How is set() implemented?

Question

I\'ve seen people say that set objects in python have O(1) membership-checking. How are they implemented internally to allow this? What sort of data structure d

Answer 1

According to this thread:

Indeed, CPython's sets are implemented as something like dictionaries with dummy values (the keys being the members of the set), with some optimization(s) that exploit this lack of values

So basically a set uses a hashtable as its underlying data structure. This explains the O(1) membership checking, since looking up an item in a hashtable is an O(1) operation, on average.

If you are so inclined you can even browse the CPython source code for set which, according to Achim Domma, is mostly a cut-and-paste from the dict implementation.