code: def to_pretty(self, format='str'):
          
          if format == 'str':
              template = '%-*s = %s'
          elif format == 'repr':
              template = '%-*s = %r'
          else:
              raise ValueError('unrecognized value for "format": %r' % format)
  
          d = self.__dict__
          maxlen = 0
  
          for k in six.iterkeys(d):
              maxlen = max(maxlen, len(k))
  
          return '\n'.join(template % (maxlen, k, d[k])
                           for k in sorted(six.iterkeys(d)))
docs: Return a string with a prettified version of this object’s contents.
  
          The format is a multiline string where each line is of the form ``key
          = value``. If the *format* argument is equal to ``"str"``, each
          ``value`` is the stringification of the value; if it is ``"repr"``, it
          is its :func:`repr`.
  
          Calling :func:`str` on a :class:`Holder` returns a slightly different
          pretty stringification that uses a textual representation similar to a
          Python :class:`dict` literal.