Python Async/Await Patterns

Purpose: Guide async Python development with clear patterns, pitfall avoidance, and concurrency strategy selection.

When to use: Writing async code, debugging async issues, choosing between async/threading/multiprocessing, testing async functions.

For detailed examples and advanced patterns: See reference.md

---

Core Principles

1. Async is for I/O, not CPU - Use async for network/disk, not heavy computation
2. Never block the event loop - Blocking calls kill async performance
3. await everything awaitable - Missing await creates silent bugs
4. Use async context managers - Proper resource cleanup in async code
5. Choose the right concurrency model - async vs threading vs multiprocessing

---

Quick Decision Framework

Workload Type	Use	Why
I/O-bound (network, files)	asyncio	Single thread handles thousands of concurrent I/O operations
CPU-bound (computation)	multiprocessing	Bypasses GIL, uses multiple CPU cores
Mixed I/O + blocking libs	ThreadPoolExecutor + asyncio	Run blocking I/O in threads, coordinate with asyncio
Legacy blocking code	run_in_executor()	Integrate blocking code into async codebase

For detailed comparison and examples: See reference.md Decision Framework section

---

Async/Await Basics

async def and await

# Coroutine function (must await to execute)
async def fetch_data(url: str) -> dict:
    """Fetch data from URL asynchronously."""
    async with aiohttp.ClientSession() as session:
        async with session.get(url) as response:
            return await response.json()

# await suspends execution until operation completes
async def process_user(user_id: int) -> dict:
    user = await db.fetch_user(user_id)      # Suspend here
    profile = await api.fetch_profile(user)  # And here
    return {"user": user, "profile": profile}

# Running async code (entry point)
import asyncio

async def main():
    result = await fetch_data("https://api.example.com")
    print(result)

asyncio.run(main())  # Python 3.7+

---

Essential asyncio Patterns

Pattern 1: Concurrent Execution with gather()

# Run multiple coroutines concurrently
async def fetch_multiple_users(user_ids: list[int]) -> list[dict]:
    """Fetch multiple users concurrently."""
    tasks = [fetch_user(uid) for uid in user_ids]
    return await asyncio.gather(*tasks)

# With error handling
async def fetch_with_errors(user_ids: list[int]) -> list[dict]:
    """Continue on error, return None for failures."""
    tasks = [fetch_user(uid) for uid in user_ids]
    results = await asyncio.gather(*tasks, return_exceptions=True)
    return [r for r in results if not isinstance(r, Exception)]

Pattern 2: Fire and Forget with create_task()

# Start background task without waiting
async def handle_request(request: dict) -> dict:
    """Handle request and log asynchronously."""
    asyncio.create_task(log_request(request))  # Fire and forget
    return await process_request(request)      # Don't wait for logging

Pattern 3: Timeouts with wait_for()

# Add timeout to any coroutine
async def fetch_with_timeout(url: str, timeout: float = 5.0) -> dict:
    """Fetch with timeout."""
    try:
        return await asyncio.wait_for(fetch_data(url), timeout=timeout)
    except asyncio.TimeoutError:
        LOG.warning(f"Timeout fetching {url}")
        raise

Pattern 4: Run Blocking Code with run_in_executor()

# Run blocking I/O in thread pool
async def read_file_async(path: str) -> str:
    """Read file without blocking event loop."""
    loop = asyncio.get_running_loop()

    def blocking_read():
        with open(path, 'r') as f:
            return f.read()

    return await loop.run_in_executor(None, blocking_read)

# Run CPU-bound work in process pool
async def compute_intensive_async(data: list[int]) -> int:
    """Run CPU-bound work in process pool."""
    from concurrent.futures import ProcessPoolExecutor

    loop = asyncio.get_running_loop()
    with ProcessPoolExecutor() as pool:
        return await loop.run_in_executor(pool, compute_heavy, data)

Pattern 5: Rate Limiting with Semaphore

# Limit concurrent operations
async def fetch_with_rate_limit(urls: list[str], max_concurrent: int = 5):
    """Limit concurrent requests."""
    semaphore = asyncio.Semaphore(max_concurrent)

    async def fetch_one(url: str):
        async with semaphore:
            return await fetch_data(url)

    tasks = [fetch_one(url) for url in urls]
    return await asyncio.gather(*tasks)

---

Async Context Managers

# Using async context managers
async def fetch_users() -> list[dict]:
    """Fetch users using async context manager."""
    async with aiohttp.ClientSession() as session:
        async with session.get("https://api.example.com") as response:
            return await response.json()

