@abstractmethod<EOL><INDENT>def filter(self, value, model=None, context=None):<DEDENT>
raise NotImplemented<EOL>
Filter
Abstract filtering method: implement this in your concrete
filters. Accepts a value and returns filtered value.

:param value:               a value to filter
:param model:               parent model of the property
:param context:             parent model or custom context
:return:                    filtered value