Before: runtime ale_linters/python/mypy.vim silent! execute 'cd /testplugin/test/command_callback' let g:dir = getcwd() After: silent execute 'cd ' . fnameescape(g:dir) " Set the file to something else, " or we'll cause issues when running other tests silent file 'dummy.py' unlet! g:dir call ale#linter#Reset() let g:ale_python_mypy_executable = 'mypy' let g:ale_python_mypy_options = '' let g:ale_python_mypy_use_global = 0 Execute(The mypy callbacks should return the correct default values): AssertEqual \ 'mypy', \ ale_linters#python#mypy#GetExecutable(bufnr('')) AssertEqual \ 'cd ''' . g:dir . ''' && ''mypy'' --show-column-numbers ' \ . '--shadow-file %s %t %s', \ ale_linters#python#mypy#GetCommand(bufnr('')) Execute(The mypy executable should be configurable, and escaped properly): let g:ale_python_mypy_executable = 'executable with spaces' AssertEqual \ 'executable with spaces', \ ale_linters#python#mypy#GetExecutable(bufnr('')) AssertEqual \ 'cd ''' . g:dir . ''' && ''executable with spaces'' --show-column-numbers ' \ . '--shadow-file %s %t %s', \ ale_linters#python#mypy#GetCommand(bufnr('')) Execute(The mypy command callback should let you set options): let g:ale_python_mypy_options = '--some-option' AssertEqual \ 'cd ''' . g:dir . ''' && ''mypy'' --show-column-numbers --some-option ' \ . '--shadow-file %s %t %s', \ ale_linters#python#mypy#GetCommand(bufnr('')) Execute(The mypy command should switch directories to the detected project root): silent execute 'file ' . fnameescape(g:dir . '/python_paths/no_virtualenv/subdir/foo/bar.py') AssertEqual \ 'mypy', \ ale_linters#python#mypy#GetExecutable(bufnr('')) AssertEqual \ 'cd ''' . g:dir . '/python_paths/no_virtualenv/subdir'' && ''mypy'' --show-column-numbers ' \ . '--shadow-file %s %t %s', \ ale_linters#python#mypy#GetCommand(bufnr('')) Execute(The mypy callbacks should detect virtualenv directories and switch to the project root): silent execute 'file ' . fnameescape(g:dir . '/python_paths/with_virtualenv/subdir/foo/bar.py') AssertEqual \ g:dir . '/python_paths/with_virtualenv/env/bin/mypy', \ ale_linters#python#mypy#GetExecutable(bufnr('')) AssertEqual \ 'cd ''' . g:dir . '/python_paths/with_virtualenv/subdir'' && ''' \ . g:dir . '/python_paths/with_virtualenv/env/bin/mypy'' --show-column-numbers ' \ . '--shadow-file %s %t %s', \ ale_linters#python#mypy#GetCommand(bufnr('')) Execute(You should able able to use the global mypy instead): silent execute 'file ' . fnameescape(g:dir . '/python_paths/with_virtualenv/subdir/foo/bar.py') let g:ale_python_mypy_use_global = 1 AssertEqual \ 'mypy', \ ale_linters#python#mypy#GetExecutable(bufnr('')) AssertEqual \ 'cd ''' . g:dir . '/python_paths/with_virtualenv/subdir'' && ''mypy'' --show-column-numbers ' \ . '--shadow-file %s %t %s', \ ale_linters#python#mypy#GetCommand(bufnr(''))