I was tinkering around with Python&#39;s `set` and `frozenset` collection types.

Initially, I assumed that `frozenset` would provide a better lookup performance than `set`, as its immutable and thus could exploit the structure of the stored items.

However, this does not seem to be the case, regarding the following experiment:

    import random
    import time
    import sys

    def main(n):
        numbers = []
        for _ in xrange(n):
            numbers.append(random.randint(0, sys.maxint))
        set_ = set(numbers)
        frozenset_ = frozenset(set_)

        start = time.time()
        for number in numbers:
            number in set_
        set_duration = time.time() - start

        start = time.time()
        for number in numbers:
            number in frozenset_
        frozenset_duration = time.time() - start

        print &quot;set      : %.3f&quot; % set_duration
        print &quot;frozenset: %.3f&quot; % frozenset_duration


    if __name__ == &quot;__main__&quot;:
        n = int(sys.argv[1])
        main(n)


I executed this code using both CPython and PyPy, which gave the following results:

    &gt; pypy set.py 100000000
    set      : 6.156
    frozenset: 6.166

    &gt; python set.py 100000000
    set      : 16.824
    frozenset: 17.248

It seems that `frozenset` is actually slower regarding the lookup performance, both in CPython and in PyPy. Does anybody have an idea why this is the case? I did not look into the implementations.

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Set vs. frozenset performance