when would a frozenset be needed?

[buran]

I see, however this well may be an error in the example code, given the comment

# at this point, *numbers* still has 1_000_000 key/values, and
# *numbers_without_997_multiples* is a copy of *numbers* without
# values that are multiples of 997.

i.e. copy definitely has changed

[Skaperen]

so the author did not intend to del twice, didn't realize that 591221 would do that, and should have used just 500000 numbers in his example (or a pair of larger primes)?

[buran]

I cannot be sure, but yes, it really looks like this.