Switching between production and test file in the same directory back and forth! (Works for Ruby & Python)

vimrc

function! OpenTestOrProduction()
  let current_file_without_extension = expand("%:r")
  let filename_parts = split(current_file_without_extension, "_")
  let target_file = ""
  if IsInTestFile()
    let main_name_parts = filename_parts[0:-2]
    let target_file = CreateTargeFilename(main_name_parts)
  else
    let target_file = CreateTargeTestFilename(current_file_without_extension)
  end

  call OpenFileForEditIfExists(target_file)
endfunction

function IsInTestFile()
  let current_file = expand("%")
  return match(current_file, '_spec.rb$') != -1 || match(current_file, '_tests.py$') != -1
endfunction

function CreateTargeFilename(filename_parts)
  let extension = expand("%:e")
  if extension == "rb"
    return join(a:filename_parts, "_") . ".rb"
  endif
  return join(a:filename_parts, "_") . ".py"
endfunction

function CreateTargeTestFilename(filename_without_extension)
  let extension = expand("%:e")
  if extension == "rb"
    return a:filename_without_extension . "_spec.rb"
  endif
  return a:filename_without_extension . "_tests.py"
endfunction

function! OpenFileForEditIfExists(filename)
  if filereadable(a:filename)
    exe "edit " . a:filename
  else
    echo "File: " . a:filename "does NOT exist!"
  end
endfunction

You can map it to something like following:

**map <leader>o :call OpenTestOrProduction()<CR>**