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Flymake helper for OpenStack's Hacking Tool
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fly-hack - OpenStack hacking support for flymake

Emacs has a built in inbuffer syntax checker called flymake. On every substantial buffer change it runs a syntax checker script and highlights incorrect lines in the code. There is plenty of documentation out there for setting up flake8 as the flymake checker for python, which is a great first step.

However, in OpenStack there are a lot of projects. Every project enforces a different level of flake8 through custom ignores. We also extend flake8 with additional rules from the hacking project. Which, again, some projects ignore. We also allow projects to build their own in tree hacking rules.

Would it be possible to make the flymake flake8 integration respect and adapt to all of these rules? So that every flymake error is a real error for that project, one that will bounce your code?

The answer seems to be yes, and is this project.

fly-hack in action


First some warnings. This code is very much in a works for me state. Suitability for anyone else is unknown.

While I'm a long time emacs user, I'm a poor elisp coder, so I expect a bunch of this to be in barely working state.

Pull requests are welcomed, as long as it doesn't break my config.


  • cp fly-hack.el to some part of your elisp tree where it can be loaded from

  • add the following to your .emacs:

    ;;; flymake base mode is availabe in emacs
    (require 'flymake)
    ;;; currently the helper has some logic based on the temp
    ;;; directory being the following
    (setq temporary-file-directory "~/.emacs.d/tmp/")
    (require 'fly-hack)
    ;;; set to the path where lives. It needs to be a
    ;;; writable directory for it's fallback flake8.
    (setq fly-hack-helper "~/code/fly-hack/")
  • evaluate said buffer, open a python file, and you should be off and running.

Theory of Function

Given that all of this may break horribly for you, let me explain what it's supposed to do in the hopes that you can figure out why it might not be working for you.

This mode disables flymake in place syntax checking, and instead relies on copying the active buffers to a full path in a temporary directory. The reason for this is to play nicely with tramp, which lets you open files over an ssh connection. I edit locally for unit test level things, but remotely when working on live devstacks. The tramp support isn't first class, but at least it doesn't hang or throw errors.

The bulk of the work is done by the custom flymake helper This attempts to do the following things:

  • figure out where in your local directory structure the editted file might be.
  • ascend the directory tree looking for a .tox/pep8 directory, which means we'll have a localized flake8 (+ hacking + local rules) ready to go. If we find that, use that flake8 for our syntax run.
  • if we don't find a flake8, make one. This is done by creating a .venv in the same directory as and installing flake8 into it. We only do this once, so if results start looking weird, you can delete the .venv and let it build it again.
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