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() let b:bin_dir = has('win32') ? 'Scripts' : 'bin' After: Restore unlet! b:bin_dir 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': 'cd ' . ale#Escape(ale#path#Simplify(g:dir . '/python_paths/with_virtualenv/subdir/foo')) . ' && ' \ . ale#Escape(ale#path#Simplify(g:dir . '/python_paths/with_virtualenv/env/' . b:bin_dir . '/isort')) . ' -', \ }, \ ale#fixers#isort#Fix(bufnr(''))