def by_type(self, identifier_class):
myidentifiers = set()<EOL>for identifier in self.source_identifiers.values():<EOL><INDENT>if (isinstance(identifier, identifier_class)):<EOL><INDENT>myidentifiers.update([identifier])<EOL><DEDENT><DEDENT>return set(myidentifiers)<EOL>
returns all unique instances of a type of identifier

:param identifier_class: the class to match identifier with
:returns: a tuple