TimLai666 Async Python Patterns
Comprehensive guidance for implementing asynchronous Python applications using asyncio, concurrent programming patterns, and async/await for building high-performance, non-blocking systems.
When to Use This Skill
- Building async web APIs (FastAPI, aiohttp, Sanic)
- Implementing concurrent I/O operations (database, file, network)
- Creating web scrapers with concurrent requests
- Developing real-time applications (WebSocket servers, chat systems)
- Processing multiple independent tasks simultaneously
- Building microservices with async communication
- Optimizing I/O-bound workloads
- Implementing async background tasks and queues
Sync vs Async Decision Guide
Before adopting async, consider whether it's the right choice for your use case.
| Use Case | Recommended Approach |
|---|---|
| Many concurrent network/DB calls | asyncio |
| CPU-bound computation | multiprocessing or thread pool |
| Mixed I/O + CPU | Offload CPU work with asyncio.to_thread() |
| Simple scripts, few connections | Sync (simpler, easier to debug) |
| Web APIs with high concurrency | Async frameworks (FastAPI, aiohttp) |
Key Rule: Stay fully sync or fully async within a call path. Mixing creates hidden blocking and complexity.
Core Concepts
1. Event Loop
The event loop is the heart of asyncio, managing and scheduling asynchronous tasks.
Key characteristics:
- Single-threaded cooperative multitasking
- Schedules coroutines for execution
- Handles I/O operations without blocking
- Manages callbacks and futures
2. Coroutines
Functions defined with async def that can be paused and resumed.
Syntax:
async def my_coroutine():
result = await some_async_operation()
return result
3. Tasks
Scheduled coroutines that run concurrently on the event loop.
4. Futures
Low-level objects representing eventual results of async operations.
5. Async Context Managers
Resources that support async with for proper cleanup.
6. Async Iterators
Objects that support async for for iterating over async data sources.
Quick Start
import asyncio
async def main():
print("Hello")
await asyncio.sleep(1)
print("World")
# Python 3.7+
asyncio.run(main())
Fundamental Patterns
Pattern 1: Basic Async/Await
import asyncio
async def fetch_data(url: str) -> dict:
"""Fetch data from URL asynchronously."""
await asyncio.sleep(1) # Simulate I/O
return {"url": url, "data": "result"}
async def main():
result = await fetch_data("https://api.example.com")
print(result)
asyncio.run(main())
Pattern 2: Concurrent Execution with gather()
import asyncio
from typing import List
async def fetch_user(user_id: int) -> dict:
"""Fetch user data."""
await asyncio.sleep(0.5)
return {"id": user_id, "name": f"User {user_id}"}
async def fetch_all_users(user_ids: List[int]) -> List[dict]:
"""Fetch multiple users concurrently."""
tasks = [fetch_user(uid) for uid in user_ids]
results = await asyncio.gather(*tasks)
return results
async def main():
user_ids = [1, 2, 3, 4, 5]
users = await fetch_all_users(user_ids)
print(f"Fetched {len(users)} users")
asyncio.run(main())
Pattern 3: Task Creation and Management
import asyncio
async def background_task(name: str, delay: int):
"""Long-running background task."""
print(f"{name} started")
await asyncio.sleep(delay)
print(f"{name} completed")
return f"Result from {name}"
async def main():
# Create tasks
task1 = asyncio.create_task(background_task("Task 1", 2))
task2 = asyncio.create_task(background_task("Task 2", 1))
# Do other work
print("Main: doing other work")
await asyncio.sleep(0.5)
# Wait for tasks
result1 = await task1
result2 = await task2
print(f"Results: {result1}, {result2}")
asyncio.run(main())
Pattern 4: Error Handling in Async Code
import asyncio
from typing import List, Optional
async def risky_operation(item_id: int) -> dict:
"""Operation that might fail."""
await asyncio.sleep(0.1)
if item_id % 3 == 0:
raise ValueError(f"Item {item_id} failed")
return {"id": item_id, "status": "success"}
async def safe_operation(item_id: int) -> Optional[dict]:
"""Wrapper with error handling."""
try:
return await risky_operation(item_id)
except ValueError as e:
print(f"Error: {e}")
return None
async def process_items(item_ids: List[int]):
"""Process multiple items with error handling."""
tasks = [safe_operation(iid) for iid in item_ids]
results = await asyncio.gather(*tasks, return_exceptions=True)
# Filter out failures
successful = [r for r in results if r is not None and not isinstance(r, Exception)]
failed = [r for r in results if isinstance(r, Exception)]
print(f"Success: {len(successful)}, Failed: {len(failed)}")
return successful
asyncio.run(process_items([1, 2, 3, 4, 5, 6]))
Pattern 5: Timeout Handling
import asyncio
async def slow_operation(delay: int) -> str:
"""Operation that takes time."""