# Creating async context manager
from contextlib import asynccontextmanager

@asynccontextmanager
async def async_transaction():
    """Async transaction context manager."""
    conn = await get_connection()
    tx = await conn.begin()
    try:
        yield conn
        await tx.commit()
    except Exception:
        await tx.rollback()
        raise
    finally:
        await conn.close()

# Usage
async def update_user(user_id: int, data: dict):
    """Update user in transaction."""
    async with async_transaction() as conn:
        await conn.execute("UPDATE users SET data = $1 WHERE id = $2", data, user_id)

---

Common Pitfalls and Solutions

Pitfall 1: Blocking the Event Loop

Problem Code	Fixed Code
time.sleep(5)	await asyncio.sleep(5)
requests.get(url)	async with aiohttp.ClientSession() as s: await s.get(url)
open(file).read()	await loop.run_in_executor(None, blocking_read)
Heavy CPU work	await loop.run_in_executor(pool, cpu_work)

Why: Blocking calls freeze ALL async tasks. Always use async versions or run in executor.

Pitfall 2: Missing await

# ❌ BAD - missing await
async def bad_fetch():
    result = fetch_data()  # Returns coroutine object, not data!
    print(result)  # <coroutine object...>

# ✅ GOOD - properly awaited
async def good_fetch():
    result = await fetch_data()  # Actually executes
    print(result)  # {'data': ...}

Detection: Python warns about unawaited coroutines. Always await coroutine functions.

Pitfall 3: Event Loop Closed

# ❌ BAD - loop closed after asyncio.run()
result1 = asyncio.run(fetch1())
result2 = asyncio.run(fetch2())  # ERROR: Event loop is closed

# ✅ GOOD - single event loop
async def main():
    result1 = await fetch1()
    result2 = await fetch2()
    return result1, result2

asyncio.run(main())

Pitfall 4: Mixing Async and Sync

# ❌ BAD - can't await in sync function
def sync_function():
    result = await fetch_data()  # SyntaxError

# ✅ GOOD - make function async
async def async_function():
    result = await fetch_data()
    return result

# ✅ GOOD - use asyncio.run (entry point only)
def sync_entry_point():
    result = asyncio.run(fetch_data())
    return result

Pitfall 5: Sequential Instead of Concurrent

# ❌ BAD - runs sequentially (slow)
async def sequential():
    results = []
    for url in urls:
        results.append(await fetch(url))  # One at a time
    return results

# ✅ GOOD - runs concurrently (fast)
async def concurrent():
    tasks = [fetch(url) for url in urls]
    return await asyncio.gather(*tasks)  # All at once

For more pitfalls and solutions: See reference.md Common Pitfalls section

---

Testing Async Code

pytest with pytest-asyncio

import pytest
from unittest.mock import AsyncMock, patch

# Mark async test
@pytest.mark.asyncio
async def test_fetch_data():
    """Test async function."""
    result = await fetch_data("https://api.example.com")
    assert result["status"] == "success"

# Async fixtures
@pytest.fixture
async def async_client():
    """Create async HTTP client."""
    async with aiohttp.ClientSession() as session:
        yield session

@pytest.mark.asyncio
async def test_with_fixture(async_client):
    """Test using async fixture."""
    async with async_client.get("https://api.example.com") as resp:
        assert resp.status == 200

# Mocking async functions
@pytest.mark.asyncio
async def test_with_mock():
    """Test with mocked async dependency."""
    mock_fetch = AsyncMock(return_value={"data": "test"})

    with patch('module.fetch_data', mock_fetch):
        result = await process_data()

    assert result["data"] == "test"
    mock_fetch.assert_awaited_once()

# Testing timeouts
@pytest.mark.asyncio
async def test_timeout():
    """Test function respects timeout."""
    with pytest.raises(asyncio.TimeoutError):
        await asyncio.wait_for(slow_operation(), timeout=1.0)

For more testing patterns: See reference.md Testing Patterns section

---

Performance Considerations

When asyncio is Fastest

# Async shines with many I/O operations
async def fetch_1000_urls(urls: list[str]) -> list[dict]:
    """Fetch 1000 URLs concurrently (seconds, not minutes)."""
    async with aiohttp.ClientSession() as session:
        tasks = [fetch_with_session(session, url) for url in urls]
        return await asyncio.gather(*tasks)

