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Gabriel Saldanha
Gabriel Saldanha

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How to write concurrent code using Python's Future

Python futures (concurrency)


Using concurrency to speed up things is quite simple in Python using the concurrent.futures module. However, it's no silver bullet and one must know when to use it.

All started with generators...

In a previous post I mentioned how you could use Python generators as a way to avoid performing extra service calls, saving up some time and resources. Here is a summary:

# Suppose you want to check if a given user_email is registered in 
# any of the following social media: Facebook, Github, or Twitter.

def has_facebook_account(user_email):

def has_github_account(user_email):

def has_twitter_account(user_email):

def has_social_account(user_email):
    calls = [
    return any((call(user_email) for call in calls))
    # (expr for i in iterable) syntax denotes a generator comprehension
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Pretty simple, huh? any iterates and evaluates each call(user_email) yield from the generator until one of them returns True. This is known as early return — you basically save up some time and resources by not performing unnecessary calls.

Some folks gave nice feedback mentioning that I should make it concurrent, i.e., I could make all the calls concurrently and early return as soon as any call returned True. "That's a good idea", I thought. I'm glad there are smarter people than myself out there.

If that's not clear why I would want to do it: suppose has_facebook_account takes too long to run (as usually happens with any I/O and network operations due to high latency) and has_github_account is pretty fast (maybe it's cached, for example). I would always need to wait for has_facebook_account return before calling has_github_account since the generator's items would be evaluated orderly. That does not sound fun.

Make it concurrent!

I am using Python's concurrent.futures module (available since version 3.2). This module consists basically of two entities: the Executor and the Future object. You should read this documentation, it is really short and straight to the point.

The Executor abstract class is responsible for scheduling a task (or callable) to be executed asynchronously (or concurrently). Scheduling a task returns a Future object, which is a reference to the task and represents its state — pending, finished, or canceled.

If you have ever worked with JavaScript Promises before, Future is very similar to a Promise: you know its execution will eventually be done, but you can not know when. The nice thing is: it is non-blocking code, meaning the Python interpreter does not have to wait until the scheduled task's execution finishes before running the next line of code.

Thus, in our scenario, we could schedule three tasks, one for querying each platform (Facebook, GitHub, and Twitter) for a user email address. This way, once any of these tasks eventually returns a value, I can early return if the value is True, since all we want to know is if the user has an account in any of these platforms.

Talk is cheap. Show me the code.

The example code below is easy to follow but I strongly suggest you read the commented lines.

import time  # I will use it to simulate latency with time.sleep
from concurrent.futures import ThreadPoolExecutor, as_completed

def has_facebook_account(user_email):
    time.sleep(5)  # 5 seconds! That is bad.
    print("Finished facebook after 5 seconds!")
    return True

def has_github_account(user_email):
    time.sleep(1)  # 1 second. Phew!
    print("Finished github after 1 second!")
    return True

def has_twitter_account(user_email):
    time.sleep(3)  # Well...
    print("Finished twitter after 3 seconds!")
    return False

# Main method that answers if a user has an account in any of the platforms
def has_social_account(user_email):
    # ThreadPoolExecutor is a subclass of Executor that uses threads.
    # max_workers is the max number of threads that will be used.
    # Since we are scheduling only 3 tasks, it does not make sense to have
    # more than 3 threads, otherwise we would be wasting resources.
    executor = ThreadPoolExecutor(max_workers=3)

    # Schedule (submit) 3 tasks (one for each social account check)
    # .submit method returns a Future object
    facebook_future = executor.submit(has_facebook_account, user_email)
    twitter_future = executor.submit(has_twitter_account, user_email)
    github_future = executor.submit(has_github_account, user_email)
    future_list = [facebook_future, github_future, twitter_future]

    # as_completed receives an iterable of Future objects
    # and yields each future once it has been completed.
    for future in as_completed(future_list):
        # .result() returns the future object return value
        future_return_value = future.result()
        if future_return_value is True:
            # I can early return once any result is True
            return True

user_email = ""
if __name__ == '__main__':
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Finished github after 1 second!
User has social account.
# The created threads will still run until completion
Finished twitter after 3 seconds!
Finished facebook after 5 seconds!
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Notice that even though facebook_future takes longer than the other two scheduled tasks to finish, however, it does not block the execution — it keeps working on its own thread. And although github_future is the last scheduled task, it is the first to finish.

Quick summary

  • Future is an object that represents a scheduled task that will eventually finish.
  • Executor is the scheduler of tasks (once a task is scheduled, it returns a Future object).
    • It can be a ThreadPoolExecutor or a ProcessPoolExecutor (using threads vs processes).
  • One can use executor.submit(callable) to schedule a task to be run asynchronously.
  • as_completed receives an iterable of Future objects and returns a generator where each yielded element is a finished task.

When would I not want to use concurrency then?

As software engineers, our job is not only knowing how to use a tool, but also when to use it. Network operations (and I/O bound operations in general) are usually a good place to use concurrent code due to their latency. But there is always a trade-off...

In the example above we traded off performance for resource usage. How so? When using generators, only the worst-case scenario would end up consuming 3 services — one call for each has_<plataform>_account. That's because we could early return True if any service returned True.

In our new example using concurrency, we are always consuming the 3 service — since the calls are made asynchronously.

"Ah, but that still could save us lots of time!", you say. It depends on the services you're consuming. In the example above I artificially made the has_facebook_account really slow — 5 times slower than the fastest alternative. But, if all the services had a similar response time and if saving resources was important (suppose that calling each service would trigger a really heavy query in the database, for instance), using a synchronous code could be a better approach.

For the sake of data: Facebook has over 2.7 billion monthly active users, while Twitter has around 330 million, and GitHub has merely 40 million users. So, it is highly likely that calling the has_facebook_account first would be enough in a huge majority of scenarios since it would return True with a much higher frequency than the other services, thus, saving lots of unnecessary calls.


Know how to write concurrent code, which is pretty easy with Python Futures. But more important: know when to do so. There are cases where the performance increase does not pay off the resource usage.

I strongly advise you to read the docs on concurrent.futures and the Chapter 17 on Luciano Ramalho's excellent Fluent Python book.

Ah, and here is the gist if you want it!

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