await asyncio.sleep(delay)
return f"Completed after {delay}s"
async def with_timeout():
"""Execute operation with timeout."""
try:
result = await asyncio.wait_for(slow_operation(5), timeout=2.0)
print(result)
except asyncio.TimeoutError:
print("Operation timed out")
asyncio.run(with_timeout())
Advanced Patterns
Pattern 6: Async Context Managers
import asyncio
from typing import Optional
class AsyncDatabaseConnection:
"""Async database connection context manager."""
def __init__(self, dsn: str):
self.dsn = dsn
self.connection: Optional[object] = None
async def __aenter__(self):
print("Opening connection")
await asyncio.sleep(0.1) # Simulate connection
self.connection = {"dsn": self.dsn, "connected": True}
return self.connection
async def __aexit__(self, exc_type, exc_val, exc_tb):
print("Closing connection")
await asyncio.sleep(0.1) # Simulate cleanup
self.connection = None
async def query_database():
"""Use async context manager."""
async with AsyncDatabaseConnection("postgresql://localhost") as conn:
print(f"Using connection: {conn}")
await asyncio.sleep(0.2) # Simulate query
return {"rows": 10}
asyncio.run(query_database())
Pattern 7: Async Iterators and Generators
import asyncio
from typing import AsyncIterator
async def async_range(start: int, end: int, delay: float = 0.1) -> AsyncIterator[int]:
"""Async generator that yields numbers with delay."""
for i in range(start, end):
await asyncio.sleep(delay)
yield i
async def fetch_pages(url: str, max_pages: int) -> AsyncIterator[dict]:
"""Fetch paginated data asynchronously."""
for page in range(1, max_pages + 1):
await asyncio.sleep(0.2) # Simulate API call
yield {
"page": page,
"url": f"{url}?page={page}",
"data": [f"item_{page}_{i}" for i in range(5)]
}
async def consume_async_iterator():
"""Consume async iterator."""
async for number in async_range(1, 5):
print(f"Number: {number}")
print("\nFetching pages:")
async for page_data in fetch_pages("https://api.example.com/items", 3):
print(f"Page {page_data['page']}: {len(page_data['data'])} items")
asyncio.run(consume_async_iterator())
Pattern 8: Producer-Consumer Pattern
import asyncio
from asyncio import Queue
from typing import Optional
async def producer(queue: Queue, producer_id: int, num_items: int):
"""Produce items and put them in queue."""
for i in range(num_items):
item = f"Item-{producer_id}-{i}"
await queue.put(item)
print(f"Producer {producer_id} produced: {item}")
await asyncio.sleep(0.1)
await queue.put(None) # Signal completion
async def consumer(queue: Queue, consumer_id: int):
"""Consume items from queue."""
while True:
item = await queue.get()
if item is None:
queue.task_done()
break
print(f"Consumer {consumer_id} processing: {item}")
await asyncio.sleep(0.2) # Simulate work
queue.task_done()
async def producer_consumer_example():
"""Run producer-consumer pattern."""
queue = Queue(maxsize=10)
# Create tasks
producers = [
asyncio.create_task(producer(queue, i, 5))
for i in range(2)
]
consumers = [
asyncio.create_task(consumer(queue, i))
for i in range(3)
]
# Wait for producers
await asyncio.gather(*producers)
# Wait for queue to be empty
await queue.join()
# Cancel consumers
for c in consumers:
c.cancel()
asyncio.run(producer_consumer_example())
Pattern 9: Semaphore for Rate Limiting
import asyncio
from typing import List
async def api_call(url: str, semaphore: asyncio.Semaphore) -> dict:
"""Make API call with rate limiting."""
async with semaphore:
print(f"Calling {url}")
await asyncio.sleep(0.5) # Simulate API call
return {"url": url, "status": 200}
async def rate_limited_requests(urls: List[str], max_concurrent: int = 5):
"""Make multiple requests with rate limiting."""
semaphore = asyncio.Semaphore(max_concurrent)
tasks = [api_call(url, semaphore) for url in urls]
results = await asyncio.gather(*tasks)
return results
async def main():
urls = [f"https://api.example.com/item/{i}" for i in range(20)]
results = await rate_limited_requests(urls, max_concurrent=3)
print(f"Completed {len(results)} requests")
asyncio.run(main())
Pattern 10: Async Locks and Synchronization
import asyncio
class AsyncCounter:
"""Thread-safe async counter."""
