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/Doc/library/datetime.rst b/Doc/library/datetime.rst
index 1ee23c2..abdc977 100644
--- a/Doc/library/datetime.rst
+++ b/Doc/library/datetime.rst
@@ -458,6 +458,13 @@
.. versionadded:: 3.7
+.. classmethod:: date.fromisocalendar(year, week, day)
+
+ Return a :class:`date` corresponding to the ISO calendar date specified by
+ year, week and day. This is the inverse of the function :meth:`date.isocalendar`.
+
+ .. versionadded:: 3.8
+
Class attributes:
@@ -854,6 +861,16 @@
.. versionadded:: 3.7
+
+.. classmethod:: datetime.fromisocalendar(year, week, day)
+
+ Return a :class:`datetime` corresponding to the ISO calendar date specified
+ by year, week and day. The non-date components of the datetime are populated
+ with their normal default values. This is the inverse of the function
+ :meth:`datetime.isocalendar`.
+
+ .. versionadded:: 3.8
+
.. classmethod:: datetime.strptime(date_string, format)
Return a :class:`.datetime` corresponding to *date_string*, parsed according to
diff --git a/Doc/whatsnew/3.8.rst b/Doc/whatsnew/3.8.rst
index 225faf8..ac57335 100644
--- a/Doc/whatsnew/3.8.rst
+++ b/Doc/whatsnew/3.8.rst
@@ -244,6 +244,16 @@
DLL) and paths added by :func:`~os.add_dll_directory`.
+datetime
+--------
+
+Added new alternate constructors :meth:`datetime.date.fromisocalendar` and
+:meth:`datetime.datetime.fromisocalendar`, which construct :class:`date` and
+:class:`datetime` objects respectively from ISO year, week number and weekday;
+these are the inverse of each class's ``isocalendar`` method.
+(Contributed by Paul Ganssle in :issue:`36004`.)
+
+
gettext
-------
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
diff --git a/Misc/NEWS.d/next/Library/2019-02-17-12-55-51.bpo-36004.hCt_KK.rst b/Misc/NEWS.d/next/Library/2019-02-17-12-55-51.bpo-36004.hCt_KK.rst
new file mode 100644
index 0000000..d2162be
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-02-17-12-55-51.bpo-36004.hCt_KK.rst
@@ -0,0 +1,4 @@
+Added new alternate constructors :meth:`datetime.date.fromisocalendar` and
+:meth:`datetime.datetime.fromisocalendar`, which construct date objects from
+ISO year, week number and weekday; these are the inverse of each class's
+``isocalendar`` method. Patch by Paul Ganssle.
diff --git a/Modules/_datetimemodule.c b/Modules/_datetimemodule.c
index b3954c9..83e43a2 100644
--- a/Modules/_datetimemodule.c
+++ b/Modules/_datetimemodule.c
@@ -3003,6 +3003,67 @@
return NULL;
}
+
+static PyObject *
+date_fromisocalendar(PyObject *cls, PyObject *args, PyObject *kw)
+{
+ static char *keywords[] = {
+ "year", "week", "day", NULL
+ };
+
+ int year, week, day;
+ if (PyArg_ParseTupleAndKeywords(args, kw, "iii:fromisocalendar",
+ keywords,
+ &year, &week, &day) == 0) {
+ if (PyErr_ExceptionMatches(PyExc_OverflowError)) {
+ PyErr_Format(PyExc_ValueError,
+ "ISO calendar component out of range");
+
+ }
+ return NULL;
+ }
+
+ // Year is bounded to 0 < year < 10000 because 9999-12-31 is (9999, 52, 5)
+ if (year < MINYEAR || year > MAXYEAR) {
+ PyErr_Format(PyExc_ValueError, "Year is out of range: %d", year);
+ return NULL;
+ }
+
+ if (week <= 0 || week >= 53) {
+ int out_of_range = 1;
+ if (week == 53) {
+ // ISO years have 53 weeks in it on years starting with a Thursday
+ // and on leap years starting on Wednesday
+ int first_weekday = weekday(year, 1, 1);
+ if (first_weekday == 3 || (first_weekday == 2 && is_leap(year))) {
+ out_of_range = 0;
+ }
+ }
+
+ if (out_of_range) {
+ PyErr_Format(PyExc_ValueError, "Invalid week: %d", week);
+ return NULL;
+ }
+ }
+
+ if (day <= 0 || day >= 8) {
+ PyErr_Format(PyExc_ValueError, "Invalid day: %d (range is [1, 7])",
+ day);
+ return NULL;
+ }
+
+ // Convert (Y, W, D) to (Y, M, D) in-place
+ int day_1 = iso_week1_monday(year);
+
+ int month = week;
+ int day_offset = (month - 1)*7 + day - 1;
+
+ ord_to_ymd(day_1 + day_offset, &year, &month, &day);
+
+ return new_date_subclass_ex(year, month, day, cls);
+}
+
+
/*
* Date arithmetic.
*/
@@ -3296,6 +3357,12 @@
METH_CLASS,
PyDoc_STR("str -> Construct a date from the output of date.isoformat()")},
+ {"fromisocalendar", (PyCFunction)(void(*)(void))date_fromisocalendar,
+ METH_VARARGS | METH_KEYWORDS | METH_CLASS,
+ PyDoc_STR("int, int, int -> Construct a date from the ISO year, week "
+ "number and weekday.\n\n"
+ "This is the inverse of the date.isocalendar() function")},
+
{"today", (PyCFunction)date_today, METH_NOARGS | METH_CLASS,
PyDoc_STR("Current date or datetime: same as "
"self.__class__.fromtimestamp(time.time()).")},