randrange(): Repaired my overly optimistic rewrite, and added comments
explaining what's wrong with the two simpler variants.
diff --git a/Lib/random.py b/Lib/random.py
index 0937ba2..1ae2553 100644
--- a/Lib/random.py
+++ b/Lib/random.py
@@ -148,7 +148,19 @@
if istop != stop:
raise ValueError, "non-integer stop for randrange()"
if step == 1 and istart < istop:
- return int(istart + self.random()*(istop - istart))
+ # Note that
+ # int(istart + self.random()*(istop - istart))
+ # instead would be incorrect. For example, consider istart
+ # = -2 and istop = 0. Then the guts would be in
+ # -2.0 to 0.0 exclusive on both ends (ignoring that random()
+ # might return 0.0), and because int() truncates toward 0, the
+ # final result would be -1 or 0 (instead of -2 or -1).
+ # istart + int(self.random()*(istop - istart))
+ # would also be incorrect, for a subtler reason: the RHS
+ # can return a long, and then randrange() would also return
+ # a long, but we're supposed to return an int (for backward
+ # compatibility).
+ return int(istart + int(self.random()*(istop - istart)))
if step == 1:
raise ValueError, "empty range for randrange()"