def __init__(self):
self.value = 0
self.lock = asyncio.Lock()
async def increment(self):
"""Safely increment counter."""
async with self.lock:
current = self.value
await asyncio.sleep(0.01) # Simulate work
self.value = current + 1
async def get_value(self) -> int:
"""Get current value."""
async with self.lock:
return self.value
async def worker(counter: AsyncCounter, worker_id: int):
"""Worker that increments counter."""
for _ in range(10):
await counter.increment()
print(f"Worker {worker_id} incremented")
async def test_counter():
"""Test concurrent counter."""
counter = AsyncCounter()
workers = [asyncio.create_task(worker(counter, i)) for i in range(5)]
await asyncio.gather(*workers)
final_value = await counter.get_value()
print(f"Final counter value: {final_value}")
asyncio.run(test_counter())
Real-World Applications
Web Scraping with aiohttp
import asyncio
import aiohttp
from typing import List, Dict
async def fetch_url(session: aiohttp.ClientSession, url: str) -> Dict:
"""Fetch single URL."""
try:
async with session.get(url, timeout=aiohttp.ClientTimeout(total=10)) as response:
text = await response.text()
return {
"url": url,
"status": response.status,
"length": len(text)
}
except Exception as e:
return {"url": url, "error": str(e)}
async def scrape_urls(urls: List[str]) -> List[Dict]:
"""Scrape multiple URLs concurrently."""
async with aiohttp.ClientSession() as session:
tasks = [fetch_url(session, url) for url in urls]
results = await asyncio.gather(*tasks)
return results
async def main():
urls = [
"https://httpbin.org/delay/1",
"https://httpbin.org/delay/2",
"https://httpbin.org/status/404",
]
results = await scrape_urls(urls)
for result in results:
print(result)
asyncio.run(main())
Async Database Operations
import asyncio
from typing import List, Optional
# Simulated async database client
class AsyncDB:
"""Simulated async database."""
async def execute(self, query: str) -> List[dict]:
"""Execute query."""
await asyncio.sleep(0.1)
return [{"id": 1, "name": "Example"}]
async def fetch_one(self, query: str) -> Optional[dict]:
"""Fetch single row."""
await asyncio.sleep(0.1)
return {"id": 1, "name": "Example"}
async def get_user_data(db: AsyncDB, user_id: int) -> dict:
"""Fetch user and related data concurrently."""
user_task = db.fetch_one(f"SELECT * FROM users WHERE id = {user_id}")
orders_task = db.execute(f"SELECT * FROM orders WHERE user_id = {user_id}")
profile_task = db.fetch_one(f"SELECT * FROM profiles WHERE user_id = {user_id}")
user, orders, profile = await asyncio.gather(user_task, orders_task, profile_task)
return {
"user": user,
"orders": orders,
"profile": profile
}
async def main():
db = AsyncDB()
user_data = await get_user_data(db, 1)
print(user_data)
asyncio.run(main())
WebSocket Server
import asyncio
from typing import Set
# Simulated WebSocket connection
class WebSocket:
"""Simulated WebSocket."""
def __init__(self, client_id: str):
self.client_id = client_id
async def send(self, message: str):
"""Send message."""
print(f"Sending to {self.client_id}: {message}")
await asyncio.sleep(0.01)
async def recv(self) -> str:
"""Receive message."""
await asyncio.sleep(1)
return f"Message from {self.client_id}"
class WebSocketServer:
"""Simple WebSocket server."""
def __init__(self):
self.clients: Set[WebSocket] = set()
async def register(self, websocket: WebSocket):
"""Register new client."""
self.clients.add(websocket)
print(f"Client {websocket.client_id} connected")
async def unregister(self, websocket: WebSocket):
"""Unregister client."""
self.clients.remove(websocket)
print(f"Client {websocket.client_id} disconnected")
async def broadcast(self, message: str):
"""Broadcast message to all clients."""
if self.clients:
tasks = [client.send(message) for client in self.clients]
await asyncio.gather(*tasks)
async def handle_client(self, websocket: WebSocket):
"""Handle individual client connection."""
await self.register(websocket)
try:
async for message in self.message_iterator(websocket):
await self.broadcast(f"{websocket.client_id}: {message}")
finally:
await self.unregister(websocket)
async def message_iterator(self, websocket: WebSocket):
"""Iterate over messages from client."""
for _ in range(3): # Simulate 3 messages
yield await websocket.recv()
Performance Best Practices
1. Use Connection Pools
import asyncio
import aiohttp
async def with_connection_pool():
"""Use connection pool for efficiency."""
