There are a couple issues out there with similar reports (21646 21629 21628).

Revert to 818f06eaa72d8e4f9ba314c1c2855613bf89f396 if you want to work around the issue.

On Fri, Oct 9, 2015 at 3:22 PM, Dima Kogan <dima@secretsauce.net> wrote:
Hi. I'm running a very recent emacs:

 https://github.com/emacs-mirror/emacs/commit/7e83f50

Opening a trivial python source file (attached) with a trivial and
seemingly-unrelated .emacs.d/init.el (also attached) makes emacs spin
its wheels. Hitting C-g doesn't work to get control back. You actually
have to kill the process.

Thanks!