bpo-36779: time.tzname returns empty string on Windows if default cod… (GH-13073)

Calling setlocale(LC_CTYPE, "") on a system where GetACP() returns CP_UTF8 results in empty strings in _tzname[].

This causes time.tzname to be an empty string.
I have reported the bug to the UCRT team and will follow up, but it will take some time get a fix into production.

In the meantime one possible workaround is to temporarily change the locale by calling setlocale(LC_CTYPE, "C") before calling _tzset and restore the current locale after if the GetACP() == CP_UTF8 or CP_UTF7

@zooba 


https://bugs.python.org/issue36779

Author: paulmon

Misc/NEWS.d/next/Windows/2019-06-11-15-41-34.bpo-36779.0TMw6f.rst

@@ -0,0 +1,2 @@
+Ensure ``time.tzname`` is correct on Windows when the active code page is
+set to CP_UTF7 or CP_UTF8.

Modules/timemodule.c

@@ -1581,6 +1581,19 @@ init_timezone(PyObject *m)
     PyModule_AddIntConstant(m, "altzone", _Py_timezone-3600);
 #endif
     PyModule_AddIntConstant(m, "daylight", _Py_daylight);
+#ifdef MS_WINDOWS
+    TIME_ZONE_INFORMATION tzinfo = {0};
+    GetTimeZoneInformation(&tzinfo);
+    otz0 = PyUnicode_FromWideChar(tzinfo.StandardName, -1);
+    if (otz0 == NULL) {
+        return -1;
+    }
+    otz1 = PyUnicode_FromWideChar(tzinfo.DaylightName, -1);
+    if (otz1 == NULL) {
+        Py_DECREF(otz0);
+        return -1;
+    }
+#else
     otz0 = PyUnicode_DecodeLocale(_Py_tzname[0], "surrogateescape");
     if (otz0 == NULL) {
         return -1;
@@ -1590,6 +1603,7 @@ init_timezone(PyObject *m)
         Py_DECREF(otz0);
         return -1;
     }
+#endif // MS_WINDOWS
     PyObject *tzname_obj = Py_BuildValue("(NN)", otz0, otz1);
     if (tzname_obj == NULL) {
         return -1;