Index: calendar.py
===================================================================
--- calendar.py	(revision 85658)
+++ calendar.py	(working copy)
@@ -95,8 +95,10 @@
 
 
 def isleap(year):
-    """Return 1 for leap years, 0 for non-leap years."""
-    return year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)
+    """Return True for leap years, False for non-leap years."""
+    if year == 0:
+        raise ValueError('year 0 does not exist')
+    return (year % 4 == 0) and (year % 100 != 0) or (year % 400 == 0)
 
 
 def leapdays(y1, y2):