Python: How to return a value from a Future

Overview

With the release of Python 3.11, working with asynchronous code has become more intuitive, particularly when it comes to dealing with futures. In this tutorial, we’ll explore how to effectively return a value from a Future object, leveraging the capabilities of Python 3.11 and newer versions. We’ll start from the basics and progressively dive into more advanced topics.

Understanding Futures in Python

Before we dive into returning values, it’s essential to understand what a Future is. In Python’s asyncio module, a Future represents an eventual result of an asynchronous operation. It acts as a placeholder that will eventually hold the actual result.

Initially, a Future can be in one of three states:

• Pending: The operation is still ongoing.
• Finished: The operation is completed, and the result is available.
• Cancelled: The operation was cancelled before completion.

Basic Example

To demonstrate basic usage of returning values from a Future, let’s create a simple asynchronous function that returns a future resolved with a value:

import asyncio

async def compute_square(x):
    return x * x

async def main():
    future = asyncio.ensure_future(compute_square(2))
    result = await future
    print(f'Result: {result}')

asyncio.run(main())

Output: Result: 4

This code snippet provides a simple yet intuitive example of how to work with futures to return values from an asynchronous operation.

Working with asyncio.gather()

The asyncio.gather() function is a powerful tool that allows for executing multiple asynchronous tasks concurrently and returning their results. Here’s how you can use it:

import asyncio

async def fetch_data(id):
    return f'Data for {id}'

async def main():
    results = await asyncio.gather(
        fetch_data(1),
        fetch_data(2),
        fetch_data(3)
    )
    for result in results:
        print(result)

asyncio.run(main())

Output: Data for 1\nData for 2\nData for 3

This demonstrates how asyncio.gather() can be used to return values from multiple futures concurrently.

Using Futures with ThreadPoolExecutor

Sometimes, you may need to execute blocking or time-consuming synchronous functions without blocking the entire application. Python’s concurrent.futures.ThreadPoolExecutor can be used in conjunction with asyncio to achieve this. Here’s how:

import asyncio
from concurrent.futures import ThreadPoolExecutor

async def main():
    with ThreadPoolExecutor(max_workers=3) as executor:
        loop = asyncio.get_running_loop()
        future = loop.run_in_executor(executor, sum, [1, 2, 3])
        result = await future
        print(f'Sum: {result}')

asyncio.run(main())

Output: Sum: 6

This pattern allows you to execute CPU-bound tasks asynchronously, returning their results without blocking the asyncio event loop.

Advanced Usage: Custom Futures

For more complex scenarios, you might find yourself needing to create custom Future objects. This is where the Future class itself comes into play. Here’s an advanced example:

import asyncio

class CustomFuture(asyncio.Future):
    def __init__(self, loop=None):
        super().__init__(loop=loop)
        self._value = None

    def set_result(self, value):
        self._value = value
        super().set_result(True)  # Signal completion with a generic result

async def use_custom_future():
    loop = asyncio.get_event_loop()
    cust_future = CustomFuture(loop=loop)
    cust_future.set_result('Custom value')

    # Await the Future's completion, ignoring its generic result
    await cust_future
    print(f'Custom Future Result: {cust_future._value}')

asyncio.run(use_custom_future())

In this example, we’ve extended the Future class to encapsulate additional logic that might be necessary for your application.

Conclusion

Working with futures in Python has become more accessible and powerful with the release of Python 3.11 and newer versions. By understanding the basics of Future objects and leveraging the provided functionalities, you can efficiently handle asynchronous operations and their results in a pythonic manner.