# Single thread handles 1000+ concurrent connections!

Optimization: Reuse Connections

# ❌ BAD - creates new session each time
async def inefficient(urls: list[str]):
    results = []
    for url in urls:
        async with aiohttp.ClientSession() as session:  # New session!
            async with session.get(url) as response:
                results.append(await response.json())
    return results

# ✅ GOOD - reuse single session
async def efficient(urls: list[str]):
    async with aiohttp.ClientSession() as session:  # Single session
        tasks = [fetch_with_session(session, url) for url in urls]
        return await asyncio.gather(*tasks)

For more optimizations: See reference.md Performance Optimization section

---

Quick Reference Commands

Basic Patterns

# Run async function
asyncio.run(main())

# Concurrent execution
results = await asyncio.gather(op1(), op2(), op3())

# Fire and forget
asyncio.create_task(background_work())

# Timeout
result = await asyncio.wait_for(operation(), timeout=5.0)

# Run blocking code
result = await loop.run_in_executor(None, blocking_func)

# Async context manager
async with AsyncResource() as resource:
    await resource.operation()

# Async iteration
async for item in async_iterator():
    await process(item)

Common Libraries

HTTP: aiohttp, httpx Database: asyncpg (PostgreSQL), motor (MongoDB), aiomysql Files: aiofiles Testing: pytest-asyncio, AsyncMock

---

Logging in Async Code

import logging

LOG = logging.getLogger(__name__)

async def async_operation():
    """Async function with logging."""
    LOG.info("Started async operation")

    try:
        result = await fetch_data()
        LOG.debug(f"Fetched {len(result)} items")
        return result
    except Exception as e:
        LOG.error(f"Error in async operation: {e}")
        raise

Note: Standard logging is thread-safe and works fine with asyncio.

---

Real-World Example: API Client

import asyncio
import aiohttp
from typing import Any

class AsyncAPIClient:
    """Async API client with connection pooling and retry."""

    def __init__(self, base_url: str, timeout: float = 30.0):
        self.base_url = base_url
        self.timeout = aiohttp.ClientTimeout(total=timeout)
        self.session = None

    async def __aenter__(self):
        """Create session on context entry."""
        self.session = aiohttp.ClientSession(timeout=self.timeout)
        return self

    async def __aexit__(self, exc_type, exc_val, exc_tb):
        """Close session on context exit."""
        if self.session:
            await self.session.close()

    async def get(self, endpoint: str, **kwargs) -> dict[str, Any]:
        """GET request with automatic retry."""
        url = f"{self.base_url}/{endpoint}"
        return await self._request_with_retry('GET', url, **kwargs)

    async def _request_with_retry(
        self,
        method: str,
        url: str,
        max_retries: int = 3,
        **kwargs
    ) -> dict[str, Any]:
        """Make request with exponential backoff retry."""
        delay = 1.0

        for attempt in range(max_retries):
            try:
                async with self.session.request(method, url, **kwargs) as resp:
                    resp.raise_for_status()
                    return await resp.json()
            except (aiohttp.ClientError, asyncio.TimeoutError) as e:
                if attempt == max_retries - 1:
                    raise
                LOG.warning(f"Request failed (attempt {attempt + 1}): {e}")
                await asyncio.sleep(delay)
                delay *= 2

# Usage
async def main():
    async with AsyncAPIClient("https://api.example.com") as client:
        user = await client.get("users/123")
        posts = await client.get("users/123/posts")
        return user, posts

asyncio.run(main())

For more real-world examples: See reference.md Real-World Examples section

---

Summary: The Rules

1. async def creates coroutine function (must await to execute)
2. await suspends coroutine until operation completes
3. Never block the event loop (use await, not sync calls)
4. asyncio.gather() runs multiple coroutines concurrently
5. create_task() starts background work without waiting
6. wait_for() adds timeout to any coroutine
7. run_in_executor() runs blocking code in thread/process pool
8. Use async context managers for proper resource cleanup
9. Test with pytest-asyncio and AsyncMock
10. Choose wisely: async for I/O, multiprocessing for CPU

---

Troubleshooting Quick Guide

Issue	Cause	Solution
Event loop closed	Multiple asyncio.run() calls	Use single asyncio.run(main())
Coroutine never executed	Missing await	Add await before coroutine call
Not concurrent	Sequential await in loop	Use asyncio.gather()
Slow performance	Blocking event loop	Use async versions or run_in_executor()
Task destroyed warning	Created task not awaited	Await task or track with callback

For detailed troubleshooting: See reference.md Troubleshooting Guide section

---

Bottom line: async is for I/O concurrency, not CPU parallelism. Never block the event loop. Always await awaitable objects.