This fixes issue5143 and includes a test.

Issue5143 notes that getpath.c and main.c pass a 'char*' where
a 'wchar_t*' is expected on OSX.

Lib/test/test_osx_env.py

+"""
+Test suite for OS X interpreter environment variables.
+"""
+
+from test.support import EnvironmentVarGuard, run_unittest
+import subprocess
+import sys
+import unittest
+
+class OSXEnvironmentVariableTestCase(unittest.TestCase):
+    def _check_sys(self, ev, cond, sv, val = '/some/path/to/python'):
+        with EnvironmentVarGuard() as evg:
+            subpc = [str(sys.executable), '-c',
+                'import sys; sys.exit(2 if "%s" %s %s else 3)' % (val, cond, sv)]
+            # ensure environment variable does not exist
+            evg.unset(ev)
+            # test that test on sys.xxx normally fails
+            rc = subprocess.call(subpc)
+            self.assertEqual(rc, 3, "expected %s not %s %s" % (ev, cond, sv))
+            # set environ variable
+            evg.set(ev, val)
+            # test that sys.xxx has been influenced by the environ value
+            rc = subprocess.call(subpc)
+            self.assertEqual(rc, 2, "expected %s %s %s" % (ev, cond, sv))
+
+    def test_pythonexecutable_sets_sys_executable(self):
+        self._check_sys('PYTHONEXECUTABLE', '==', 'sys.executable')
+
+def test_main():
+    if sys.platform == 'darwin':
+        run_unittest(OSXEnvironmentVariableTestCase)
+
+if __name__ == "__main__":
+    test_main()

Modules/getpath.c

@@ -457,6 +457,7 @@ calculate_path(void)
 #else
     unsigned long nsexeclength = MAXPATHLEN;
 #endif
+	char execpath[MAXPATHLEN+1];
 #endif
 
     if (_path) {
@@ -486,8 +487,13 @@ calculate_path(void)
       * will fail if a relative path was used. but in that case,
       * absolutize() should help us out below
       */
-     else if(0 == _NSGetExecutablePath(progpath, &nsexeclength) && progpath[0] == SEP)
-       ;
+	else if(0 == _NSGetExecutablePath(execpath, &nsexeclength) && execpath[0] == SEP) {
+		size_t r = mbstowcs(progpath, execpath, MAXPATHLEN+1);
+		if (r == (size_t)-1 || r > MAXPATHLEN) {
+			/* Could not convert execpath, or it's too long. */
+			progpath[0] = '\0';
+		}
+	}
 #endif /* __APPLE__ */
 	else if (path) {
 		while (1) {

Modules/main.c

@@ -487,10 +487,23 @@ Py_Main(int argc, wchar_t **argv)
 	   so the actual executable path is passed in an environment variable.
 	   See Lib/plat-mac/bundlebuiler.py for details about the bootstrap
 	   script. */
-	if ((p = Py_GETENV("PYTHONEXECUTABLE")) && *p != '\0')
-		Py_SetProgramName(p);
-	else
+	if ((p = Py_GETENV("PYTHONEXECUTABLE")) && *p != '\0') {
+		wchar_t* buffer;
+		size_t len = strlen(p);
+		size_t r;
+
+		buffer = malloc(len * sizeof(wchar_t));
+		if (buffer == NULL) {
+			Py_FatalError(
+			   "not enough memory to copy PYTHONEXECUTABLE");
+		}
+
+		r = mbstowcs(buffer, p, len);
+		Py_SetProgramName(buffer);
+		/* buffer is now handed off - do not free */
+	} else {
 		Py_SetProgramName(argv[0]);
+	}
 #else
 	Py_SetProgramName(argv[0]);
 #endif