bpo-36004: Add date.fromisocalendar (GH-11888)

This commit implements the first version of date.fromisocalendar, the
inverse function for date.isocalendar.

2 files changed, 111 insertions, 0 deletions

diff --git a/Lib/datetime.py b/Lib/datetime.py
index 85bfa48..0e64815 100644
--- a/Lib/datetime.py
+++ b/Lib/datetime.py
@@ -884,6 +884,40 @@ class date:
         except Exception:
             raise ValueError(f'Invalid isoformat string: {date_string!r}')
 
+    @classmethod
+    def fromisocalendar(cls, year, week, day):
+        """Construct a date from the ISO year, week number and weekday.
+
+        This is the inverse of the date.isocalendar() function"""
+        # Year is bounded this way because 9999-12-31 is (9999, 52, 5)
+        if not MINYEAR <= year <= MAXYEAR:
+            raise ValueError(f"Year is out of range: {year}")
+
+        if not 0 < week < 53:
+            out_of_range = True
+
+            if week == 53:
+                # ISO years have 53 weeks in them on years starting with a
+                # Thursday and leap years starting on a Wednesday
+                first_weekday = _ymd2ord(year, 1, 1) % 7
+                if (first_weekday == 4 or (first_weekday == 3 and
+                                           _is_leap(year))):
+                    out_of_range = False
+
+            if out_of_range:
+                raise ValueError(f"Invalid week: {week}")
+
+        if not 0 < day < 8:
+            raise ValueError(f"Invalid weekday: {day} (range is [1, 7])")
+
+        # Now compute the offset from (Y, 1, 1) in days:
+        day_offset = (week - 1) * 7 + (day - 1)
+
+        # Calculate the ordinal day for monday, week 1
+        day_1 = _isoweek1monday(year)
+        ord_day = day_1 + day_offset
+
+        return cls(*_ord2ymd(ord_day))
 
     # Conversions to string
 
@@ -2141,6 +2175,7 @@ def _isoweek1monday(year):
         week1monday += 7
     return week1monday
 
+
 class timezone(tzinfo):
     __slots__ = '_offset', '_name'
 
diff --git a/Lib/test/datetimetester.py b/Lib/test/datetimetester.py
index 617bf9a..9fe32eb 100644
--- a/Lib/test/datetimetester.py
+++ b/Lib/test/datetimetester.py
@@ -1795,6 +1795,82 @@ class TestDate(HarmlessMixedComparison, unittest.TestCase):
             with self.assertRaises(TypeError):
                 self.theclass.fromisoformat(bad_type)
 
+    def test_fromisocalendar(self):
+        # For each test case, assert that fromisocalendar is the
+        # inverse of the isocalendar function
+        dates = [
+            (2016, 4, 3),
+            (2005, 1, 2),       # (2004, 53, 7)
+            (2008, 12, 30),     # (2009, 1, 2)
+            (2010, 1, 2),       # (2009, 53, 6)
+            (2009, 12, 31),     # (2009, 53, 4)
+            (1900, 1, 1),       # Unusual non-leap year (year % 100 == 0)
+            (1900, 12, 31),
+            (2000, 1, 1),       # Unusual leap year (year % 400 == 0)
+            (2000, 12, 31),
+            (2004, 1, 1),       # Leap year
+            (2004, 12, 31),
+            (1, 1, 1),
+            (9999, 12, 31),
+            (MINYEAR, 1, 1),
+            (MAXYEAR, 12, 31),
+        ]
+
+        for datecomps in dates:
+            with self.subTest(datecomps=datecomps):
+                dobj = self.theclass(*datecomps)
+                isocal = dobj.isocalendar()
+
+                d_roundtrip = self.theclass.fromisocalendar(*isocal)
+
+                self.assertEqual(dobj, d_roundtrip)
+
+    def test_fromisocalendar_value_errors(self):
+        isocals = [
+            (2019, 0, 1),
+            (2019, -1, 1),
+            (2019, 54, 1),
+            (2019, 1, 0),
+            (2019, 1, -1),
+            (2019, 1, 8),
+            (2019, 53, 1),
+            (10000, 1, 1),
+            (0, 1, 1),
+            (9999999, 1, 1),
+            (2<<32, 1, 1),
+            (2019, 2<<32, 1),
+            (2019, 1, 2<<32),
+        ]
+
+        for isocal in isocals:
+            with self.subTest(isocal=isocal):
+                with self.assertRaises(ValueError):
+                    self.theclass.fromisocalendar(*isocal)
+
+    def test_fromisocalendar_type_errors(self):
+        err_txformers = [
+            str,
+            float,
+            lambda x: None,
+        ]
+
+        # Take a valid base tuple and transform it to contain one argument
+        # with the wrong type. Repeat this for each argument, e.g.
+        # [("2019", 1, 1), (2019, "1", 1), (2019, 1, "1"), ...]
+        isocals = []
+        base = (2019, 1, 1)
+        for i in range(3):
+            for txformer in err_txformers:
+                err_val = list(base)
+                err_val[i] = txformer(err_val[i])
+                isocals.append(tuple(err_val))
+
+        for isocal in isocals:
+            with self.subTest(isocal=isocal):
+                with self.assertRaises(TypeError):
+                    self.theclass.fromisocalendar(*isocal)
+
+
 #############################################################################
 # datetime tests