Run mypy from "root" path

[Holt59]

I have the following directory structure (standard for a python package I think):

module/
    __init__.py
    ...
tests/
    test_xxx.py
setup.cfg
setup.py

I have installed the module doing pip install -e ..

Problem

When running mypy from the command line, there are no issues with test_xxx.py. When running flycheck-mypy from emacs, I got tons of warnings similar to:

test_settings.py:7: error: Cannot find implementation or library stub for module named 'module.settings'

The problem occurs because flycheck run mypy from the tests folder and not the root one. Even if I specify a mypy config file, the error persists.

Potential fix

There is :working-directory option for flycheck checkers that could be used. Mine look like this (only works for setup.cfg currently):

(defun flycheck-mypy--find-project-root (_checker)
  "Compute an appropriate working-directory for flycheck-ruby.
This is either a parent directory containing a Gemfile, or nil."
  (and
   buffer-file-name
   (locate-dominating-file buffer-file-name "setup.cfg")))

(flycheck-define-checker python-mypy
  "Mypy syntax checker. Requires mypy>=0.3.1.

Customize `flycheck-python-mypy-args` to add specific args to default
executable.

E.g. when processing Python2 files, add \"--py2\".

See URL `http://mypy-lang.org/'."

  :command ("mypy"
            (config-file "--config-file" flycheck-mypy.ini)
            (eval flycheck-python-mypy-args)
            source-original)
  :working-directory flycheck-mypy--find-project-root
  :error-patterns
  ((error line-start (file-name) ":" line ": error:" (message) line-end)
   (warning line-start (file-name) ":" line ": note:" (message) line-end)
   (info line-start (file-name) ":" line ": note:" (message) line-end))
  :modes python-mode)

(add-to-list 'flycheck-checkers 'python-mypy t)

It should be possible to adapt this to either have a variable to define the directory, or use the directory of flycheck-mypy.ini.

[lbolla]

@Holt59 I am happy to take a MR if you have time to work on it!