code: def _describe_type(arg):
      
      if isinstance(arg, tuple):
          return "tuple of length %s" % len(arg)
      else:
          return type(arg).__name__
docs: Describe given argument, including length if it's a tuple.
  
      The purpose is to prevent nonsensical exception messages such as::
  
          expected a tuple of length 2, got tuple
          expected a tuple, got tuple
  
      by turning them into::
  
          expected a tuple of length 3, got tuple of length 2