Introducing randrange([start,] stop [,step]) -- same as

choice(range(start, stop, step)) but faster.  This addresses the
problem that randint() was accidentally defined as taking an inclusive
range (how unpythonic).

The code is longish because Tim Peters insisted that it reject
non-integral arguments while I insisted that it be not much slower
than randint(); the compromise satisfies both but is somewhat
convoluted.

Also changed randint() to be implemented through randrange().  This is
a semantic change because old randint() didn't test its arguments for
validity.  (It also makes randrange() win any contest with randint()
:-)

diff --git a/Lib/whrandom.py b/Lib/whrandom.py
index 7f33fd60799d4dca632a491a610cc2f0afe4c4bf..3bfd14f820c5aafed3afa1041cf5d48db431f37f 100644 (file)
--- a/Lib/whrandom.py
+++ b/Lib/whrandom.py
@@ -86,14 +86,53 @@ class whrandom:
                return a + (b-a) * self.random()
        #
        # Get a random integer in the range [a, b] including both end points.
+       # (Deprecated; use randrange below.)
        #
        def randint(self, a, b):
-               return a + int(self.random() * (b+1-a))
+               return self.randrange(a, b+1)
        #
        # Choose a random element from a non-empty sequence.
        #
        def choice(self, seq):
                return seq[int(self.random() * len(seq))]
+       #
+       # Choose a random item from range([start,] step[, stop]).
+       # This fixes the problem with randint() which includes the
+       # endpoint; in Python this is usually not what you want.
+       #
+       def randrange(self, start, stop=None, step=1,
+                     # Do not supply the following arguments
+                     int=int, default=None):
+               # This code is a bit messy to make it fast for the
+               # common case while still doing adequate error checking
+               istart = int(start)
+               if istart != start:
+                       raise ValueError, "non-integer arg 1 for randrange()"
+               if stop is default:
+                       if istart > 0:
+                               return int(self.random() * istart)
+                       raise ValueError, "empty range for randrange()"
+               istop = int(stop)
+               if istop != stop:
+                       raise ValueError, "non-integer stop for randrange()"
+               if step == 1:
+                       if istart < istop:
+                               return istart + int(self.random() *
+                                                  (istop - istart))
+                       raise ValueError, "empty range for randrange()"
+               istep = int(step)
+               if istep != step:
+                       raise ValueError, "non-integer step for randrange()"
+               if istep > 0:
+                       n = (istop - istart + istep - 1) / istep
+               elif istep < 0:
+                       n = (istop - istart + istep + 1) / istep
+               else:
+                       raise ValueError, "zero step for randrange()"
+
+               if n <= 0:
+                       raise ValueError, "empty range for randrange()"
+               return istart + istep*int(self.random() * n)
 
 
 # Initialize from the current time
@@ -104,3 +143,4 @@ random = _inst.random
 uniform = _inst.uniform
 randint = _inst.randint
 choice = _inst.choice
+randrange = _inst.randrange