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

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

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'