Issue #23718: Fixed parsing time in week 0 before Jan 1. Original patch by

Tamás Bence Gedai.

Lib/_strptime.py

@@ -462,6 +462,10 @@ def _strptime(data_string, format="%a %b %d %H:%M:%S %Y"):
         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)
+        if julian <= 0:
+            year -= 1
+            yday = 366 if calendar.isleap(year) else 365
+            julian += yday
     # Cannot pre-calculate datetime_date() since can change in Julian
     # calculation and thus could have different value for the day of the week
     # calculation.

Lib/test/test_strptime.py

@@ -496,14 +496,14 @@ class CalculationTests(unittest.TestCase):
     def test_week_0(self):
         def check(value, format, *expected):
             self.assertEqual(_strptime._strptime_time(value, format)[:-1], expected)
-        check('2015 0 0', '%Y %U %w', 2014, 12, 28, 0, 0, 0, 6, -3)
+        check('2015 0 0', '%Y %U %w', 2014, 12, 28, 0, 0, 0, 6, 362)
         check('2015 0 0', '%Y %W %w', 2015, 1, 4, 0, 0, 0, 6, 4)
-        check('2015 0 1', '%Y %U %w', 2014, 12, 29, 0, 0, 0, 0, -2)
-        check('2015 0 1', '%Y %W %w', 2014, 12, 29, 0, 0, 0, 0, -2)
-        check('2015 0 2', '%Y %U %w', 2014, 12, 30, 0, 0, 0, 1, -1)
-        check('2015 0 2', '%Y %W %w', 2014, 12, 30, 0, 0, 0, 1, -1)
-        check('2015 0 3', '%Y %U %w', 2014, 12, 31, 0, 0, 0, 2, 0)
-        check('2015 0 3', '%Y %W %w', 2014, 12, 31, 0, 0, 0, 2, 0)
+        check('2015 0 1', '%Y %U %w', 2014, 12, 29, 0, 0, 0, 0, 363)
+        check('2015 0 1', '%Y %W %w', 2014, 12, 29, 0, 0, 0, 0, 363)
+        check('2015 0 2', '%Y %U %w', 2014, 12, 30, 0, 0, 0, 1, 364)
+        check('2015 0 2', '%Y %W %w', 2014, 12, 30, 0, 0, 0, 1, 364)
+        check('2015 0 3', '%Y %U %w', 2014, 12, 31, 0, 0, 0, 2, 365)
+        check('2015 0 3', '%Y %W %w', 2014, 12, 31, 0, 0, 0, 2, 365)
         check('2015 0 4', '%Y %U %w', 2015, 1, 1, 0, 0, 0, 3, 1)
         check('2015 0 4', '%Y %W %w', 2015, 1, 1, 0, 0, 0, 3, 1)
         check('2015 0 5', '%Y %U %w', 2015, 1, 2, 0, 0, 0, 4, 2)
@@ -511,6 +511,20 @@ class CalculationTests(unittest.TestCase):
         check('2015 0 6', '%Y %U %w', 2015, 1, 3, 0, 0, 0, 5, 3)
         check('2015 0 6', '%Y %W %w', 2015, 1, 3, 0, 0, 0, 5, 3)
 
+        check('2009 0 0', '%Y %U %w', 2008, 12, 28, 0, 0, 0, 6, 363)
+        check('2009 0 0', '%Y %W %w', 2009, 1, 4, 0, 0, 0, 6, 4)
+        check('2009 0 1', '%Y %U %w', 2008, 12, 29, 0, 0, 0, 0, 364)
+        check('2009 0 1', '%Y %W %w', 2008, 12, 29, 0, 0, 0, 0, 364)
+        check('2009 0 2', '%Y %U %w', 2008, 12, 30, 0, 0, 0, 1, 365)
+        check('2009 0 2', '%Y %W %w', 2008, 12, 30, 0, 0, 0, 1, 365)
+        check('2009 0 3', '%Y %U %w', 2008, 12, 31, 0, 0, 0, 2, 366)
+        check('2009 0 3', '%Y %W %w', 2008, 12, 31, 0, 0, 0, 2, 366)
+        check('2009 0 4', '%Y %U %w', 2009, 1, 1, 0, 0, 0, 3, 1)
+        check('2009 0 4', '%Y %W %w', 2009, 1, 1, 0, 0, 0, 3, 1)
+        check('2009 0 5', '%Y %U %w', 2009, 1, 2, 0, 0, 0, 4, 2)
+        check('2009 0 5', '%Y %W %w', 2009, 1, 2, 0, 0, 0, 4, 2)
+        check('2009 0 6', '%Y %U %w', 2009, 1, 3, 0, 0, 0, 5, 3)
+        check('2009 0 6', '%Y %W %w', 2009, 1, 3, 0, 0, 0, 5, 3)
 
 class CacheTests(unittest.TestCase):
     """Test that caching works properly."""

Misc/ACKS

@@ -484,6 +484,7 @@ Matthieu Gautier
 Stephen M. Gava
 Xavier de Gaye
 Harry Henry Gebel
+Tamás Bence Gedai
 Marius Gedminas
 Jan-Philip Gehrcke
 Thomas Gellekum

Misc/NEWS

@@ -91,6 +91,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #23718: Fixed parsing time in week 0 before Jan 1.  Original patch by
+  Tamás Bence Gedai.
+
 - Issue #20589: Invoking Path.owner() and Path.group() on Windows now raise
   NotImplementedError instead of ImportError.