  # Same order
        async with lock2:
            pass

Production Patterns

Pattern 1: Graceful Shutdown

Complete Shutdown Example:

import asyncio
import signal
from contextlib import suppress

class Application:
    def __init__(self):
        self.should_exit = False
        self.tasks = []

    async def worker(self, name):
        try:
            while not self.should_exit:
                print(f'{name} working...')
                await asyncio.sleep(1)
        except asyncio.CancelledError:
            print(f'{name} cancelled, cleaning up...')
            raise

    def handle_signal(self, sig):
        print(f'Received signal {sig}, shutting down...')
        self.should_exit = True

    async def run(self):
        # Setup signal handlers
        loop = asyncio.get_running_loop()
        for sig in (signal.SIGTERM, signal.SIGINT):
            loop.add_signal_handler(
                sig,
                lambda s=sig: self.handle_signal(s)
            )

        # Start workers
        self.tasks = [
            asyncio.create_task(self.worker(f'Worker-{i}'))
            for i in range(3)
        ]

        # Wait for shutdown signal
        while not self.should_exit:
            await asyncio.sleep(0.1)

        # Cancel all tasks
        for task in self.tasks:
            task.cancel()

        # Wait for cancellation to complete
        await asyncio.gather(*self.tasks, return_exceptions=True)

        print('Shutdown complete')

# Run application
app = Application()
asyncio.run(app.run())

Pattern 2: Background Tasks with Application Lifecycle

aiohttp Application with Background Tasks:

import asyncio
from contextlib import suppress
from aiohttp import web

async def listen_to_redis(app):
    """Background task that listens to Redis"""
    # Simulated Redis listening
    try:
        while True:
            # Process messages
            await asyncio.sleep(1)
            print('Processing Redis message...')
    except asyncio.CancelledError:
        print('Redis listener stopped')
        raise

async def background_tasks(app):
    """Cleanup context for managing background tasks"""
    # Startup: Create background task
    app['redis_listener'] = asyncio.create_task(listen_to_redis(app))

    yield  # App is running

    # Cleanup: Cancel background task
    app['redis_listener'].cancel()
    with suppress(asyncio.CancelledError):
        await app['redis_listener']

# Setup application
app = web.Application()
app.cleanup_ctx.append(background_tasks)

Pattern 3: Retry Logic with Exponential Backoff

import asyncio
import aiohttp
from typing import Any, Callable

async def retry_with_backoff(
    coro_func: Callable,
    max_retries: int = 3,
    base_delay: float = 1.0,
    max_delay: float = 60.0,
    *args,
    **kwargs
) -> Any:
    """
    Retry async function with exponential backoff

    Args:
        coro_func: Async function to retry
        max_retries: Maximum number of retries
        base_delay: Initial delay between retries
        max_delay: Maximum delay between retries
    """
    for attempt in range(max_retries):
        try:
            return await coro_func(*args, **kwargs)
        except Exception as e:
            if attempt == max_retries - 1:
                # Last attempt failed
                raise

            # Calculate delay with exponential backoff
            delay = min(base_delay * (2 ** attempt), max_delay)

            print(f'Attempt {attempt + 1} failed: {e}')
            print(f'Retrying in {delay:.1f} seconds...')

            await asyncio.sleep(delay)

# Usage
async def unstable_api_call():
    async with aiohttp.ClientSession() as session:
        async with session.get('http://unstable-api.com') as resp:
            return await resp.json()

async def main():
    result = await retry_with_backoff(
        unstable_api_call,
        max_retries=5,
        base_delay=1.0
    )
    return result

Pattern 4: Circuit Breaker

import asyncio
from datetime import datetime, timedelta
from enum import Enum

class CircuitState(Enum):
    CLOSED = "closed"      # Normal operation
    OPEN = "open"          # Failing, reject requests
    HALF_OPEN = "half_open"  # Testing if recovered

class CircuitBreaker:
    def __init__(
        self,
        failure_threshold: int = 5,
        recovery_timeout: float = 60.0,
        success_threshold: int = 2
    ):
        self.failure_threshold = failure_threshold
        self.recovery_timeout = recovery_timeout
        self.success_threshold = success_threshold

        self.failure_count = 0
        self.success_count = 0
        self.state = CircuitState.CLOSED
        self.opened_at = None

