async-python-patterns

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())

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())

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())

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]))

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())

# Wrong - returns coroutine object, doesn't execute
result = async_function()
 
# Correct
result = await async_function()

# Wrong - blocks event loop
import time
async def bad():
    time.sleep(1)  # Blocks!
 
# Correct
async def good():
    await asyncio.sleep(1)  # Non-blocking

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

# 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())

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)