connector = aiohttp.TCPConnector(limit=100, limit_per_host=10)
async with aiohttp.ClientSession(connector=connector) as session:
tasks = [session.get(f"https://api.example.com/item/{i}") for i in range(50)]
responses = await asyncio.gather(*tasks)
return responses
2. Batch Operations
async def batch_process(items: List[str], batch_size: int = 10):
"""Process items in batches."""
for i in range(0, len(items), batch_size):
batch = items[i:i + batch_size]
tasks = [process_item(item) for item in batch]
await asyncio.gather(*tasks)
print(f"Processed batch {i // batch_size + 1}")
async def process_item(item: str):
"""Process single item."""
await asyncio.sleep(0.1)
return f"Processed: {item}"
3. Avoid Blocking Operations
Never block the event loop with synchronous operations. A single blocking call stalls all concurrent tasks.
# BAD - blocks the entire event loop
async def fetch_data_bad():
import time
import requests
time.sleep(1) # Blocks!
response = requests.get(url) # Also blocks!
# GOOD - use async-native libraries (e.g., httpx for async HTTP)
import httpx
async def fetch_data_good(url: str):
await asyncio.sleep(1)
async with httpx.AsyncClient() as client:
response = await client.get(url)
Wrapping Blocking Code with asyncio.to_thread() (Python 3.9+):
When you must use synchronous libraries, offload to a thread pool:
import asyncio
from pathlib import Path
async def read_file_async(path: str) -> str:
"""Read file without blocking event loop."""
# asyncio.to_thread() runs sync code in a thread pool
return await asyncio.to_thread(Path(path).read_text)
async def call_sync_library(data: dict) -> dict:
"""Wrap a synchronous library call."""
# Useful for sync database drivers, file I/O, CPU work
return await asyncio.to_thread(sync_library.process, data)
Lower-level approach with run_in_executor():
import asyncio
import concurrent.futures
from typing import Any
def blocking_operation(data: Any) -> Any:
"""CPU-intensive blocking operation."""
import time
time.sleep(1)
return data * 2
async def run_in_executor(data: Any) -> Any:
"""Run blocking operation in thread pool."""
loop = asyncio.get_running_loop()
with concurrent.futures.ThreadPoolExecutor() as pool:
result = await loop.run_in_executor(pool, blocking_operation, data)
return result
async def main():
results = await asyncio.gather(*[run_in_executor(i) for i in range(5)])
print(results)
asyncio.run(main())
Common Pitfalls
1. Forgetting await
# Wrong - returns coroutine object, doesn't execute
result = async_function()
# Correct
result = await async_function()
2. Blocking the Event Loop
# Wrong - blocks event loop
import time
async def bad():
time.sleep(1) # Blocks!
# Correct
async def good():
await asyncio.sleep(1) # Non-blocking
3. Not Handling Cancellation
async def cancelable_task():
"""Task that handles cancellation."""
try:
while True:
await asyncio.sleep(1)
print("Working...")
except asyncio.CancelledError:
print("Task cancelled, cleaning up...")
# Perform cleanup
raise # Re-raise to propagate cancellation
4. Mixing Sync and Async Code
# Wrong - can't call async from sync directly
def sync_function():
result = await async_function() # SyntaxError!
# Correct
def sync_function():
result = asyncio.run(async_function())
Testing Async Code
import asyncio
import pytest
# Using pytest-asyncio
@pytest.mark.asyncio
async def test_async_function():
"""Test async function."""
result = await fetch_data("https://api.example.com")
assert result is not None
@pytest.mark.asyncio
async def test_with_timeout():
"""Test with timeout."""
with pytest.raises(asyncio.TimeoutError):
await asyncio.wait_for(slow_operation(5), timeout=1.0)
Resources
- Python asyncio documentation: https://docs.python.org/3/library/asyncio.html
- aiohttp: Async HTTP client/server
- FastAPI: Modern async web framework
- asyncpg: Async PostgreSQL driver
- motor: Async MongoDB driver
Best Practices Summary
- Use asyncio.run() for entry point (Python 3.7+)
- Always await coroutines to execute them
- Limit concurrency with semaphores - unbounded
gather()can exhaust resources - Implement proper error handling with try/except
- Use timeouts to prevent hanging operations
- Pool connections for better performance
- Never block the event loop - use
asyncio.to_thread()for sync code - Use semaphores for rate limiting external API calls
- Handle task cancellation properly - always re-raise
CancelledError - Test async code with pytest-asyncio
- Stay consistent - fully sync or fully async, avoid mixing