Switch to new method of detecting ISO long years

This is equivalent but uses only existing helper functions and in many cases will be slightly more efficient.
python · vstinner · Apr 29, 2019 · Feb 15, 2019 · Feb 17, 2019 · Feb 17, 2019
commit 86edee5eda5acb9975a6c1b8d20322edab4cc24a
diff --git a/Lib/datetime.py b/Lib/datetime.py
@@ -891,9 +891,17 @@ def fromisocalendar(cls, year, week, day):
             raise ValueError(f"Year is out of range: {year}")
 
         if not 0 < week < 53:
-            if not (week == 53 and
-                    _iso_long_year_helper(year) == 4 or
+            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:
@@ -2153,10 +2161,6 @@ def __reduce__(self):
 datetime.resolution = timedelta(microseconds=1)
 
 
-def _iso_long_year_helper(year):
-    return (year + (year // 4) - (year // 100) + (year // 400)) % 7
-
-
 def _isoweek1monday(year):
     # Helper to calculate the day number of the Monday starting week 1
     # XXX This could be done more efficiently
@@ -2500,7 +2504,7 @@ def _name_from_offset(delta):
          _format_time, _format_offset, _is_leap, _isoweek1monday, _math,
          _ord2ymd, _time, _time_class, _tzinfo_class, _wrap_strftime, _ymd2ord,
          _divide_and_round, _parse_isoformat_date, _parse_isoformat_time,
-         _parse_hh_mm_ss_ff, _iso_long_year_helper)
+         _parse_hh_mm_ss_ff)
     # XXX Since import * above excludes names that start with _,
     # docstring does not get overwritten. In the future, it may be
     # appropriate to maintain a single module level docstring and

diff --git a/Modules/_datetimemodule.c b/Modules/_datetimemodule.c
@@ -3004,12 +3004,9 @@ date_fromisoformat(PyObject *cls, PyObject *dtstr)
 }
 
 
-static int _iso_long_year_helper(int year) {
-    return ((year + (year / 4) - (year / 100) + (year / 400)) % 7);
-}
-
 static PyObject *
-date_fromisocalendar(PyObject *cls, PyObject *args, PyObject *kw) {
+date_fromisocalendar(PyObject *cls, PyObject *args, PyObject *kw)
+{
     static char *keywords[] = {
         "year", "week", "day", NULL
     };
@@ -3033,9 +3030,17 @@ date_fromisocalendar(PyObject *cls, PyObject *args, PyObject *kw) {
     }
 
     if (week <= 0 || week >= 53) {
-        if (!(week == 53 &&
-             (_iso_long_year_helper(year) == 4 ||
-              _iso_long_year_helper(year - 1) == 3))) {
+        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;
         }