Note (Oct. 2020): as of v3.9, Python has officially deprecated random.sample() working on sets, with the official guidance being to explicitly convert the set to a list or tuple before passing it in, though this doesn’t solve the efficiency problems.
>>> random.sample(set('abcdefghijklmnopqrstuvwxyz'), 1)
['f']
Documentation: https://docs.python.org/3/library/random.html#random.sample
Note that choosing random elements from a set is extremely inefficient no matter how you do it – it takes time proportional to the size of the set, or worse if the set’s underlying hash table is sparse due to removed elements.
Instead, you should probably use a different data structure that supports this operation efficiently.