Before: Save g:ale_python_isort_executable " Use an invalid global executable, so we don't match it. let g:ale_python_isort_executable = 'xxxinvalid' call ale#test#SetDirectory('/testplugin/test/fixers') silent cd .. silent cd command_callback let g:dir = getcwd() After: Restore call ale#test#RestoreDirectory() Execute(The isort callback should return the correct default values): AssertEqual \ 0, \ ale#fixers#isort#Fix(bufnr('')) silent execute 'file ' . fnameescape(g:dir . '/python_paths/with_virtualenv/subdir/foo/bar.py') AssertEqual \ {'command': "'" . g:dir . "/python_paths/with_virtualenv/env/bin/isort' -" }, \ ale#fixers#isort#Fix(bufnr(''))