Change pickling behavior of isocalendar() objects

In order to leave IsocalendarDate as a private class and to improve what
backwards compatibility is offered for pickling the result of a
datetime.isocalendar() call, add a __reduce__ method to the named tuples
that reduces them to plain tuples.

Also add a test to this effect.

Author: pganssle

Lib/datetime.py

@@ -1232,9 +1232,12 @@ def week(self):
     def weekday(self):
         return self[2]
 
+    def __reduce__(self):
+        return (tuple, (tuple(self),))
+
     def __repr__(self):
         return (f'{self.__class__.__name__}'
-            f'(year={self[0]}, week={self[1]}, weekday={self[2]})')
+                f'(year={self[0]}, week={self[1]}, weekday={self[2]})')
 
 
 _tzinfo_class = tzinfo

Lib/test/datetimetester.py

@@ -2,6 +2,7 @@
 
 See http://www.zope.org/Members/fdrake/DateTimeWiki/TestCases
 """
+import io
 import itertools
 import bisect
 import copy
@@ -1380,6 +1381,20 @@ def test_isocalendar(self):
                 self.assertEqual((t.year, t.week, t.weekday), exp_iso)
                 self.assertEqual(d.isocalendar(), exp_iso)
 
+    def test_isocalendar_pickling(self):
+        """Test that the result of datetime.isocalendar() can be pickled.
+
+        The result of a round trip should be a plain tuple.
+        """
+        d = self.theclass(2019, 1, 1)
+        f = io.BytesIO()
+        pickle.dump(d.isocalendar(), f)
+        f.seek(0)
+
+        res = pickle.load(f)
+
+        self.assertEqual(res, (2019, 1, 2))
+
     def test_iso_long_years(self):
         # Calculate long ISO years and compare to table from
         # http://www.phys.uu.nl/~vgent/calendar/isocalendar.htm

Modules/_datetimemodule.c

@@ -3245,9 +3245,53 @@ static PyStructSequence_Desc struct_iso_calendar_date_desc = {
     3
 };
 
-static int initialized;
+static int isocalendardate_initialized;
 static PyTypeObject StructIsoCalendarDateType;
 
+// Required for pickling as a tuple
+static PyObject *
+isocalendardate_reduce(PyObject *self, PyObject *Py_UNUSED(ignored))
+{
+    // Construct the tuple that this reduces to
+    PyObject * reduce_tuple = Py_BuildValue(
+        "O((OOO))", &PyTuple_Type,
+        PyStructSequence_GET_ITEM(self, 0),
+        PyStructSequence_GET_ITEM(self, 1),
+        PyStructSequence_GET_ITEM(self, 2)
+    );
+
+    return reduce_tuple;
+}
+
+/* Method to add our custom reduce type to the IsoCalendarDate type */
+static int
+_initialize_isocalendardate() {
+    if (isocalendardate_initialized) {
+        return 0;
+    }
+
+    if (PyStructSequence_InitType2(&StructIsoCalendarDateType,
+                                   &struct_iso_calendar_date_desc) < 0) {
+        return -1;
+    }
+
+    // Add our custom __reduce__
+    PyMethodDef *method = StructIsoCalendarDateType.tp_methods;
+    int reduce_missing = 1;
+    while (method != NULL) {
+        if (strcmp("__reduce__", method->ml_name) == 0) {
+            reduce_missing = 0;
+            method->ml_meth = (PyCFunction)isocalendardate_reduce;
+            method->ml_doc = "Reduce IsoCalendarDate to a simple tuple";
+
+            break;
+        }
+    }
+
+    assert(!reduce_missing);
+    return 0;
+}
+
 static PyObject *
 date_isocalendar(PyDateTime_Date *self, PyObject *Py_UNUSED(ignored))
 {
@@ -6554,15 +6598,10 @@ PyInit__datetime(void)
     PyModule_AddIntMacro(m, MAXYEAR);
 
     /* IsoCalendarDate */
-    if (!initialized) {
-        if (PyStructSequence_InitType2(&StructIsoCalendarDateType,
-                                       &struct_iso_calendar_date_desc) < 0) {
-            return NULL;
-        }
-        initialized = 1;
+    if (_initialize_isocalendardate()) {
+        return NULL;
     }
     Py_INCREF((PyObject *) &StructIsoCalendarDateType);
-    PyModule_AddObject(m, "IsoCalendarDate", (PyObject *) &StructIsoCalendarDateType);
 
     Py_INCREF(&PyDateTime_DateType);
     PyModule_AddObject(m, "date", (PyObject *) &PyDateTime_DateType);