Issue #16309: Make PYTHONPATH= behavior the same as if PYTHONPATH not set at all.

Thanks to Armin Rigo and Alexey Kachayev.

Lib/test/test_cmd_line.py

@@ -216,6 +216,23 @@ class CmdLineTest(unittest.TestCase):
         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()

Misc/ACKS

@@ -587,6 +587,7 @@ Sijin Joseph
 Andreas Jung
 Tattoo Mabonzo K.
 Bohuslav Kabrda
+Alexey Kachayev
 Bob Kahn
 Kurt B. Kaiser
 Tamito Kajiyama

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 3.4.0 Alpha 1?
 Core and Builtins
 -----------------
 
+- Issue #16309: Make PYTHONPATH="" behavior the same as if PYTHONPATH
+  not set at all.
+
 - Issue #10189: Improve the error reporting of SyntaxErrors related to global
   and nonlocal statements.
 

Modules/getpath.c

@@ -699,13 +699,11 @@ calculate_path(void)
      */
     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;