Fix incorrect iso week-years (w)

babel/dates.py

@@ -1439,6 +1439,16 @@ def format_week(self, char: str, num: int) -> str:
                 date = self.value - datetime.timedelta(days=day_of_year)
                 week = self.get_week_number(self.get_day_of_year(date),
                                             date.weekday())
+            elif self.locale.min_week_days == 4 and self.locale.first_week_day == 0:
+                # HACK: EOY days being punted into next year can occur
+                #       in all week numbering situations, it's the
+                #       reverse (underflowing the current year) which
+                #       can't (requires min_week_days > 1, which is
+                #       only widely used by ISO calendaring), so
+                #       checking for ISO-ish locales is not correct
+                _, num_weeks, _ = self.value.replace(month=12, day=28).isocalendar()
+                if week > num_weeks:
+                    week = 1
             return self.format(week, num)
         else:  # week of month
             week = self.get_week_number(self.value.day)
@@ -1637,16 +1647,6 @@ def get_week_number(self, day_of_period: int, day_of_week: int | None = None) ->
         if 7 - first_day >= self.locale.min_week_days:
             week_number += 1
 
-        if self.locale.first_week_day == 0:
-            # Correct the weeknumber in case of iso-calendar usage (first_week_day=0).
-            # If the weeknumber exceeds the maximum number of weeks for the given year
-            # we must count from zero.For example the above calculation gives week 53
-            # for 2018-12-31. By iso-calender definition 2018 has a max of 52
-            # weeks, thus the weeknumber must be 53-52=1.
-            max_weeks = datetime.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

@@ -48,30 +48,18 @@ def test_abbreviated_month_alias(self):
 
     def test_week_of_year_first(self):
         d = date(2006, 1, 8)
-        assert dates.DateTimeFormat(d, locale='de_DE')['w'] == '1'
         assert dates.DateTimeFormat(d, locale='en_US')['ww'] == '02'
 
-    def test_week_of_year_first_with_year(self):
-        d = date(2006, 1, 1)
-        fmt = dates.DateTimeFormat(d, locale='de_DE')
-        assert fmt['w'] == '52'
-        assert fmt['YYYY'] == '2005'
-
     def test_week_of_year_last(self):
         d = date(2006, 12, 26)
-        assert dates.DateTimeFormat(d, locale='de_DE')['w'] == '52'
         assert dates.DateTimeFormat(d, locale='en_US')['w'] == '52'
 
     def test_week_of_year_last_us_extra_week(self):
         d = date(2005, 12, 26)
-        assert dates.DateTimeFormat(d, locale='de_DE')['w'] == '52'
         assert dates.DateTimeFormat(d, locale='en_US')['w'] == '53'
 
     def test_week_of_year_de_first_us_last_with_year(self):
         d = date(2018, 12, 31)
-        fmt = dates.DateTimeFormat(d, locale='de_DE')
-        assert fmt['w'] == '1'
-        assert fmt['YYYY'] == '2019'
         fmt = dates.DateTimeFormat(d, locale='en_US')
         assert fmt['w'] == '53'
         assert fmt['yyyy'] == '2018'
@@ -742,3 +730,15 @@ def test_en_gb_first_weekday():
 
 def test_issue_798():
     assert dates.format_timedelta(timedelta(), format='narrow', locale='es_US') == '0s'
+
+# 200 years + 20 days / year (-10 +10) ~ 4000 tests
+@pytest.mark.parametrize('date,weekyear,week', [
+    pytest.param(day, *day.isocalendar()[:2], id=str(day))
+    for year in range(1900, 2101)
+    # check +- 10 days around jan 1st
+    for d in range(-10, 11)
+    for day in [date(year, 1, 1) + timedelta(days=d)]
+])
+def test_iso_week_exhaustive(date, weekyear, week):
+    fmt = dates.DateTimeFormat(date, locale='de_DE')
+    assert (fmt['YYYY'], fmt['w']) == (str(weekyear), str(week))

tox.ini

@@ -18,7 +18,7 @@ deps =
 allowlist_externals = make
 commands = make clean-cldr test
 setenv =
-    PYTEST_FLAGS=--cov=babel --cov-report=xml:{env:COVERAGE_XML_PATH:.coverage_cache}/coverage.{envname}.xml
+    PYTEST_FLAGS={env:PYTEST_FLAGS:} --cov=babel --cov-report=xml:{env:COVERAGE_XML_PATH:.coverage_cache}/coverage.{envname}.xml
 passenv =
     BABEL_*
     PYTEST_*