Regarding the purpose of the “Readline completer” module, I agree it is inconvenient. But the documentation <https://docs.python.org/3.5/library/rlcompleter.html> does say “without readline, the Completer class . . . can still be used”. I ran into this problem in Issue 25419 where Serhiy pointed it out.

OS X is apparently the main platform where Editline is supported. There are even #ifdef __APPLE__ bits to work around bugs. However, people also seem to use Editline on Free BSD.

Testing your current patch on Linux + Editline, it is worse than the 3.5 behaviour. Pressing Tab once appears to work, until you type the next key. It seems to invoke the history search mode (normally done with Ctrl+R), except the “bck:” indicator is not immediately shown. So the next key brings up an old history line, rather than being inserted directly.

But if this works fine on OS X’s Editline, then maybe my setup is not right. I also see annoying buffering or flushing problems with my Editline setup. I am using Arch Linux’s libedit-20150325_3.1-1 package.

Testing with Linux + Gnu Readline, it seems to work as advertised.

I haven’t heard of Editline or Gnu Readline being used on Windows, certainly not in normal setups.