How do I write a sequence of promises in Python?

Here's a similar program using asyncio and the async/await syntax:

import asyncio
import random

async def alpha(x):
    await asyncio.sleep(0.2)
    return x + 1 

async def bravo(x):
    await asyncio.sleep(0.2)
    return random.randint(0, 1000) + x

async def charlie(x):
    if x % 2 == 0:
        return x
    raise ValueError(x, 'is odd')

async def run():
    try:
        number = await charlie(await bravo(await alpha(42)))
    except ValueError as exc:
        print('error:', exc.args[0])
    else:
        print('success:', number)

if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    loop.run_until_complete(run())
    loop.close()

EDIT: If you're interested in reactive streams, you might consider using aiostream.

Here's a simple example:

import asyncio
from aiostream import stream, pipe

async def main():
    # This stream computes 11² + 13² in 1.5 second
    xs = (
        stream.count(interval=0.1)      # Count from zero every 0.1 s
        | pipe.skip(10)                 # Skip the first 10 numbers
        | pipe.take(5)                  # Take the following 5
        | pipe.filter(lambda x: x % 2)  # Keep odd numbers
        | pipe.map(lambda x: x ** 2)    # Square the results
        | pipe.accumulate()             # Add the numbers together
    )
    print('11² + 13² = ', await xs)

if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main())
    loop.close()

More examples in the documentation.

Disclaimer: I am the project maintainer.


You're in luck, Python 3.4 and above include asyncio, although the feature you are looking for (Future) is available in Python 3.5 and up.

From your own link about asyncio: "This version is only relevant for Python 3.3, which does not include asyncio in its stdlib."

Example:

import asyncio


async def some_coroutine():
    await asyncio.sleep(1)
    return 'done'


def process_result(future):
    print('Task returned:', future.result())


loop = asyncio.get_event_loop()
task = loop.create_task(some_coroutine())
task.add_done_callback(process_result)
loop.run_until_complete()