fixes python-babel#619 wrong weeknumber for 31.12.2018

The weeknumber was calculated to 53, but by definition the value
must compute to 1.

the fix will compute the weeknumber by using date.isocalendar if
locale.first_week_day == 0.

Also the computation of the year format 'YYYY' is replaced by isocalendar.

Tests added.

babel/dates.py

@@ -1319,9 +1319,7 @@ def format_era(self, char, num):
     def format_year(self, char, num):
         value = self.value.year
         if char.isupper():
-            week = self.get_week_number(self.get_day_of_year())
-            if week == 0:
-                value -= 1
+            value = self.value.isocalendar()[0]
         year = self.format(value, num)
         if num == 2:
             year = year[-2:]
@@ -1505,8 +1503,15 @@ def get_week_number(self, day_of_period, day_of_week=None):
         if first_day < 0:
             first_day += 7
         week_number = (day_of_period + first_day - 1) // 7
+
         if 7 - first_day >= self.locale.min_week_days:
             week_number += 1
+
+        if self.locale.first_week_day == 0:
+            max_weeks = date(year=self.value.year, day=28, month=12).isocalendar()[1]
+            if week_number > max_weeks:
+                week_number -= max_weeks
+
         return week_number
 
 

tests/test_dates.py

@@ -79,6 +79,15 @@ def test_week_of_year_last_us_extra_week(self):
         fmt = dates.DateTimeFormat(d, locale='en_US')
         self.assertEqual('53', fmt['w'])
 
+    def test_week_of_year_de_first_us_last_with_year(self):
+        d = date(2018,12,31)
+        fmt = dates.DateTimeFormat(d, locale='de_DE')
+        self.assertEqual('1', fmt['w'])
+        self.assertEqual('2019', fmt['YYYY'])
+        fmt = dates.DateTimeFormat(d, locale='en_US')
+        self.assertEqual('53', fmt['w'])
+        self.assertEqual('2018',fmt['yyyy'])
+
     def test_week_of_month_first(self):
         d = date(2006, 1, 8)
         fmt = dates.DateTimeFormat(d, locale='de_DE')