Python: What's the use case for set.pop()?

Question

The Python built-in type set has a method called pop(), from the docs:

Remove and return an arbitrary element from the set. Raises KeyError if the set is empty.

I couldn't think of any use case for this feature, it seems to be an attempt to implement the list interface.

Why is this part of the Python standard?

Answer 1

You'd use it when you have a pool of jobs to process in no particular order. Jobs should only be executed once, but once executed can be added again:

jobs = {job1, job2, job3}

while jobs:
    job = jobs.pop()
    job.process()

where job.process() potentially adds more jobs to the pile. Once all jobs have been processed, the set will be empty and the loop is done.

Or, for a less contrived, real-world example, see the DependencyFinder.find() method from the distlib library (used by pip, for example), which uses a todo set to track distribution providers that still need processing.

The method is a mirror of the dict.popitem() method, which explicitly mentions set algorithms:

popitem() is useful to destructively iterate over a dictionary, as often used in set algorithms.

Before set was added to the language, you'd use a dictionary with None values to emulate a set.