gh-137165: Standardized non-zero-padded date formatting for datetime.strftime

Doc/library/datetime.rst

@@ -2919,6 +2919,17 @@ Notes:
    for  formats ``%d``, ``%m``, ``%H``, ``%I``, ``%M``, ``%S``, ``%j``, ``%U``,
    ``%W``, and ``%V``. Format ``%y`` does require a leading zero.
 
+   For formatting with the :meth:`~.datetime.strftime` method, leading
+   zeroes are included by default for formats ``%d``, ``%m``, ``%H``,
+   ``%I``, ``%M``, ``%S``, ``%j``, ``%U``, ``%W``, ``%V`` and ``%y``.
+   The ``%-`` flag (for example, ``%-d``) produces non-zero-padded
+   output on all platforms.
+
+   .. versionchanged:: 3.16
+      The ``%-`` flag is now supported on all platforms.  It is emulated
+      where the platform C library lacks it, including on Windows, Android
+      and OpenBSD, and for ``%-y`` on Apple platforms, FreeBSD and NetBSD.
+
 (10)
    When parsing a month and day using :meth:`~.datetime.strptime`, always
    include a year in the format.  If the value you need to parse lacks a year,

Lib/_pydatetime.py

@@ -213,6 +213,11 @@ def _need_normalize_century():
             _normalize_century = True
     return _normalize_century
 
+def _make_dash_replacement(ch, timetuple):
+    fmt = '%' + ch
+    val = _time.strftime(fmt, timetuple)
+    return val.lstrip('0') or '0'
+
 # 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.
@@ -291,6 +296,13 @@ def _wrap_strftime(object, format, timetuple):
                         push('{:04}'.format(year))
                         if ch == 'F':
                             push('-{:02}-{:02}'.format(*timetuple[1:3]))
+                # Emulate the %- flag (no zero padding) for the documented
+                # numeric codes, so behavior is consistent on all platforms.
+                # Some libcs lack the flag (Windows, Android, OpenBSD),
+                # others ignore it for %-y (Apple, FreeBSD, NetBSD).
+                elif ch == '-' and i < n and format[i] in 'dmHIMSjUWVy':
+                    push(_make_dash_replacement(format[i], timetuple))
+                    i += 1
                 else:
                     push('%')
                     push(ch)

Lib/test/datetimetester.py

@@ -1598,6 +1598,17 @@ def test_strftime(self):
         self.assertEqual(t.strftime(""), "") # SF bug #761337
         self.assertEqual(t.strftime('x'*1000), 'x'*1000) # SF bug #1556784
 
+        # gh-137165: The %- flag (no zero padding) is emulated for the
+        # documented numeric codes on all platforms, so expectations are
+        # the same everywhere.  In particular, %-y gives "5" here even
+        # though Apple, FreeBSD and NetBSD libcs still zero-pad it natively.
+        self.assertEqual(t.strftime("m:%-m d:%-d y:%-y"), "m:3 d:2 y:5")
+        if sys.platform == 'win32':
+            # Windows' native %# flag still passes through to the platform.
+            self.assertEqual(t.strftime("m:%#m d:%#d y:%#y"), "m:3 d:2 y:5")
+
+        self.assertEqual(t.strftime("%-j. %-U. %-W. %-V."), "61. 9. 9. 9.")
+
         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
@@ -1651,6 +1662,29 @@ def test_strftime_trailing_percent(self):
             _time.strftime("m:03 d:02 y:05 %", t.timetuple()),
         )
 
+    def test_strftime_dash_flag_passthrough(self):
+        # gh-137165: The %- flag is only emulated for the documented numeric
+        # codes.  Anything else after %- is left to the platform's strftime,
+        # so check that datetime's strftime acts the same as time.strftime.
+        t = self.theclass(2005, 3, 2)
+        tt = t.timetuple()
+
+        def outcome(func, *args):
+            try:
+                return func(*args)
+            except ValueError:
+                return ValueError
+
+        for fmt in ['%-', '%-#', '%-e', '%-Y', '%-z', '%-Z', '%-%d', '%-\0',
+                    '%-é', '%-Ť', '%-中']:  # low byte of 'Ť' is 'd'
+            with self.subTest(fmt=fmt):
+                self.assertEqual(outcome(t.strftime, fmt),
+                                 outcome(_time.strftime, fmt, tt))
+
+        # Format codes handled by datetime itself still work after %-.
+        self.assertEqual(outcome(t.strftime, '%-%f'),
+                         outcome(_time.strftime, '%-000000', tt))
+
     def test_format(self):
         dt = self.theclass(2007, 9, 10)
         self.assertEqual(dt.__format__(''), str(dt))
@@ -4115,6 +4149,14 @@ def test_strftime(self):
         # A naive object replaces %z, %:z and %Z with empty strings.
         self.assertEqual(t.strftime("'%z' '%:z' '%Z'"), "'' '' ''")
 
