gh-69142: add %:z strftime format code (gh-95983)

datetime.isoformat generates the tzoffset with colons, but there
was no format code to make strftime output the same format.

for simplicity and consistency the %:z formatting behaves mostly
as %z, with the exception of adding colons. this includes the
dynamic behaviour of adding seconds and microseconds only when
needed (when not 0).

this fixes the still open "generate" part of this issue:

https://github.com/python/cpython/issues/69142

Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>

5 files changed, 92 insertions, 55 deletions

diff --git a/Doc/library/datetime.rst b/Doc/library/datetime.rst
index 0f04237..c3a66a4 100644
--- a/Doc/library/datetime.rst
+++ b/Doc/library/datetime.rst
@@ -2443,6 +2443,11 @@ convenience. These parameters all correspond to ISO 8601 date values.
 |           | Week 01 is the week containing |                        |       |
 |           | Jan 4.                         |                        |       |
 +-----------+--------------------------------+------------------------+-------+
+| ``%:z``   | UTC offset in the form         | (empty), +00:00,       | \(6)  |
+|           | ``±HH:MM[:SS[.ffffff]]``       | -04:00, +10:30,        |       |
+|           | (empty string if the object is | +06:34:15,             |       |
+|           | naive).                        | -03:07:12.345216       |       |
++-----------+--------------------------------+------------------------+-------+
 
 These may not be available on all platforms when used with the :meth:`strftime`
 method. The ISO 8601 year and ISO 8601 week directives are not interchangeable
@@ -2458,6 +2463,9 @@ differences between platforms in handling of unsupported format specifiers.
 .. versionadded:: 3.6
    ``%G``, ``%u`` and ``%V`` were added.
 
+.. versionadded:: 3.12
+   ``%:z`` was added.
+
 Technical Detail
 ^^^^^^^^^^^^^^^^
 
@@ -2530,8 +2538,8 @@ Notes:
    available).
 
 (6)
-   For a naive object, the ``%z`` and ``%Z`` format codes are replaced by empty
-   strings.
+   For a naive object, the ``%z``, ``%:z`` and ``%Z`` format codes are replaced
+   by empty strings.
 
    For an aware object:
 
@@ -2557,6 +2565,10 @@ Notes:
       For example, ``'+01:00:00'`` will be parsed as an offset of one hour.
       In addition, providing ``'Z'`` is identical to ``'+00:00'``.
 
+   ``%:z``
+      Behaves exactly as ``%z``, but has a colon separator added between
+      hours, minutes and seconds.
+
    ``%Z``
       In :meth:`strftime`, ``%Z`` is replaced by an empty string if
       :meth:`tzname` returns ``None``; otherwise ``%Z`` is replaced by the
diff --git a/Lib/datetime.py b/Lib/datetime.py
index 00ded32..007114a 100644
--- a/Lib/datetime.py
+++ b/Lib/datetime.py
@@ -179,7 +179,7 @@ def _format_time(hh, mm, ss, us, timespec='auto'):
     else:
         return fmt.format(hh, mm, ss, us)
 
-def _format_offset(off):
+def _format_offset(off, sep=':'):
     s = ''
     if off is not None:
         if off.days < 0:
@@ -189,9 +189,9 @@ def _format_offset(off):
             sign = "+"
         hh, mm = divmod(off, timedelta(hours=1))
         mm, ss = divmod(mm, timedelta(minutes=1))
-        s += "%s%02d:%02d" % (sign, hh, mm)
+        s += "%s%02d%s%02d" % (sign, hh, sep, mm)
         if ss or ss.microseconds:
-            s += ":%02d" % ss.seconds
+            s += "%s%02d" % (sep, ss.seconds)
 
             if ss.microseconds:
                 s += '.%06d' % ss.microseconds
@@ -202,9 +202,10 @@ def _wrap_strftime(object, format, timetuple):
     # Don't call utcoffset() or tzname() unless actually needed.
     freplace = None  # the string to use for %f
     zreplace = None  # the string to use for %z
+    colonzreplace = None  # the string to use for %:z
     Zreplace = None  # the string to use for %Z
 
