Chapter 17. Concurrency with Futures

The people bashing threads are typically system programmers which have in mind use cases that the typical application programmer will never encounter in her life. […] In 99% of the use cases an application programmer is likely to run into, the simple pattern of spawning a bunch of independent threads and collecting the results in a queue is everything one needs to know.1

Michele Simionato, Python deep thinker

This chapter focuses on the concurrent.futures library introduced in Python 3.2, but also available for Python 2.5 and newer as the futures package on PyPI. This library encapsulates the pattern described by Michele Simionato in the preceding quote, making it almost trivial to use.

Here I also introduce the concept of “futures”—objects representing the asynchronous execution of an operation. This powerful idea is the foundation not only of concurrent.futures but also of the asyncio package, which we’ll cover in Chapter 18.

We’ll start with a motivating example.

Example: Web Downloads in Three Styles

To handle network I/O efficiently, you need concurrency, as it involves high latency—so instead of wasting CPU cycles waiting, it’s better to do something else until a response comes back from the network.

To make this last point with code, I wrote three simple programs to download images of 20 country flags from the Web. The first one,, runs sequentially: it only requests the next image when the previous one is downloaded and saved ...

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