wehub-resource-sync
446 lines
12 KiB
Markdown
446 lines
12 KiB
Markdown
# async-python-patterns — detailed worked examples
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## Advanced Patterns
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### Pattern 6: Async Context Managers
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```python
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import asyncio
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from typing import Optional
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class AsyncDatabaseConnection:
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"""Async database connection context manager."""
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def __init__(self, dsn: str):
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self.dsn = dsn
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self.connection: Optional[object] = None
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async def __aenter__(self):
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print("Opening connection")
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await asyncio.sleep(0.1) # Simulate connection
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self.connection = {"dsn": self.dsn, "connected": True}
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return self.connection
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async def __aexit__(self, exc_type, exc_val, exc_tb):
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print("Closing connection")
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await asyncio.sleep(0.1) # Simulate cleanup
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self.connection = None
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async def query_database():
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"""Use async context manager."""
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async with AsyncDatabaseConnection("postgresql://localhost") as conn:
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print(f"Using connection: {conn}")
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await asyncio.sleep(0.2) # Simulate query
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return {"rows": 10}
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asyncio.run(query_database())
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```
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### Pattern 7: Async Iterators and Generators
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```python
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import asyncio
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from typing import AsyncIterator
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async def async_range(start: int, end: int, delay: float = 0.1) -> AsyncIterator[int]:
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"""Async generator that yields numbers with delay."""
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for i in range(start, end):
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await asyncio.sleep(delay)
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yield i
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async def fetch_pages(url: str, max_pages: int) -> AsyncIterator[dict]:
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"""Fetch paginated data asynchronously."""
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for page in range(1, max_pages + 1):
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await asyncio.sleep(0.2) # Simulate API call
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yield {
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"page": page,
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"url": f"{url}?page={page}",
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"data": [f"item_{page}_{i}" for i in range(5)]
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}
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async def consume_async_iterator():
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"""Consume async iterator."""
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async for number in async_range(1, 5):
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print(f"Number: {number}")
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print("\nFetching pages:")
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async for page_data in fetch_pages("https://api.example.com/items", 3):
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print(f"Page {page_data['page']}: {len(page_data['data'])} items")
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asyncio.run(consume_async_iterator())
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```
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### Pattern 8: Producer-Consumer Pattern
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```python
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import asyncio
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from asyncio import Queue
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from typing import Optional
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async def producer(queue: Queue, producer_id: int, num_items: int):
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"""Produce items and put them in queue."""
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for i in range(num_items):
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item = f"Item-{producer_id}-{i}"
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await queue.put(item)
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print(f"Producer {producer_id} produced: {item}")
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await asyncio.sleep(0.1)
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await queue.put(None) # Signal completion
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async def consumer(queue: Queue, consumer_id: int):
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"""Consume items from queue."""
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while True:
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item = await queue.get()
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if item is None:
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queue.task_done()
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break
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print(f"Consumer {consumer_id} processing: {item}")
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await asyncio.sleep(0.2) # Simulate work
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queue.task_done()
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async def producer_consumer_example():
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"""Run producer-consumer pattern."""
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queue = Queue(maxsize=10)
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# Create tasks
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producers = [
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asyncio.create_task(producer(queue, i, 5))
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for i in range(2)
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]
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consumers = [
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asyncio.create_task(consumer(queue, i))
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for i in range(3)
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]
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# Wait for producers
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await asyncio.gather(*producers)
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# Wait for queue to be empty
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await queue.join()
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# Cancel consumers
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for c in consumers:
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c.cancel()
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asyncio.run(producer_consumer_example())
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```
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### Pattern 9: Semaphore for Rate Limiting
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```python
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import asyncio
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from typing import List
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async def api_call(url: str, semaphore: asyncio.Semaphore) -> dict:
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"""Make API call with rate limiting."""
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async with semaphore:
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print(f"Calling {url}")
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await asyncio.sleep(0.5) # Simulate API call
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return {"url": url, "status": 200}
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async def rate_limited_requests(urls: List[str], max_concurrent: int = 5):
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"""Make multiple requests with rate limiting."""