-    # Scan format for %z and %Z escapes, replacing as needed.
+    # Scan format for %z, %:z and %Z escapes, replacing as needed.
     newformat = []
     push = newformat.append
     i, n = 0, len(format)
@@ -222,26 +223,28 @@ def _wrap_strftime(object, format, timetuple):
                     newformat.append(freplace)
                 elif ch == 'z':
                     if zreplace is None:
-                        zreplace = ""
                         if hasattr(object, "utcoffset"):
-                            offset = object.utcoffset()
-                            if offset is not None:
-                                sign = '+'
-                                if offset.days < 0:
-                                    offset = -offset
-                                    sign = '-'
-                                h, rest = divmod(offset, timedelta(hours=1))
-                                m, rest = divmod(rest, timedelta(minutes=1))
-                                s = rest.seconds
-                                u = offset.microseconds
-                                if u:
-                                    zreplace = '%c%02d%02d%02d.%06d' % (sign, h, m, s, u)
-                                elif s:
-                                    zreplace = '%c%02d%02d%02d' % (sign, h, m, s)
-                                else:
-                                    zreplace = '%c%02d%02d' % (sign, h, m)
+                            zreplace = _format_offset(object.utcoffset(), sep="")
+                        else:
+                            zreplace = ""
                     assert '%' not in zreplace
                     newformat.append(zreplace)
+                elif ch == ':':
+                    if i < n:
+                        ch2 = format[i]
+                        i += 1
+                        if ch2 == 'z':
+                            if colonzreplace is None:
+                                if hasattr(object, "utcoffset"):
+                                    colonzreplace = _format_offset(object.utcoffset(), sep=":")
+                                else:
+                                    colonzreplace = ""
+                            assert '%' not in colonzreplace
+                            newformat.append(colonzreplace)
+                        else:
+                            push('%')
+                            push(ch)
+                            push(ch2)
                 elif ch == 'Z':
                     if Zreplace is None:
                         Zreplace = ""
diff --git a/Lib/test/datetimetester.py b/Lib/test/datetimetester.py
index 7e7f4f3..bba9669 100644
--- a/Lib/test/datetimetester.py
+++ b/Lib/test/datetimetester.py
@@ -1463,8 +1463,8 @@ class TestDate(HarmlessMixedComparison, unittest.TestCase):
         # test that unicode input is allowed (issue 2782)
         self.assertEqual(t.strftime("%m"), "03")
 
-        # A naive object replaces %z and %Z w/ empty strings.
-        self.assertEqual(t.strftime("'%z' '%Z'"), "'' ''")
+        # A naive object replaces %z, %:z and %Z w/ empty strings.
+        self.assertEqual(t.strftime("'%z' '%:z' '%Z'"), "'' '' ''")
 
         #make sure that invalid format specifiers are handled correctly
         #self.assertRaises(ValueError, t.strftime, "%e")
@@ -1528,7 +1528,7 @@ class TestDate(HarmlessMixedComparison, unittest.TestCase):
 
         for fmt in ["m:%m d:%d y:%y",
                     "m:%m d:%d y:%y H:%H M:%M S:%S",
-                    "%z %Z",
+                    "%z %:z %Z",
                     ]:
             self.assertEqual(dt.__format__(fmt), dt.strftime(fmt))
             self.assertEqual(a.__format__(fmt), dt.strftime(fmt))
@@ -2134,7 +2134,7 @@ class TestDateTime(TestDate):
 
         for fmt in ["m:%m d:%d y:%y",
                     "m:%m d:%d y:%y H:%H M:%M S:%S",
-                    "%z %Z",
+                    "%z %:z %Z",
                     ]:
             self.assertEqual(dt.__format__(fmt), dt.strftime(fmt))
             self.assertEqual(a.__format__(fmt), dt.strftime(fmt))
@@ -2777,6 +2777,7 @@ class TestDateTime(TestDate):
             tz = timezone(-timedelta(hours=2, seconds=s, microseconds=us))
             t = t.replace(tzinfo=tz)
             self.assertEqual(t.strftime("%z"), "-0200" + z)
+            self.assertEqual(t.strftime("%:z"), "-02:00:" + z)
 
         # bpo-34482: Check that surrogates don't cause a crash.
         try:
@@ -3515,8 +3516,8 @@ class TestTime(HarmlessMixedComparison, unittest.TestCase):
     def test_strftime(self):
         t = self.theclass(1, 2, 3, 4)
         self.assertEqual(t.strftime('%H %M %S %f'), "01 02 03 000004")
-        # A naive object replaces %z and %Z with empty strings.
-        self.assertEqual(t.strftime("'%z' '%Z'"), "'' ''")
+        # A naive object replaces %z, %:z and %Z with empty strings.
+        self.assertEqual(t.strftime("'%z' '%:z' '%Z'"), "'' '' ''")
 
         # bpo-34482: Check that surrogates don't cause a crash.
         try:
@@ -3934,10 +3935,10 @@ class TestTimeTZ(TestTime, TZInfoBase, unittest.TestCase):
         self.assertEqual(repr(t4), d + "(0, 0, 0, 40)")
         self.assertEqual(repr(t5), d + "(0, 0, 0, 40, tzinfo=utc)")
 
-        self.assertEqual(t1.strftime("%H:%M:%S %%Z=%Z %%z=%z"),
-                                     "07:47:00 %Z=EST %z=-0500")
-        self.assertEqual(t2.strftime("%H:%M:%S %Z %z"), "12:47:00 UTC +0000")
-        self.assertEqual(t3.strftime("%H:%M:%S %Z %z"), "13:47:00 MET +0100")
+        self.assertEqual(t1.strftime("%H:%M:%S %%Z=%Z %%z=%z %%:z=%:z"),
+                                     "07:47:00 %Z=EST %z=-0500 %:z=-05:00")
+        self.assertEqual(t2.strftime("%H:%M:%S %Z %z %:z"), "12:47:00 UTC +0000 +00:00")
+        self.assertEqual(t3.strftime("%H:%M:%S %Z %z %:z"), "13:47:00 MET +0100 +01:00")
 
