code: def is_url(default_scheme='http', **kwargs):
      
      def converter(value):
          if value is None:
              return value
          if '://' not in value and default_scheme:
              value = '://'.join((default_scheme, value.strip()))
          try:
              return uris.validate(value)
          except uris.ValidationError as e:
              raise Invalid(e.message)
      return converter
docs: Return a converter that converts a clean string to an URL.