Before: Save g:ale_dart_dartfmt_executable Save g:ale_dart_dartfmt_options " Use an invalid global executable, so we don't match it. let g:ale_dart_dartfmt_executable = 'xxxinvalid' let g:ale_dart_dartfmt_options = '' call ale#test#SetDirectory('/testplugin/test/fixers') After: Restore call ale#test#RestoreDirectory() Execute(The dartfmt callback should return the correct default values): call ale#test#SetFilename('../test-files/dart/testfile.dart') AssertEqual \ { \ 'read_temporary_file': 1, \ 'command': ale#Escape('xxxinvalid') \ . ' -w' \ . ' %t', \ }, \ ale#fixers#dartfmt#Fix(bufnr('')) Execute(The dartfmt callback should include custom dartfmt options): let g:ale_dart_dartfmt_options = "-l 80" call ale#test#SetFilename('../test-files/dart/testfile.dart') AssertEqual \ { \ 'read_temporary_file': 1, \ 'command': ale#Escape('xxxinvalid') \ . ' -w' \ . ' ' . g:ale_dart_dartfmt_options \ . ' %t', \ }, \ ale#fixers#dartfmt#Fix(bufnr(''))