gh-122272: guarantee specifiers %F and %C for datetime.strftime to be 0-padded

Lib/_pydatetime.py

@@ -215,6 +215,17 @@ def _need_normalize_century():
             _normalize_century = True
     return _normalize_century
 
+_supports_c99 = None
+def _can_support_c99():
+    global _supports_c99
+    if _supports_c99 is None:
+        try:
+            _supports_c99 = (
+                _time.strftime("%F", (1900, 1, 1, 0, 0, 0, 0, 1, 0)) == "1900-01-01")
+        except ValueError:
+            _supports_c99 = False
+    return _supports_c99
+
 # Correctly substitute for %z and %Z escapes in strftime formats.
 def _wrap_strftime(object, format, timetuple):
     # Don't call utcoffset() or tzname() unless actually needed.
@@ -272,14 +283,20 @@ def _wrap_strftime(object, format, timetuple):
                                 # strftime is going to have at this: escape %
                                 Zreplace = s.replace('%', '%%')
                     newformat.append(Zreplace)
-                elif ch in 'YG' and object.year < 1000 and _need_normalize_century():
-                    # Note that datetime(1000, 1, 1).strftime('%G') == '1000' so
-                    # year 1000 for %G can go on the fast path.
+                # Note that datetime(1000, 1, 1).strftime('%G') == '1000' so
+                # year 1000 for %G can go on the fast path.
+                elif ((ch in 'YG' or ch in 'FC' and _can_support_c99()) and
+                        object.year < 1000 and _need_normalize_century()):
                     if ch == 'G':
                         year = int(_time.strftime("%G", timetuple))
                     else:
                         year = object.year
-                    push('{:04}'.format(year))
+                    if ch == 'C':
+                        push('{:02}'.format(year // 100))
+                    else:
+                        push('{:04}'.format(year))
+                        if ch == 'F':
+                            push('-{:02}-{:02}'.format(*timetuple[1:3]))
                 else:
                     push('%')
                     push(ch)

Lib/test/datetimetester.py

@@ -1710,13 +1710,22 @@ def test_strftime_y2k(self):
             (1000, 0),
             (1970, 0),
         )
-        for year, offset in dataset:
-            for specifier in 'YG':
+        specifiers = 'YG'
+        if _time.strftime('%F', (1900, 1, 1, 0, 0, 0, 0, 1, 0)) == '1900-01-01':
+            specifiers += 'FC'
+        for year, g_offset in dataset:
+            for specifier in specifiers:
                 with self.subTest(year=year, specifier=specifier):
                     d = self.theclass(year, 1, 1)
                     if specifier == 'G':
-                        year += offset
-                    self.assertEqual(d.strftime(f"%{specifier}"), f"{year:04d}")
+                        year += g_offset
+                    if specifier == 'C':
+                        expected = f"{year // 100:02d}"
+                    else:
+                        expected = f"{year:04d}"
+                        if specifier == 'F':
+                            expected += f"-01-01"
+                    self.assertEqual(d.strftime(f"%{specifier}"), expected)
 
     def test_replace(self):
         cls = self.theclass

Misc/NEWS.d/next/Library/2024-07-30-04-27-55.gh-issue-122272.6Wwa1V.rst

@@ -0,0 +1,2 @@
+On some platforms such as Linux, year with century was not 0-padded when formatted by :meth:`~.datetime.strftime` with C99-specific specifiers ``'%C'`` or ``'%F'``. The 0-padding behavior is now guaranteed when the format specifiers ``'%C'`` and ``'%F'`` are supported by the C library.
+Patch by Ben Hsing

Modules/_datetimemodule.c

@@ -1853,7 +1853,12 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
 
 #ifdef Py_NORMALIZE_CENTURY
     /* Buffer of maximum size of formatted year permitted by long. */
-    char buf[SIZEOF_LONG*5/2+2];
+    char buf[SIZEOF_LONG * 5 / 2 + 2
+#ifdef Py_STRFTIME_C99_SUPPORT
+    /* Need 6 more to accomodate dashes, 2-digit month and day for %F. */
+             + 6
+#endif
+    ];
 #endif
 
     assert(object && format && timetuple);
@@ -1950,11 +1955,20 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
             ntoappend = PyBytes_GET_SIZE(freplacement);
         }
 #ifdef Py_NORMALIZE_CENTURY
-        else if (ch == 'Y' || ch == 'G') {
+        else if (ch == 'Y' || ch == 'G'
+#ifdef Py_STRFTIME_C99_SUPPORT
+                 || ch == 'F' || ch == 'C'
+#endif
+        ) {
             /* 0-pad year with century as necessary */
-            PyObject *item = PyTuple_GET_ITEM(timetuple, 0);
+            if (!PyTuple_Check(timetuple)) {
+                goto Done;
+            }
+            PyObject *item = PyTuple_GetItem(timetuple, 0);
+            if (item == NULL) {
+                goto Done;
+            }
             long year_long = PyLong_AsLong(item);
-
             if (year_long == -1 && PyErr_Occurred()) {
                 goto Done;
             }
@@ -1980,8 +1994,16 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
                     goto Done;
                 }
             }
-
-            ntoappend = PyOS_snprintf(buf, sizeof(buf), "%04ld", year_long);
+            ntoappend = PyOS_snprintf(buf, sizeof(buf),
+#ifdef Py_STRFTIME_C99_SUPPORT
+                                      ch == 'F' ? "%04ld-%%m-%%d" :
+#endif
+                                      "%04ld", year_long);
+#ifdef Py_STRFTIME_C99_SUPPORT
+            if (ch == 'C') {
+                ntoappend -= 2;
+            }
+#endif
             ptoappend = buf;
         }
 #endif

configure.ac

@@ -6700,6 +6700,34 @@ then
   [Define if year with century should be normalized for strftime.])
 fi
 
+AC_CACHE_CHECK([whether C99-specific strftime specifiers are supported], [ac_cv_strftime_c99_support], [
+AC_RUN_IFELSE([AC_LANG_SOURCE([[
+#include <time.h>
+#include <string.h>
+
+int main(void)
+{
+  char full_date[11];
+  struct tm date = {
+    .tm_year = 0,
+    .tm_mon = 0,
+    .tm_mday = 1
+  };
+  if (strftime(full_date, sizeof(full_date), "%F", &date) && !strcmp(full_date, "1900-01-01")) {
+    return 0;
+  }
+  return 1;
+}
+]])],
+[ac_cv_strftime_c99_support=yes],
+[ac_cv_strftime_c99_support=no],
+[ac_cv_strftime_c99_support=no])])
+if test "$ac_cv_strftime_c99_support" = yes
+then
+  AC_DEFINE([Py_STRFTIME_C99_SUPPORT], [1],
+  [Define if C99-specific strftime specifiers are supported.])
+fi
+
 dnl check for ncursesw/ncurses and panelw/panel
 dnl NOTE: old curses is not detected.
 dnl have_curses=[no, yes]

pyconfig.h.in

@@ -1698,6 +1698,9 @@
 /* Define if you want to enable internal statistics gathering. */
 #undef Py_STATS
 
+/* Define if C99-specific strftime specifiers are supported. */
+#undef Py_STRFTIME_C99_SUPPORT
+
 /* The version of SunOS/Solaris as reported by `uname -r' without the dot. */
 #undef Py_SUNOS_VERSION