code: def set_mypy_path(mypy_path):
      
      original = os.environ.get(mypy_path_env_var)
      if original is None:
          new_mypy_path = mypy_path
      elif not original.startswith(mypy_path):
          new_mypy_path = mypy_path + os.pathsep + original
      else:
          new_mypy_path = None
      if new_mypy_path is not None:
          logger.log(mypy_path_env_var + ":", new_mypy_path)
          os.environ[mypy_path_env_var] = new_mypy_path
docs: Prepend to MYPYPATH.