[issue46705] Memory optimization for set.issubset

[Jack Nguyen]

New submission from Jack Nguyen <jackyeenguyen at gmail.com>:

I noticed that the set.issubset cpython implementation casts its iterable argument to a set. In some cases, casting the whole iterable to a set is unnecessary (see https://bugs.python.org/issue18032). Although the latter suggestion is to perform early termination, my suggestion is to use the intersection instead.

    # PyAnySet_Check coming from the cpython source code.
    def issubset(self, other):
        # Intersection suggestion:
        if not PyAnySet_Check(other):
            return len(self.intersection(other)) == len(self)
        # Usual implementation for sets.
        else:
            return ...

The main advantage that this implementation has is its memory performance, using only O(min(len(self), len(other))) memory, since it never stores elements it does not need.

I'm assuming that set construction costs O(n) set.__contains__ calls. This implementation uses len(other) calls to self.__contains__ and tmp.__contains__, where tmp = set(other). The current implementation uses len(self) + len(other) calls to tmp.__contains__.

Thus, I suspect the current implementation only has a chance at running noticeably faster when len(self) << len(other), where it performs fewer calls to set.__contains__. This is, however, also where the proposed implementation has significantly superior memory performance.

----------
components: Interpreter Core
messages: 412966
nosy: panda1200
priority: normal
severity: normal
status: open
title: Memory optimization for set.issubset
type: performance
versions: Python 3.10, Python 3.11, Python 3.7, Python 3.8, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue46705>
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