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>

Modules/_datetimemodule.c

@@ -1506,6 +1506,27 @@ format_utcoffset(char *buf, size_t buflen, const char *sep,
     return 0;
 }
 
+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)
 {
@@ -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;