+        # gh-137165: %-H, %-M and %-S are emulated on all platforms.
+        self.assertEqual(t.strftime('%-H %-M %-S %f'), "1 2 3 000004")
+        if sys.platform == 'win32':
+            self.assertEqual(t.strftime('%#H %#M %#S %f'), "1 2 3 000004")
+
+        t_zero = self.theclass(0, 0, 0, 4)
+        self.assertEqual(t_zero.strftime('%-H %-M %-S %f'), "0 0 0 000004")
+
         # bpo-34482: Check that surrogates don't cause a crash.
         try:
             t.strftime('%H\ud800%M')
@@ -4124,6 +4166,15 @@ def test_strftime(self):
         # gh-85432: The parameter was named "fmt" in the pure-Python impl.
         t.strftime(format="%f")
 
+    def test_strftime_dash_flag_12_hour(self):
+        # gh-137165: Midnight and noon are both 12 in the 12-hour clock.
+        for hour, expected in ((0, '12'), (1, '1'), (12, '12'),
+                               (13, '1'), (23, '11')):
+            with self.subTest(hour=hour):
+                t = self.theclass(hour)
+                self.assertEqual(t.strftime('%-I'), expected)
+                self.assertEqual(t.strftime('%I'), expected.zfill(2))
+
     def test_strftime_special(self):
         t = self.theclass(1, 2, 3, 4)
         s1 = t.strftime('%I%p%Z')

Misc/NEWS.d/next/Library/2025-09-18-17-08-08.gh-issue-137165.sJase0.rst

@@ -0,0 +1,3 @@
+Standardized non-zero-padded numeric formatting for dates and times in
+:meth:`datetime.datetime.strftime`, :meth:`datetime.date.strftime` and
+:meth:`datetime.time.strftime` across all platforms.

Modules/_datetimemodule.c

@@ -1876,6 +1876,42 @@ make_freplacement(PyObject *object)
     return PyUnicode_FromString(freplacement);
 }
 
+/* The documented numeric format codes for which the %- flag (no zero
+ * padding) is emulated on all platforms, so behavior is consistent.
+ * Some libcs lack the flag (Windows, Android, OpenBSD), others ignore
+ * it for %-y (Apple, FreeBSD, NetBSD).  Their output is digits only,
+ * so it needs no escaping before being stuffed into the format.
+ */
+static int
+is_dash_emulated(Py_UCS4 ch)
+{
+    switch (ch) {
+        case 'd': case 'm': case 'H': case 'I': case 'M': case 'S':
+        case 'j': case 'U': case 'W': case 'V': case 'y':
+            return 1;
+        default:
+            return 0;
+    }
+}
+
+static PyObject *
+make_dash_replacement(PyObject *strftime, Py_UCS4 ch, PyObject *timetuple)
+{
+    assert(is_dash_emulated(ch));
+    char fmt[3] = {'%', (char)ch, '\0'};
+    PyObject *res = PyObject_CallFunction(strftime, "sO", fmt, timetuple);
+    if (res == NULL) {
+        return NULL;
+    }
+
+    PyObject *stripped = PyObject_CallMethod(res, "lstrip", "s", "0");
+    Py_DECREF(res);
+    if (stripped != NULL && PyUnicode_GET_LENGTH(stripped) == 0) {
+        Py_SETREF(stripped, PyUnicode_FromString("0"));
+    }
+    return stripped;
+}
+
 /* I sure don't want to reproduce the strftime code from the time module,
  * so this imports the module and calls it.  All the hair is due to
  * giving special meanings to the %z, %:z, %Z and %f format codes via a
@@ -1893,6 +1929,7 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
     PyObject *colonzreplacement = NULL; /* py string, replacement for %:z */
     PyObject *Zreplacement = NULL;      /* py string, replacement for %Z */
     PyObject *freplacement = NULL;      /* py string, replacement for %f */
+    PyObject *dash_replacement = NULL;  /* py string, replacement for %- */
 
     assert(object && format && timetuple);
     assert(PyUnicode_Check(format));
@@ -2047,6 +2084,21 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
             }
             continue;
         }
+        else if (ch == '-' && i < flen
+                 && is_dash_emulated(PyUnicode_READ_CHAR(format, i)))
+        {
+            /* %-d etc. -> no zero padding, emulated on all platforms */
+            Py_UCS4 next_ch = PyUnicode_READ_CHAR(format, i);
+            i++;
+
+            Py_XDECREF(dash_replacement);
+            dash_replacement = make_dash_replacement(strftime, next_ch,
+                                                     timetuple);
+            if (dash_replacement == NULL) {
+                goto Error;
+            }
+            replacement = dash_replacement;
+        }
         else {
             /* percent followed by something else */
             continue;
@@ -2085,6 +2137,7 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
     Py_XDECREF(zreplacement);
     Py_XDECREF(colonzreplacement);
     Py_XDECREF(Zreplacement);
+    Py_XDECREF(dash_replacement);
     RELEASE_CURRENT_STATE(st, current_mod);
     return result;