    async def call(self, coro_func, *args, **kwargs):
        if self.state == CircuitState.OPEN:
            # Check if should try recovery
            if datetime.now() - self.opened_at > timedelta(seconds=self.recovery_timeout):
                self.state = CircuitState.HALF_OPEN
                self.success_count = 0
            else:
                raise Exception('Circuit breaker is OPEN')

        try:
            result = await coro_func(*args, **kwargs)
            self._on_success()
            return result
        except Exception as e:
            self._on_failure()
            raise

    def _on_success(self):
        self.failure_count = 0

        if self.state == CircuitState.HALF_OPEN:
            self.success_count += 1
            if self.success_count >= self.success_threshold:
                self.state = CircuitState.CLOSED
                self.success_count = 0

    def _on_failure(self):
        self.failure_count += 1

        if self.failure_count >= self.failure_threshold:
            self.state = CircuitState.OPEN
            self.opened_at = datetime.now()

# Usage
async def flaky_service():
    # Simulated flaky service
    import random
    await asyncio.sleep(0.1)
    if random.random() < 0.5:
        raise Exception('Service error')
    return 'Success'

async def main():
    breaker = CircuitBreaker(failure_threshold=3, recovery_timeout=5.0)

    for i in range(20):
        try:
            result = await breaker.call(flaky_service)
            print(f'Request {i}: {result} - State: {breaker.state.value}')
        except Exception as e:
            print(f'Request {i}: Failed - State: {breaker.state.value}')

        await asyncio.sleep(0.5)

Pattern 5: WebSocket with Multiple Event Sources

Handling Parallel WebSocket and Background Events:

import asyncio
from aiohttp import web

async def read_subscription(ws, redis):
    """Background task reading from Redis and sending to WebSocket"""
    # Simulated Redis subscription
    channel = await redis.subscribe('channel:1')

    try:
        # Simulate receiving messages
        for i in range(10):
            await asyncio.sleep(1)
            message = f'Redis message {i}'
            await ws.send_str(message)
    finally:
        await redis.unsubscribe('channel:1')

async def websocket_handler(request):
    """WebSocket handler with parallel event sources"""
    ws = web.WebSocketResponse()
    await ws.prepare(request)

    # Create background task for Redis subscription
    redis = request.app['redis']
    task = asyncio.create_task(read_subscription(ws, redis))

    try:
        # Handle incoming WebSocket messages
        async for msg in ws:
            if msg.type == web.WSMsgType.TEXT:
                # Process incoming message
                await ws.send_str(f'Echo: {msg.data}')
            elif msg.type == web.WSMsgType.ERROR:
                print(f'WebSocket error: {ws.exception()}')
    finally:
        # Cleanup: Cancel background task
        task.cancel()

    return ws

Best Practices

Testing Async Code

Using pytest-asyncio:

import pytest
import asyncio

@pytest.mark.asyncio
async def test_async_function():
    result = await async_operation()
    assert result == 'expected'

@pytest.mark.asyncio
async def test_with_fixture(aiohttp_client):
    client = await aiohttp_client(create_app())
    resp = await client.get('/')
    assert resp.status == 200

Manual Event Loop Setup:

import asyncio
import unittest

class TestAsyncCode(unittest.TestCase):
    def setUp(self):
        self.loop = asyncio.new_event_loop()
        asyncio.set_event_loop(self.loop)

    def tearDown(self):
        self.loop.close()

    def test_coroutine(self):
        async def test_impl():
            result = await async_function()
            self.assertEqual(result, 'expected')

        self.loop.run_until_complete(test_impl())

Debugging Async Code

Enable Debug Mode:

import asyncio
import warnings

# Enable asyncio debug mode
asyncio.run(main(), debug=True)

# Or manually
loop = asyncio.get_event_loop()
loop.set_debug(True)
loop.run_until_complete(main())

What debug mode detects:

• Coroutines that were never awaited
• Callbacks taking too long
• Tasks destroyed while pending

Logging Slow Callbacks:

import asyncio
import logging

logging.basicConfig(level=logging.DEBUG)

loop = asyncio.get_event_loop()
loop.slow_callback_duration = 0.1  # 100ms threshold
loop.set_debug(True)

Documentation

Documenting Async Functions:

async def fetch_user_data(user_id: int) -> dict:
    """
    Fetch user data from the database.

