Fix time.strptime's %U support. Basically rewrote the algorithm to be more

generic so that one only has to shift certain values based on whether the week
was specified to start on Monday or Sunday.  Cut out a lot of edge case code
compared to the previous version.  Also broke algorithm out into its own
function (that is private to the module).

Fixes bug #1643943 (thanks Biran Nahas for the report).

Lib/_strptime.py

@@ -273,6 +273,27 @@ _TimeRE_cache = TimeRE()
 _CACHE_MAX_SIZE = 5 # Max number of regexes stored in _regex_cache
 _regex_cache = {}
 
+def _calc_julian_from_U_or_W(year, week_of_year, day_of_week, week_starts_Mon):
+    """Calculate the Julian day based on the year, week of the year, and day of
+    the week, with week_start_day representing whether the week of the year
+    assumes the week starts on Sunday or Monday (6 or 0)."""
+    first_weekday = datetime_date(year, 1, 1).weekday()
+    # If we are dealing with the %U directive (week starts on Sunday), it's
+    # easier to just shift the view to Sunday being the first day of the
+    # week.
+    if not week_starts_Mon:
+        first_weekday = (first_weekday + 1) % 7
+        day_of_week = (day_of_week + 1) % 7
+    # Need to watch out for a week 0 (when the first day of the year is not
+    # the same as that specified by %U or %W).
+    week_0_length = (7 - first_weekday) % 7
+    if week_of_year == 0:
+        return 1 + day_of_week - first_weekday
+    else:
+        days_to_week = week_0_length + (7 * (week_of_year - 1))
+        return 1 + days_to_week + day_of_week
+
+
 def strptime(data_string, format="%a %b %d %H:%M:%S %Y"):
     """Return a time struct based on the input string and the format string."""
     global _TimeRE_cache, _regex_cache
@@ -385,10 +406,10 @@ def strptime(data_string, format="%a %b %d %H:%M:%S %Y"):
         elif group_key in ('U', 'W'):
             week_of_year = int(found_dict[group_key])
             if group_key == 'U':
-                # U starts week on Sunday
+                # U starts week on Sunday.
                 week_of_year_start = 6
             else:
-                # W starts week on Monday
+                # W starts week on Monday.
                 week_of_year_start = 0
         elif group_key == 'Z':
             # Since -1 is default value only need to worry about setting tz if
@@ -406,42 +427,20 @@ def strptime(data_string, format="%a %b %d %H:%M:%S %Y"):
                         tz = value
                         break
     # If we know the week of the year and what day of that week, we can figure
-    # out the Julian day of the year
-    # Calculations below assume 0 is a Monday
+    # out the Julian day of the year.
     if julian == -1 and week_of_year != -1 and weekday != -1:
-        # Calculate how many days in week 0
-        first_weekday = datetime_date(year, 1, 1).weekday()
-        preceeding_days = 7 - first_weekday
-        if preceeding_days == 7:
-            preceeding_days = 0
-        # Adjust for U directive so that calculations are not dependent on
-        # directive used to figure out week of year
-        if weekday == 6 and week_of_year_start == 6:
-            week_of_year -= 1
-        # If a year starts and ends on a Monday but a week is specified to
-        # start on a Sunday we need to up the week to counter-balance the fact
-        # that with %W that first Monday starts week 1 while with %U that is
-        # week 0 and thus shifts everything by a week
-        if weekday == 0 and first_weekday == 0 and week_of_year_start == 6:
-            week_of_year += 1
-        # If in week 0, then just figure out how many days from Jan 1 to day of
-        # week specified, else calculate by multiplying week of year by 7,
-        # adding in days in week 0, and the number of days from Monday to the
-        # day of the week
-        if week_of_year == 0:
-            julian = 1 + weekday - first_weekday
-        else:
-            days_to_week = preceeding_days + (7 * (week_of_year - 1))
-            julian = 1 + days_to_week + weekday
+        week_starts_Mon = True if week_of_year_start == 0 else False
+        julian = _calc_julian_from_U_or_W(year, week_of_year, weekday,
+                                            week_starts_Mon)
     # Cannot pre-calculate datetime_date() since can change in Julian
-    #calculation and thus could have different value for the day of the week
-    #calculation
+    # calculation and thus could have different value for the day of the week
+    # calculation.
     if julian == -1:
         # Need to add 1 to result since first day of the year is 1, not 0.
         julian = datetime_date(year, month, day).toordinal() - \
                   datetime_date(year, 1, 1).toordinal() + 1
     else:  # Assume that if they bothered to include Julian day it will
-           #be accurate
+           # be accurate.
         datetime_result = datetime_date.fromordinal((julian - 1) + datetime_date(year, 1, 1).toordinal())
         year = datetime_result.year
         month = datetime_result.month

Lib/test/test_strptime.py

@@ -463,6 +463,10 @@ class CalculationTests(unittest.TestCase):
                                         "of the year")
         test_helper((1917, 12, 31), "Dec 31 on Monday with year starting and "
                                         "ending on Monday")
+        test_helper((2007, 01, 07), "First Sunday of 2007")
+        test_helper((2007, 01, 14), "Second Sunday of 2007")
+        test_helper((2006, 12, 31), "Last Sunday of 2006")
+        test_helper((2006, 12, 24), "Second to last Sunday of 2006")
 
 
 class CacheTests(unittest.TestCase):

Misc/NEWS

@@ -123,6 +123,8 @@ Core and builtins
 Library
 -------
 
+- Bug #1643943: Fix time.strptime's support for the %U directive.
+
 - Patch #1643874: memory leak in ctypes fixed.
 
 - Patch #1627441: close sockets properly in urllib2.
@@ -130,12 +132,12 @@ Library
 - Bug #494589: make ntpath.expandvars behave according to its docstring.
 
 - Changed platform module API python_version_tuple() to actually
-  return a tuple (it used to return a list)
+  return a tuple (it used to return a list).
 
 - Added new platform module APIs python_branch(), python_revision(),
-  python_implementation() and linux_distribution()
+  python_implementation() and linux_distribution().
 
-- Added support for IronPython and Jython to the platform module
+- Added support for IronPython and Jython to the platform module.
 
 - The sets module has been deprecated.  Use the built-in set/frozenset types
   instead.