diff -r 19b2c54e5f09 Doc/library/pty.rst
--- a/Doc/library/pty.rst	Wed Nov 12 10:23:44 2014 -0500
+++ b/Doc/library/pty.rst	Thu Nov 13 13:00:21 2014 -0500
@@ -43,7 +43,9 @@

    The functions *master_read* and *stdin_read* should be functions which read from
    a file descriptor. The defaults try to read 1024 bytes each time they are
-   called.
+   called. Returning an empty string will be interpreted as an EOF
+   condition. In order to silently ignore input or output, *None*
+   should be returned from the appropriate *_read* function.

    .. versionchanged:: 3.4
       :func:`spawn` now returns the status value from :func:`os.waitpid`
diff -r 19b2c54e5f09 Lib/pty.py
--- a/Lib/pty.py	Wed Nov 12 10:23:44 2014 -0500
+++ b/Lib/pty.py	Thu Nov 13 13:00:21 2014 -0500
@@ -136,13 +136,17 @@
         rfds, wfds, xfds = select(fds, [], [])
         if master_fd in rfds:
             data = master_read(master_fd)
-            if not data:  # Reached EOF.
+            if data == None: # Don't copy
+                continue
+            elif not data:  # Reached EOF.
                 fds.remove(master_fd)
             else:
                 os.write(STDOUT_FILENO, data)
         if STDIN_FILENO in rfds:
             data = stdin_read(STDIN_FILENO)
-            if not data:
+            if data == None: # Don't copy
+                continue
+            elif not data:
                 fds.remove(STDIN_FILENO)
             else:
                 _writen(master_fd, data)