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

Doc/library/datetime.rst

@@ -2785,6 +2785,13 @@ Notes:
    for  formats ``%d``, ``%m``, ``%H``, ``%I``, ``%M``, ``%S``, ``%j``, ``%U``,
    ``%W``, and ``%V``. Format ``%y`` does require a leading zero.
 
+   When used 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``) will produce non-zero-padded
+   output, except for ``%-y`` on Apple platforms and FreeBSD,
+   which is still zero-padded.
+
 (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.
@@ -284,6 +289,19 @@ 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:
+                            if sys.platform in ['win32', 'android']:
+                                push(_make_dash_replacement(next_ch, timetuple))
+                            else:
+                                push('%-' + next_ch)
+                    else:
+                        push('%-')
                 else:
                     push('%')
                     push(ch)

Lib/test/datetimetester.py

@@ -15,6 +15,7 @@
 import textwrap
 import unittest
 import warnings
+import platform
 
 from array import array
 
@@ -1589,6 +1590,21 @@ def test_strftime(self):
         self.assertEqual(t.strftime(""), "") # SF bug #761337
         self.assertEqual(t.strftime('x'*1000), 'x'*1000) # SF bug #1556784
 
+        # See gh-137165
+        if platform.system() in ("Darwin", "iOS", "FreeBSD"):
+            self.assertEqual(t.strftime("m:%-m d:%-d y:%-y"), "m:3 d:2 y:05")
+        else:
+            if platform.system() == "Windows":
+                self.assertEqual(t.strftime("m:%#m d:%#d y:%#y"), "m:3 d:2 y:5")
+            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.")
+
+        # unsupported %-format specifiers are passed through unchanged.
+        self.assertEqual(t.strftime("%-1"), "%-1")
+        self.assertEqual(t.strftime("%--"), "%--")
+        self.assertEqual(t.strftime("%-#"), "%-#")
+
         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
@@ -4019,6 +4035,14 @@ def test_strftime(self):
         # A naive object replaces %z, %:z and %Z with empty strings.
         self.assertEqual(t.strftime("'%z' '%:z' '%Z'"), "'' '' ''")
 
+        # See gh-137165
+        self.assertEqual(t.strftime('%-H %-M %-S %f'), "1 2 3 000004")
+        if platform.system() == 'Windows':
+            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')

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
+:func:`datetime.datetime.strftime` and :func:`datetime.date.strftime` across
+all platforms.

Modules/_datetimemodule.c

@@ -1857,6 +1857,54 @@ make_freplacement(PyObject *object)
     return PyUnicode_FromString(freplacement);
 }
 
+#if defined(MS_WINDOWS) || defined(__ANDROID__)
+static PyObject *
+make_dash_replacement(PyObject *object, Py_UCS4 ch, PyObject *timetuple)
+{
+    PyObject *strftime = NULL;
+    PyObject *fmt_obj = NULL;
+    PyObject *res = NULL;
+    PyObject *stripped = NULL;
+
+    strftime = PyImport_ImportModuleAttrString("time", "strftime");
+    if (strftime == NULL) {
+        goto error;
+    }
+
+    fmt_obj = PyUnicode_FromFormat("%%%c", (char)ch);
+    if (fmt_obj == NULL) {
+        goto error;
+    }
+
+    res = PyObject_CallFunctionObjArgs(strftime, fmt_obj, timetuple, NULL);
+    if (res == NULL) {
+        goto error;
+    }
+
+    stripped = PyObject_CallMethod(res, "lstrip", "s", "0");
+    if (stripped == NULL) {
+        goto error;
+    }
+
+    if (PyUnicode_GET_LENGTH(stripped) == 0) {
+        Py_DECREF(stripped);
+        stripped = PyUnicode_FromString("0");
+    }
+
+    Py_DECREF(fmt_obj);
+    Py_DECREF(strftime);
+    Py_DECREF(res);
+    return stripped;
+
+error:
+    Py_XDECREF(fmt_obj);
+    Py_XDECREF(strftime);
+    Py_XDECREF(res);
+    Py_XDECREF(stripped);
+    return NULL;
+}
+#endif
+
 /* 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
@@ -1874,6 +1922,7 @@ 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));
@@ -2003,6 +2052,31 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
             }
             continue;
         }
+        /* non-0-pad Windows and Android support */
+        else if (ch == '-' && i < flen) {
+            Py_UCS4 next_ch = PyUnicode_READ_CHAR(format, i);
+            i++;
+
+            if (strchr("dmHIMSjUWVy", (int)next_ch) == NULL) {
+                replacement = PyUnicode_FromFormat("%%%%-%c", (char)next_ch);
+                if (replacement == NULL) {
+                    goto Error;
+                }
+            }
+            else {
+                #if defined(MS_WINDOWS) || defined(__ANDROID__)
+                Py_XDECREF(dash_replacement);
+
+                dash_replacement = make_dash_replacement(object, next_ch, timetuple);
+                if (dash_replacement == NULL) {
+                    goto Error;
+                }
+                replacement = dash_replacement;
+                #else
+                continue;
+                #endif
+            }
+        }
         else {
             /* percent followed by something else */
             continue;
@@ -2041,6 +2115,7 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
     Py_XDECREF(zreplacement);
     Py_XDECREF(colonzreplacement);
     Py_XDECREF(Zreplacement);
+    Py_XDECREF(dash_replacement);
     Py_XDECREF(strftime);
     return result;