Index: Doc/library/urllib.parse.rst
===================================================================
--- Doc/library/urllib.parse.rst	(revision 88386)
+++ Doc/library/urllib.parse.rst	(working copy)
@@ -506,9 +506,9 @@
 .. function:: urlencode(query, doseq=False, safe='', encoding=None, errors=None)
 
    Convert a mapping object or a sequence of two-element tuples, which may
-   either be a :class:`str` or a :class:`bytes`,  to a "percent-encoded" string,
-   suitable to pass to :func:`urlopen` above as the optional *data* argument.
-   This is useful to pass a dictionary of form fields to a ``POST`` request.
+   either be a :class:`str` or a :class:`bytes`,  to a "percent-encoded" string.
+   The resultant string must be converted to bytes before it is passed to
+   :func:`urlopen` above as the optional *data* argument.
    The resulting string is a series of ``key=value`` pairs separated by ``'&'``
    characters, where both *key* and *value* are quoted using :func:`quote_plus`
    above. When a sequence of two-element tuples is used as the *query*
Index: Lib/urllib/request.py
===================================================================
--- Lib/urllib/request.py	(revision 88386)
+++ Lib/urllib/request.py	(working copy)
@@ -1048,6 +1048,8 @@
 
         if request.data is not None:  # POST
             data = request.data
+            if isinstance(data, str):
+                raise ValueError("POST data should be bytes")
             if not request.has_header('Content-type'):
                 request.add_unredirected_header(
                     'Content-type',
Index: Lib/test/test_urllib2.py
===================================================================
--- Lib/test/test_urllib2.py	(revision 88386)
+++ Lib/test/test_urllib2.py	(working copy)
@@ -794,6 +794,10 @@
         http.raise_on_endheaders = True
         self.assertRaises(urllib.error.URLError, h.do_open, http, req)
 
+        # Check for ValueError on Bad POST data.
+        req = Request("http://example.com/","badpost")
+        self.assertRaises(ValueError, h.do_request_, req)
+
         # check adding of standard headers
         o.addheaders = [("Spam", "eggs")]
         for data in b"", None:  # POST, GET
@@ -837,10 +841,11 @@
             else:
                 newreq = h.do_request_(req)
 
-        # A file object
+        # A file object.
+        # Test only Content-Length attribute of request.
 
-        file_obj = io.StringIO()
-        file_obj.write("Something\nSomething\nSomething\n")
+        file_obj = io.BytesIO()
+        file_obj.write(b"Something\nSomething\nSomething\n")
 
         for headers in {}, {"Content-Length": 30}:
             req = Request("http://example.com/", file_obj, headers)
@@ -863,7 +868,6 @@
             newreq = h.do_request_(req)
             self.assertEqual(int(newreq.get_header('Content-length')),16)
 
-
     def test_http_doubleslash(self):
         # Checks the presence of any unnecessary double slash in url does not
         # break anything. Previously, a double slash directly after the host