SF bug #801342: Bug (documentation or real, your choice) in random.sample.

random.sample() uses one of two algorithms depending on the ratio of the
sample size to the population size.  One of the algorithms accepted any
iterable population argument so long as it defined __len__().  The other
had a stronger requirement that the population argument be indexable.

While it met the documentation specifications which insisted that the
population argument be a sequence, it made random.sample() less usable
with sets.  So, the second algorithm was modified to coerce non-indexable
iterables and dictionaries into a tuple before proceeding.

Lib/random.py

@@ -258,6 +258,10 @@ class Random(_random.Random):
                 result[i] = pool[j]
                 pool[j] = pool[n-i-1]   # move non-selected item into vacancy
         else:
+            try:
+                n > 0 and (population[0], population[n//2], population[n-1])
+            except (TypeError, KeyError):   # handle sets and dictionaries
+                population = tuple(population)
             selected = {}
             for i in xrange(k):
                 j = _int(random() * n)