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semaphore = asyncio.Semaphore(max_concurrent)
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tasks = [api_call(url, semaphore) for url in urls]
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results = await asyncio.gather(*tasks)
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return results
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async def main():
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urls = [f"https://api.example.com/item/{i}" for i in range(20)]
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results = await rate_limited_requests(urls, max_concurrent=3)
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print(f"Completed {len(results)} requests")
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asyncio.run(main())
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```
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### Pattern 10: Async Locks and Synchronization
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```python
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import asyncio
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class AsyncCounter:
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"""Thread-safe async counter."""
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def __init__(self):
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self.value = 0
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self.lock = asyncio.Lock()
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async def increment(self):
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"""Safely increment counter."""
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async with self.lock:
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current = self.value
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await asyncio.sleep(0.01) # Simulate work
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self.value = current + 1
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async def get_value(self) -> int:
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"""Get current value."""
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async with self.lock:
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return self.value
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async def worker(counter: AsyncCounter, worker_id: int):
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"""Worker that increments counter."""
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for _ in range(10):
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await counter.increment()
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print(f"Worker {worker_id} incremented")
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async def test_counter():
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"""Test concurrent counter."""
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counter = AsyncCounter()
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workers = [asyncio.create_task(worker(counter, i)) for i in range(5)]
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await asyncio.gather(*workers)
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final_value = await counter.get_value()
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print(f"Final counter value: {final_value}")
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asyncio.run(test_counter())
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```
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## Real-World Applications
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### Web Scraping with aiohttp
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```python
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import asyncio
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import aiohttp
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from typing import List, Dict
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async def fetch_url(session: aiohttp.ClientSession, url: str) -> Dict:
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"""Fetch single URL."""
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try:
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async with session.get(url, timeout=aiohttp.ClientTimeout(total=10)) as response:
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text = await response.text()
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return {
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"url": url,
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"status": response.status,
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"length": len(text)
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}
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except Exception as e:
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return {"url": url, "error": str(e)}
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async def scrape_urls(urls: List[str]) -> List[Dict]:
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"""Scrape multiple URLs concurrently."""
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async with aiohttp.ClientSession() as session:
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tasks = [fetch_url(session, url) for url in urls]
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results = await asyncio.gather(*tasks)
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return results
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async def main():
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urls = [
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"https://httpbin.org/delay/1",
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"https://httpbin.org/delay/2",
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"https://httpbin.org/status/404",
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]
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results = await scrape_urls(urls)
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for result in results:
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print(result)
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asyncio.run(main())
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```
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### Async Database Operations
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```python
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import asyncio
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from typing import List, Optional
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# Simulated async database client
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class AsyncDB:
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"""Simulated async database."""
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async def execute(self, query: str) -> List[dict]:
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"""Execute query."""
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await asyncio.sleep(0.1)
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return [{"id": 1, "name": "Example"}]
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async def fetch_one(self, query: str) -> Optional[dict]:
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"""Fetch single row."""
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await asyncio.sleep(0.1)
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return {"id": 1, "name": "Example"}
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async def get_user_data(db: AsyncDB, user_id: int) -> dict:
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"""Fetch user and related data concurrently."""
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user_task = db.fetch_one(f"SELECT * FROM users WHERE id = {user_id}")
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orders_task = db.execute(f"SELECT * FROM orders WHERE user_id = {user_id}")
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profile_task = db.fetch_one(f"SELECT * FROM profiles WHERE user_id = {user_id}")
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user, orders, profile = await asyncio.gather(user_task, orders_task, profile_task)
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return {
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"user": user,
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"orders": orders,
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"profile": profile
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}
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async def main():
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db = AsyncDB()
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user_data = await get_user_data(db, 1)
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print(user_data)
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asyncio.run(main())
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```
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### WebSocket Server
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```python
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import asyncio
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from typing import Set
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# Simulated WebSocket connection
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class WebSocket:
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"""Simulated WebSocket."""
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def __init__(self, client_id: str):
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self.client_id = client_id
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async def send(self, message: str):
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"""Send message."""
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print(f"Sending to {self.client_id}: {message}")
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await asyncio.sleep(0.01)
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async def recv(self) -> str:
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"""Receive message."""
