Today I learned about the generator method close.

The generator method close

Python generators have a method called close that “closes” the generator. Closing the generator means that if you call next on it again, it will raise StopIteration, which is essentially the same as if the generator had been exhausted completely.

Here is a trivial example that shows this method in use:

>>> squares = (num ** 2 for num in range(10))
>>> for square in squares:
...     print(square)
...     if square > 20:
...         squares.close()
...
0
1
4
16
25

I learned this from a poster session I attended at EuroPython 2023 by Maxim Danilov.

An infinite loop for that is cancellable

In his poster session, Maxim proposed a really interesting pattern with an infinite loop for that could easily get “cancelled” by calling the method close on the infinite generator. Maxim said there were examples in the standard library where this example could be interesting, but sadly I don't remember exactly where he said we could do it.

Essentially, the pattern uses the infinite iterator iter(int, 1) to bootstrap a generator and then it uses the method close:

import random

infinity = (_ for _ in iter(int, 1))
for _ in infinity:
    print("v")
    if random.random() < 0.05:
        infinity.close()
        print("Loop `for` has been cancelled.")

If you run this piece of code, you will get output like this:

v
v
v
v
v
v
v
Loop `for` has been cancelled.

If I understood correctly, Maxim claimed that there was a common pattern with some infinite loops implemented in terms of while True:. In certain situations, those loops can get messy because of the logic used inside the loop to determine when to use the keyword break!

This is relevant because calling close on the generator that we are iterating over will prevent the next iteration but it will finish running the code of the current iteration. On the other hand, the keyword break will exit the loop altogether as soon as it is encountered, skipping the remainder of the code in the loop.

This was a very specific use case for close, but close can be used with any generator; it doesn't have to be an infinite one.

That's it for now! Stay tuned and I'll see you around!

Become a better Python 🐍 developer 🚀

+35 chapters. +400 pages. Hundreds of examples. Over 30,000 readers!

My book “Pydon'ts” teaches you how to write elegant, expressive, and Pythonic code, to help you become a better developer. >>> Download it here 🐍🚀.