gh-122272: Guarantee specifiers %F and %C for datetime.strftime to be 0-padded (GH-122436)

2 files changed, 34 insertions, 8 deletions

diff --git a/Lib/_pydatetime.py b/Lib/_pydatetime.py
index 27cacb8..78432d4 100644
--- a/Lib/_pydatetime.py
+++ b/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)
diff --git a/Lib/test/datetimetester.py b/Lib/test/datetimetester.py
index 38de110..0265643 100644
--- a/Lib/test/datetimetester.py
+++ b/Lib/test/datetimetester.py
@@ -1710,13 +1710,22 @@ class TestDate(HarmlessMixedComparison, unittest.TestCase):
             (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