diff -r b2bd62d1644f Lib/test/test_cmd_line.py
--- a/Lib/test/test_cmd_line.py	Fri Nov 02 22:07:26 2012 +0200
+++ b/Lib/test/test_cmd_line.py	Sat Nov 03 13:01:20 2012 +0200
@@ -216,6 +216,23 @@
         self.assertIn(path1.encode('ascii'), out)
         self.assertIn(path2.encode('ascii'), out)
 
+    def test_empty_PYTHONPATH_issue16309(self):
+        """On Posix, it is documented that setting PATH to the 
+        empty string is equivalent to not setting PATH at all, 
+        which is an exception to the rule that in a string like 
+        "/bin::/usr/bin" the empty string in the middle gets 
+        interpreted as '.'"""
+        code = """if 1:
+            import sys
+            path = ":".join(sys.path)
+            path = path.encode("ascii", "backslashreplace")
+            sys.stdout.buffer.write(path)"""
+        rc1, out1, err1 = assert_python_ok('-c', code, PYTHONPATH="")
+        rc2, out2, err2 = assert_python_ok('-c', code)
+        # regarding to Posix specification, outputs should be equal
+        # for empty and unset PYTHONPATH
+        self.assertEquals(out1, out2)
+
     def test_displayhook_unencodable(self):
         for encoding in ('ascii', 'latin-1', 'utf-8'):
             env = os.environ.copy()
diff -r b2bd62d1644f Modules/getpath.c
--- a/Modules/getpath.c	Fri Nov 02 22:07:26 2012 +0200
+++ b/Modules/getpath.c	Sat Nov 03 13:01:20 2012 +0200
@@ -699,13 +699,11 @@
      */
     bufsz = 0;
 
-    if (_rtpypath) {
+    if (_rtpypath && _rtpypath[0] != '\0') {
         size_t rtpypath_len;
         rtpypath = _Py_char2wchar(_rtpypath, &rtpypath_len);
         if (rtpypath != NULL)
             bufsz += rtpypath_len + 1;
-        else
-            _rtpypath = NULL;
     }
 
     defpath = _pythonpath;