is_str(x) yields True if x is a string object or a 0-dim numpy array of a string and yields
      False otherwise.

def is_str(arg):
    '''
    is_str(x) yields True if x is a string object or a 0-dim numpy array of a string and yields
      False otherwise. 
    '''
    return (isinstance(arg, six.string_types) or
            is_npscalar(arg, 'string') or
            is_npvalue(arg, 'string'))