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await asyncio.sleep(1)
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return f"Message from {self.client_id}"
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class WebSocketServer:
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"""Simple WebSocket server."""
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def __init__(self):
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self.clients: Set[WebSocket] = set()
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async def register(self, websocket: WebSocket):
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"""Register new client."""
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self.clients.add(websocket)
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print(f"Client {websocket.client_id} connected")
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async def unregister(self, websocket: WebSocket):
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"""Unregister client."""
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self.clients.remove(websocket)
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print(f"Client {websocket.client_id} disconnected")
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async def broadcast(self, message: str):
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"""Broadcast message to all clients."""
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if self.clients:
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tasks = [client.send(message) for client in self.clients]
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await asyncio.gather(*tasks)
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async def handle_client(self, websocket: WebSocket):
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"""Handle individual client connection."""
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await self.register(websocket)
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try:
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async for message in self.message_iterator(websocket):
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await self.broadcast(f"{websocket.client_id}: {message}")
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finally:
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await self.unregister(websocket)
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async def message_iterator(self, websocket: WebSocket):
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"""Iterate over messages from client."""
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for _ in range(3): # Simulate 3 messages
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yield await websocket.recv()
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```
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## Performance Best Practices
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### 1. Use Connection Pools
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```python
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import asyncio
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import aiohttp
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async def with_connection_pool():
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"""Use connection pool for efficiency."""
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connector = aiohttp.TCPConnector(limit=100, limit_per_host=10)
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async with aiohttp.ClientSession(connector=connector) as session:
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tasks = [session.get(f"https://api.example.com/item/{i}") for i in range(50)]
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responses = await asyncio.gather(*tasks)
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return responses
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```
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### 2. Batch Operations
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```python
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async def batch_process(items: List[str], batch_size: int = 10):
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"""Process items in batches."""
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for i in range(0, len(items), batch_size):
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batch = items[i:i + batch_size]
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tasks = [process_item(item) for item in batch]
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await asyncio.gather(*tasks)
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print(f"Processed batch {i // batch_size + 1}")
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async def process_item(item: str):
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"""Process single item."""
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await asyncio.sleep(0.1)
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return f"Processed: {item}"
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```
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### 3. Avoid Blocking Operations
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Never block the event loop with synchronous operations. A single blocking call stalls all concurrent tasks.
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```python
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# BAD - blocks the entire event loop
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async def fetch_data_bad():
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import time
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import requests
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time.sleep(1) # Blocks!
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response = requests.get(url) # Also blocks!
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# GOOD - use async-native libraries (e.g., httpx for async HTTP)
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import httpx
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async def fetch_data_good(url: str):
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await asyncio.sleep(1)
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async with httpx.AsyncClient() as client:
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response = await client.get(url)
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```
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**Wrapping Blocking Code with `asyncio.to_thread()` (Python 3.9+):**
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When you must use synchronous libraries, offload to a thread pool:
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```python
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import asyncio
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from pathlib import Path
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async def read_file_async(path: str) -> str:
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"""Read file without blocking event loop."""
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# asyncio.to_thread() runs sync code in a thread pool
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return await asyncio.to_thread(Path(path).read_text)
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async def call_sync_library(data: dict) -> dict:
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"""Wrap a synchronous library call."""
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# Useful for sync database drivers, file I/O, CPU work
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return await asyncio.to_thread(sync_library.process, data)
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```
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**Lower-level approach with `run_in_executor()`:**
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```python
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import asyncio
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import concurrent.futures
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from typing import Any
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def blocking_operation(data: Any) -> Any:
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"""CPU-intensive blocking operation."""
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import time
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time.sleep(1)
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return data * 2
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async def run_in_executor(data: Any) -> Any:
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"""Run blocking operation in thread pool."""
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loop = asyncio.get_running_loop()
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with concurrent.futures.ThreadPoolExecutor() as pool:
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result = await loop.run_in_executor(pool, blocking_operation, data)
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return result
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async def main():
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results = await asyncio.gather(*[run_in_executor(i) for i in range(5)])
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print(results)
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asyncio.run(main())
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```
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