Python: Using ‘async for’ to iterate over an asynchronous iterator

Introduction

Recent versions of Python introduce several enhancements that enrich the developer’s toolbox, particularly in asynchronous programming. Among these features, using ‘async for’ to iterate over an asynchronous iterator stands out as a highly useful pattern for handling sequential operations that might involve waiting for external events or data. In this guide, we’ll dive into how to effectively use ‘async for’ in your Python 3.11 (and newer) projects, including practical examples to demonstrate its benefits and subtleties.

First, let’s establish what asynchronous iterators are. In synchronous Python code, an iterator is an object that implements the __iter__() and __next__() methods, allowing you to iterate over its elements in a loop. An asynchronous iterator, on the other hand, implements __aiter__() and __anext__() methods. The key difference is that __anext__() is an asynchronous function, allowing for operations like network requests or file reads to be performed without blocking the executing thread.

Creating an Asynchronous Iterator

To get started, let’s create a simple asynchronous iterator class that yields numbers from 0 to n asynchronously:

class AsyncRange:
    def __init__(self, n):
        self.n = n
        self.i = 0

    async def __aiter__(self):
        return self

    async def __anext__(self):
        if self.i < self.n:
            result = self.i
            self.i += 1
            await asyncio.sleep(1) # Simulating an asynchronous operation
            return result
        else:
            raise StopAsyncIteration

To use this iterator, you’d typically employ an ‘async for’ loop in an asynchronous function:

import asyncio

async def main():
    async for number in AsyncRange(5):
        print(f"Number: {number}")

asyncio.run(main())

This prints numbers 0 through 4, each after waiting for a second, demonstrating how ‘async for’ can be used to process elements of an asynchronous iterator one by one, without blocking.

Handling Asynchronous Iteration Over External Resources

Iterating asynchronously is particularly beneficial when dealing with external resources, such as API calls, database queries, or file system operations. Let’s expand our example to make asynchronous API calls:

import aiohttp
import asyncio

class AsyncAPICaller:
    def __init__(self, urls):
        self.urls = urls

    async def __aiter__(self):
        return self

    async def __anext__(self):
        if not self.urls:
            raise StopAsyncIteration

        url = self.urls.pop(0)
        async with aiohttp.ClientSession() as session:
            async with session.get(url) as response:
                data = await response.text()
                return data

Using this class in an ‘async for’ loop allows you to handle each API response as it arrives, potentially improving the responsiveness and performance of your application:

async def main():
    urls = ["http://example.com", "http://another.example"]
    async for response in AsyncAPICaller(urls):
        print(response)

asyncio.run(main())

Best Practices and Common Pitfalls

While ‘async for’ is a powerful tool, there are best practices to follow and common pitfalls to avoid:

• Ensure all operations within the ‘async for’ body are non-blocking. Mixing synchronous and asynchronous code improperly can nullify the benefits of asynchronous iteration.
• Handle exceptions meticulously. Asynchronous code can be more susceptible to uncaught exceptions due to its non-linear execution flow. Use try-except blocks wisely.
• Be mindful of memory consumption. If your iterator produces a large number of elements, consider implementing backpressure mechanisms or limiting concurrency.

Employing ‘async for’ with asynchronous iterators in Python allows developers to write concise, readable, and efficient asynchronous code. By understanding the mechanics of asynchronous iteration and following best practices, you can leverage this feature to build responsive and high-performance applications.