[Waterluvian]

Sets are my #1 favourite Python built-in. I used to have these tedious imperative functions for doing change detection on collections. Then I learned sets and it turned into obvious code like:

added = b - a

removed = a - b

repeated = a & b

Etc.

Of course this is set theory and not Python specific. But still. Learn sets!

[ehsankia]

Pycon 2019 about how to use Sets more in your every day code: https://www.youtube.com/watch?v=tGAngdU_8D8

[joatmon-snoo]

frozenset is a really nice one for enforcing immutabolity :)

[ForHackernews]

Frozensets can also be keys in a dictionary.