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Expand Up		@@ -458,6 +458,13 @@ Other constructors, all class methods:
		.. 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:

Expand Down Expand Up		@@ -854,6 +861,16 @@ Other constructors, all class methods:

		.. 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
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10 changes: 10 additions & 0 deletions 10 Doc/whatsnew/3.8.rst

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Expand Up		@@ -219,6 +219,16 @@ where the DLL is stored (if a full or partial path is used to load the initial
		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
		-------

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35 changes: 35 additions & 0 deletions 35 Lib/datetime.py

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Expand Up		@@ -884,6 +884,40 @@ def fromisoformat(cls, date_string):
		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

Expand Down Expand Up		@@ -2141,6 +2175,7 @@ def _isoweek1monday(year):
		week1monday += 7
		return week1monday


		class timezone(tzinfo):
		__slots__ = '_offset', '_name'

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76 changes: 76 additions & 0 deletions 76 Lib/test/datetimetester.py

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -1795,6 +1795,82 @@ def test_fromisoformat_fails_typeerror(self):
		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),
Comment thread Outdated vstinner Apr 28, 2019 Copy link Copy Markdown Member There was a problem hiding this comment. Choose a reason for hiding this comment The reason will be displayed to describe this comment to others. Learn more. Maybe use MINYEAR/MAXYEAR here. Sorry, something went wrong. Uh oh! There was an error while loading. Please reload this page. All reactions pganssle Apr 29, 2019 Copy link Copy Markdown Member Author There was a problem hiding this comment. Choose a reason for hiding this comment The reason will be displayed to describe this comment to others. Learn more. Instead of converting these over, I have just added a `MINYEAR` and `MAXYEAR` test. Even though it's redundant, I like to have the explicit callout of the current boundaries, so that if `MINYEAR` or `MAXYEAR` get modified in a way that breaks backwards compatibility, it will raise an error. Sorry, something went wrong. Uh oh! There was an error while loading. Please reload this page. All reactions
		(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,
		]
Comment thread Outdated vstinner Apr 28, 2019 Copy link Copy Markdown Member There was a problem hiding this comment. Choose a reason for hiding this comment The reason will be displayed to describe this comment to others. Learn more. Would it be possible to use 3 loops to test all combinations, rather than generate these combinations manualy? Sorry, something went wrong. Uh oh! There was an error while loading. Please reload this page. All reactions pganssle Apr 29, 2019 Copy link Copy Markdown Member Author There was a problem hiding this comment. Choose a reason for hiding this comment The reason will be displayed to describe this comment to others. Learn more. OK, I've converted it over. I think it's a little harder to understand what's going on in the version where the test cases are generated (compared to manual), but using a loop also has its advantages. Sorry, something went wrong. Uh oh! There was an error while loading. Please reload this page. All reactions

		# 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

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4 changes: 4 additions & 0 deletions 4 Misc/NEWS.d/next/Library/2019-02-17-12-55-51.bpo-36004.hCt_KK.rst

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Original file line number	Diff line number	Diff line change
		@@ -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.

67 changes: 67 additions & 0 deletions 67 Modules/_datetimemodule.c

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -3003,6 +3003,67 @@ date_fromisoformat(PyObject cls, PyObject dtstr)
		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.
		*/
Expand Down Expand Up		@@ -3296,6 +3357,12 @@ static PyMethodDef date_methods[] = {
		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()).")},
Expand Down

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bpo-36004: Add date.fromisocalendar #11888

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vstinner Apr 28, 2019

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