gh-137165: Unify datetime.strftime behavior for unsupported non-zero-padded specifiers

Lib/_pydatetime.py

@@ -291,6 +291,16 @@ def _wrap_strftime(object, format, timetuple):
                         push('{:04}'.format(year))
                         if ch == 'F':
                             push('-{:02}-{:02}'.format(*timetuple[1:3]))
+                elif ch == '-':
+                    if i < n:
+                        next_ch = format[i]
+                        i += 1
+                        if next_ch not in 'dmHIMSjUWVy':
+                            push('%%-' + next_ch)
+                        else:
+                           push('%-' + next_ch)
+                    else:
+                        push('%-')
                 else:
                     push('%')
                     push(ch)

Lib/test/datetimetester.py

@@ -1598,6 +1598,15 @@ def test_strftime(self):
         self.assertEqual(t.strftime(""), "") # SF bug #761337
         self.assertEqual(t.strftime('x'*1000), 'x'*1000) # SF bug #1556784
 
+        # unsupported %-format specifiers are passed through unchanged.
+        self.assertEqual(t.strftime("%-1"), "%-1")
+        self.assertEqual(t.strftime("%--"), "%--")
+        self.assertEqual(t.strftime("%-#"), "%-#")
+        # non-ASCII specifiers are unsupported too: echo without narrowing.
+        self.assertEqual(t.strftime("%-é"), "%-é")
+        self.assertEqual(t.strftime("%-Ť"), "%-Ť")  # low byte aliases 'd'
+        self.assertEqual(t.strftime("%-中"), "%-中")
+
         self.assertRaises(TypeError, t.strftime) # needs an arg
         self.assertRaises(TypeError, t.strftime, "one", "two") # too many args
         self.assertRaises(TypeError, t.strftime, 42) # arg wrong type

Misc/NEWS.d/next/Library/2026-10-21-02-40-00.gh-issue-137165.QXa3Pg.rst

@@ -0,0 +1,4 @@
+Unify :meth:`datetime.date.strftime`, :meth:`datetime.time.strftime` and
+:meth:`datetime.datetime.strftime` behavior for unsupported non-zero-padded
+(``%-``) format specifiers across platforms: unknown specifiers are now echoed
+back literally instead of raising :exc:`ValueError` or returning partial output.

Modules/_datetimemodule.c

@@ -2047,6 +2047,23 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
             }
             continue;
         }
+        else if (ch == '-' && i < flen) {
+            Py_UCS4 next_ch = PyUnicode_READ_CHAR(format, i);
+            i++;
+
+            /* Only ASCII can name a conversion: strchr() would narrow
+               anything else to a byte, so echo it back literally. */
+            if (next_ch > 127 ||
+                strchr("dmHIMSjUWVy", (int)next_ch) == NULL) {
+                replacement = PyUnicode_FromFormat("%%%%-%c", (int)next_ch);
+                if (replacement == NULL) {
+                    goto Error;
+                }
+            }
+            else {
+                continue;
+            }
+        }
         else {
             /* percent followed by something else */
             continue;