Remove the random=None nonsense from sample() before it gets set in stone.

It was once available so that faster generators could be substituted.  Now,
that is less necessary and preferrably done via subclassing.

Also, clarified and shortened the comments for sample().

Lib/random.py

@@ -207,7 +207,7 @@ class Random(CoreGenerator):
             j = int(random() * (i+1))
             x[i], x[j] = x[j], x[i]
 
-    def sample(self, population, k, random=None, int=int):
+    def sample(self, population, k, int=int):
         """Chooses k unique random elements from a population sequence.
 
         Returns a new list containing elements from the population while
@@ -223,37 +223,30 @@ class Random(CoreGenerator):
         To choose a sample in a range of integers, use xrange as an argument.
         This is especially fast and space efficient for sampling from a
         large population:   sample(xrange(10000000), 60)
-
-        Optional arg random is a 0-argument function returning a random
-        float in [0.0, 1.0); by default, the standard random.random.
         """
 
         # Sampling without replacement entails tracking either potential
-        # selections (the pool) or previous selections.
+        # selections (the pool) in a list or previous selections in a
+        # dictionary.
 
-        # Pools are stored in lists which provide __getitem__ for selection
-        # and provide a way to remove selections.  But each list.remove()
-        # rebuilds the entire list, so it is better to rearrange the list,
-        # placing non-selected elements at the head of the list.  Tracking
-        # the selection pool is only space efficient with small populations.
-
-        # Previous selections are stored in dictionaries which provide
-        # __contains__ for detecting repeat selections.  Discarding repeats
-        # is efficient unless most of the population has already been chosen.
-        # So, tracking selections is fast only with small sample sizes.
+        # When the number of selections is small compared to the population,
+        # then tracking selections is efficient, requiring only a small
+        # dictionary and an occasional reselection.  For a larger number of
+        # selections, the pool tracking method is preferred since the list takes
+        # less space than the dictionary and it doesn't suffer from frequent
+        # reselections.
 
         n = len(population)
         if not 0 <= k <= n:
             raise ValueError, "sample larger than population"
-        if random is None:
-            random = self.random
+        random = self.random
         result = [None] * k
         if n < 6 * k:     # if n len list takes less space than a k len dict
             pool = list(population)
             for i in xrange(k):         # invariant:  non-selected at [0,n-i)
                 j = int(random() * (n-i))
                 result[i] = pool[j]
-                pool[j] = pool[n-i-1]
+                pool[j] = pool[n-i-1]   # move non-selected item into vacancy
         else:
             selected = {}
             for i in xrange(k):