    Args:
        user_id: The unique identifier of the user

    Returns:
        Dictionary containing user data

    Raises:
        UserNotFoundError: If user doesn't exist
        DatabaseError: If database connection fails

    Example:
        >>> async def main():
        ...     user = await fetch_user_data(123)
        ...     print(user['name'])

    Note:
        This function must be called within an async context.
        Connection pooling is handled automatically.
    """
    async with get_db_connection() as conn:
        return await conn.fetch_one(
            'SELECT * FROM users WHERE id = $1',
            user_id
        )

Complete Examples

Example 1: Parallel HTTP Requests

import asyncio
import aiohttp
import time

async def fetch(session, url):
    """Fetch a single URL"""
    async with session.get(url) as response:
        return {
            'url': url,
            'status': response.status,
            'length': len(await response.text())
        }

async def fetch_all(urls):
    """Fetch multiple URLs concurrently"""
    async with aiohttp.ClientSession() as session:
        tasks = [fetch(session, url) for url in urls]
        results = await asyncio.gather(*tasks)
        return results

async def main():
    urls = [
        'http://python.org',
        'http://docs.python.org',
        'http://pypi.org',
        'http://github.com/python',
        'http://www.python.org/dev/peps/'
    ]

    start = time.perf_counter()
    results = await fetch_all(urls)
    elapsed = time.perf_counter() - start

    for result in results:
        print(f"{result['url']}: {result['status']} ({result['length']} bytes)")

    print(f"\nFetched {len(urls)} URLs in {elapsed:.2f} seconds")

asyncio.run(main())

Example 2: Rate-Limited API Client

import asyncio
import aiohttp
from typing import List, Dict, Any

class RateLimitedClient:
    def __init__(self, rate_limit: int = 10):
        """
        Args:
            rate_limit: Maximum concurrent requests
        """
        self.semaphore = asyncio.Semaphore(rate_limit)
        self.session = None

    async def __aenter__(self):
        self.session = aiohttp.ClientSession()
        return self

    async def __aexit__(self, *args):
        await self.session.close()
        # Allow connections to close
        await asyncio.sleep(0)

    async def fetch(self, url: str) -> Dict[str, Any]:
        """Fetch URL with rate limiting"""
        async with self.semaphore:
            print(f'Fetching {url}')
            async with self.session.get(url) as resp:
                return {
                    'url': url,
                    'status': resp.status,
                    'data': await resp.json()
                }

    async def fetch_all(self, urls: List[str]) -> List[Dict[str, Any]]:
        """Fetch all URLs with rate limiting"""
        tasks = [self.fetch(url) for url in urls]
        return await asyncio.gather(*tasks, return_exceptions=True)

async def main():
    urls = [f'https://api.github.com/users/{user}'
            for user in ['python', 'django', 'flask', 'requests', 'aiohttp']]

    async with RateLimitedClient(rate_limit=2) as client:
        results = await client.fetch_all(urls)

        for result in results:
            if isinstance(result, Exception):
                print(f'Error: {result}')
            else:
                print(f"User: {result['data'].get('login', 'unknown')}")

asyncio.run(main())

Example 3: Database Connection Pool

import asyncio
from typing import List, Any

class AsyncConnectionPool:
    def __init__(self, size: int = 10):
        self.pool = asyncio.Queue(maxsize=size)
        self.size = size

    async def init(self):
        """Initialize connection pool"""
        for i in range(self.size):
            conn = await self._create_connection(i)
            await self.pool.put(conn)

    async def _create_connection(self, conn_id: int):
        """Create a database connection (simulated)"""
        await asyncio.sleep(0.1)  # Simulate connection time
        return {'id': conn_id, 'connected': True}

    async def acquire(self):
        """Acquire connection from pool"""
        return await self.pool.get()

    async def release(self, conn):
        """Release connection back to pool"""
        await self.pool.put(conn)

    async def execute(self, query: str) -> Any:
        """Execute query using pooled connection"""
        conn = await self.acquire()
        try:
            # Simulate query execution
            await asyncio.sleep(0.05)
            return f"Query '{query}' executed on connection {conn['id']}"
        finally:
            await self.release(conn)

    async def close(self):
        """Close all connections"""
        while not self.pool.empty():
            conn = await self.pool.get()
            # Close connection (simulated)
            conn['connected'] = False

async def worker(pool: AsyncConnectionPool, worker_id: int):
    """Worker that executes queries"""
    for i in range(5):
        result = await pool.execute(f'SELECT * FROM table WHERE id={i}')
        print(f'Worker {worker_id}: {result}')

async def main():
    # Create and initialize pool
    pool = AsyncConnectionPool(size=5)
    await pool.init()