         yuck = FixedOffset(-1439, "%z %Z %%z%%Z")
         t1 = time(23, 59, tzinfo=yuck)
diff --git a/Misc/NEWS.d/next/Library/2022-08-14-18-59-54.gh-issue-69142.6is5Pq.rst b/Misc/NEWS.d/next/Library/2022-08-14-18-59-54.gh-issue-69142.6is5Pq.rst
new file mode 100644
index 0000000..0db8b37
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2022-08-14-18-59-54.gh-issue-69142.6is5Pq.rst
@@ -0,0 +1 @@
+Add ``%:z`` strftime format code (generates tzoffset with colons as separator), see :ref:`strftime-strptime-behavior`.
diff --git a/Modules/_datetimemodule.c b/Modules/_datetimemodule.c
index eca7c6b..d86418a 100644
--- a/Modules/_datetimemodule.c
+++ b/Modules/_datetimemodule.c
@@ -1507,6 +1507,27 @@ format_utcoffset(char *buf, size_t buflen, const char *sep,
 }
 
 static PyObject *
+make_somezreplacement(PyObject *object, char *sep, PyObject *tzinfoarg)
+{
+    char buf[100];
+    PyObject *tzinfo = get_tzinfo_member(object);
+
+    if (tzinfo == Py_None || tzinfo == NULL) {
+        return PyBytes_FromStringAndSize(NULL, 0);
+    }
+   
+    assert(tzinfoarg != NULL);
+    if (format_utcoffset(buf,
+                         sizeof(buf),
+                         sep,
+                         tzinfo,
+                         tzinfoarg) < 0)
+        return NULL;
+        
+    return PyBytes_FromStringAndSize(buf, strlen(buf));
+}
+
+static PyObject *
 make_Zreplacement(PyObject *object, PyObject *tzinfoarg)
 {
     PyObject *temp;
@@ -1566,7 +1587,7 @@ make_freplacement(PyObject *object)
 
 /* 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 and %f format codes via a
+ * giving special meanings to the %z, %:z, %Z and %f format codes via a
  * preprocessing step on the format string.
  * tzinfoarg is the argument to pass to the object's tzinfo method, if
  * needed.
@@ -1578,6 +1599,7 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
     PyObject *result = NULL;            /* guilty until proved innocent */
 
     PyObject *zreplacement = NULL;      /* py string, replacement for %z */
+    PyObject *colonzreplacement = NULL; /* py string, replacement for %:z */
     PyObject *Zreplacement = NULL;      /* py string, replacement for %Z */
     PyObject *freplacement = NULL;      /* py string, replacement for %f */
 
@@ -1632,32 +1654,29 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
         }
         /* A % has been seen and ch is the character after it. */
         else if (ch == 'z') {
+            /* %z -> +HHMM */
             if (zreplacement == NULL) {
-                /* format utcoffset */
-                char buf[100];
-                PyObject *tzinfo = get_tzinfo_member(object);
-                zreplacement = PyBytes_FromStringAndSize("", 0);
-                if (zreplacement == NULL) goto Done;
-                if (tzinfo != Py_None && tzinfo != NULL) {
-                    assert(tzinfoarg != NULL);
-                    if (format_utcoffset(buf,
-                                         sizeof(buf),
-                                         "",
-                                         tzinfo,
-                                         tzinfoarg) < 0)
-                        goto Done;
-                    Py_DECREF(zreplacement);
-                    zreplacement =
-                      PyBytes_FromStringAndSize(buf,
-                                               strlen(buf));
-                    if (zreplacement == NULL)
-                        goto Done;
-                }
+                zreplacement = make_somezreplacement(object, "", tzinfoarg);
+                if (zreplacement == NULL)
+                    goto Done;
             }
             assert(zreplacement != NULL);
+            assert(PyBytes_Check(zreplacement));
             ptoappend = PyBytes_AS_STRING(zreplacement);
             ntoappend = PyBytes_GET_SIZE(zreplacement);
         }
+        else if (ch == ':' && *pin == 'z' && pin++) {
+            /* %:z -> +HH:MM */
+            if (colonzreplacement == NULL) {
+                colonzreplacement = make_somezreplacement(object, ":", tzinfoarg);
+                if (colonzreplacement == NULL)
+                    goto Done;
+            }
+            assert(colonzreplacement != NULL);
+            assert(PyBytes_Check(colonzreplacement));
+            ptoappend = PyBytes_AS_STRING(colonzreplacement);
+            ntoappend = PyBytes_GET_SIZE(colonzreplacement);
+        }
         else if (ch == 'Z') {
             /* format tzname */
             if (Zreplacement == NULL) {
@@ -1686,7 +1705,7 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
             ntoappend = PyBytes_GET_SIZE(freplacement);
         }
         else {
-            /* percent followed by neither z nor Z */
+            /* percent followed by something else */
             ptoappend = pin - 2;
             ntoappend = 2;
         }
@@ -1733,6 +1752,7 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
  Done:
     Py_XDECREF(freplacement);
     Py_XDECREF(zreplacement);
+    Py_XDECREF(colonzreplacement);
     Py_XDECREF(Zreplacement);
     Py_XDECREF(newfmt);
     return result;