How to retrieve an element from a set without removing it?

Question

Suppose the following:

>>> s = set([1, 2, 3])

How do I get a value (any value) out of s without doing s.pop()

Answer 1

Since you want a random element, this will also work:

>>> import random
>>> s = set([1,2,3])
>>> random.sample(s, 1)
[2]

The documentation doesn't seem to mention performance of random.sample. From a really quick empirical test with a huge list and a huge set, it seems to be constant time for a list but not for the set. Also, iteration over a set isn't random; the order is undefined but predictable:

>>> list(set(range(10))) == range(10)
True 

If randomness is important and you need a bunch of elements in constant time (large sets), I'd use random.sample and convert to a list first:

>>> lst = list(s) # once, O(len(s))?
...
>>> e = random.sample(lst, 1)[0] # constant time