def Instance(expected, message="<STR_LIT>"):
@wraps(Instance)<EOL>def built(value):<EOL><INDENT>if not isinstance(value, expected):<EOL><INDENT>raise Error(message.format(expected.__name__))<EOL><DEDENT>return value<EOL><DEDENT>return built<EOL>
Creates a validator that checks if the given value is an instance of
``expected``.

A custom message can be specified with ``message``.