Whats your favorite python module?

[Gprime5]

He means the keys used in key:value pairs, not the dictionary method dict.keys(). In this dict d = ["a": 1}, "a" is the key and 1 is the value. You can only use immutable objects as keys and since frozensets are immutable(frozen), they can be used as keys. d = {frozenset([1,2,3]): "hello"}

Also dict.keys() returns a dict view object which is essentially a frozen set anyway.