    # Run multiple workers concurrently
    await asyncio.gather(*[
        worker(pool, i) for i in range(10)
    ])

    # Cleanup
    await pool.close()

asyncio.run(main())

Example 4: Real-Time Data Processor

import asyncio
import random
from datetime import datetime

class DataProcessor:
    def __init__(self):
        self.queue = asyncio.Queue()
        self.processed = 0
        self.errors = 0

    async def producer(self, producer_id: int):
        """Produce data items"""
        for i in range(10):
            await asyncio.sleep(random.uniform(0.1, 0.5))
            item = {
                'producer_id': producer_id,
                'item_id': i,
                'timestamp': datetime.now(),
                'data': random.randint(1, 100)
            }
            await self.queue.put(item)
            print(f'Producer {producer_id} generated item {i}')

        # Signal completion
        await self.queue.put(None)

    async def consumer(self, consumer_id: int):
        """Consume and process data items"""
        while True:
            item = await self.queue.get()

            if item is None:
                # Propagate sentinel
                await self.queue.put(None)
                break

            try:
                # Simulate processing
                await asyncio.sleep(random.uniform(0.05, 0.2))

                # Process item
                result = item['data'] * 2
                print(f"Consumer {consumer_id} processed: {item['item_id']} -> {result}")

                self.processed += 1
            except Exception as e:
                print(f'Consumer {consumer_id} error: {e}')
                self.errors += 1
            finally:
                self.queue.task_done()

    async def monitor(self):
        """Monitor processing statistics"""
        while True:
            await asyncio.sleep(2)
            print(f'\n=== Stats: Processed={self.processed}, Errors={self.errors}, Queue={self.queue.qsize()} ===\n')

    async def run(self, num_producers: int = 3, num_consumers: int = 5):
        """Run the data processor"""
        # Start monitor
        monitor_task = asyncio.create_task(self.monitor())

        # Start producers and consumers
        await asyncio.gather(
            *[self.producer(i) for i in range(num_producers)],
            *[self.consumer(i) for i in range(num_consumers)]
        )

        # Cancel monitor
        monitor_task.cancel()

        print(f'\nFinal Stats: Processed={self.processed}, Errors={self.errors}')

async def main():
    processor = DataProcessor()
    await processor.run(num_producers=3, num_consumers=5)

asyncio.run(main())

Example 5: Async File I/O with aiofiles

import asyncio
import aiofiles
from pathlib import Path

async def write_file(path: str, content: str):
    """Write content to file asynchronously"""
    async with aiofiles.open(path, 'w') as f:
        await f.write(content)

async def read_file(path: str) -> str:
    """Read file content asynchronously"""
    async with aiofiles.open(path, 'r') as f:
        return await f.read()

async def process_files(file_paths: list):
    """Process multiple files concurrently"""
    tasks = [read_file(path) for path in file_paths]
    contents = await asyncio.gather(*tasks)

    # Process contents
    results = []
    for path, content in zip(file_paths, contents):
        result = {
            'path': path,
            'lines': len(content.split('\n')),
            'words': len(content.split()),
            'chars': len(content)
        }
        results.append(result)

    return results

async def main():
    # Create test files
    test_files = ['test1.txt', 'test2.txt', 'test3.txt']

    # Write files concurrently
    await asyncio.gather(*[
        write_file(f, f'Content of file {f}\n' * 10)
        for f in test_files
    ])

    # Process files
    results = await process_files(test_files)

    for result in results:
        print(f"{result['path']}: {result['lines']} lines, "
              f"{result['words']} words, {result['chars']} chars")

    # Cleanup
    for f in test_files:
        Path(f).unlink(missing_ok=True)

# asyncio.run(main())  # Uncomment to run (requires aiofiles)

Resources

• Python asyncio Documentation: https://docs.python.org/3/library/asyncio.html
• aiohttp Documentation: https://docs.aiohttp.org/
• Real Python asyncio Guide: https://realpython.com/async-io-python/
• PEP 492 - Coroutines with async and await syntax: https://www.python.org/dev/peps/pep-0492/
• asyncio Cheat Sheet: https://www.pythonsheets.com/notes/python-asyncio.html
• Effective Python: Item 60 - Consider asyncio: https://effectivepython.com/

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Skill Version: 1.0.0 Last Updated: October 2025 Skill Category: Concurrency, Performance, Async Programming Compatible With: Python 3.7+, aiohttp, asyncio, uvloop