bpo-36004: Add date.fromisocalendar (GH-11888)
This commit implements the first version of date.fromisocalendar, the
inverse function for date.isocalendar.
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 @@
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 @@
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 @@
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