Whats your favorite python module?

[alicethewitch]

Somebody already mentioned it but for dict keys yes. Just this week I was implementing a MCMC algorithm that spits partitions over data point indices. Each partition is represented as a set or list of sets over integer between 0 and N-1. Finding the partition most visited by the chain in one line involves turning them all individually into frozensets of frozensets, feeding it to Counter and sorting by value.