async-python-patterns

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

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."""
    f

---

*Content truncated.*