Currently filters check updates via the __call__ method. When switching from function-based filters to class-based filters, this was mainly done to maintain backwards compatibility with filter(update). However with some of the new filters functionalities, this leads to ugly workarounds like

I therefore propose to refactor to something along the lines of

class BaseFilter:
    def __call__(self, *args, **kwargs):
        return self.__class__(*args, **kwargs)

    @abstractmethod
    def check_update(update/method):
        ...

This matches the Handler.check_update interface and allows for filters having a custom __init__, wich can still be called through instances of those filters.

However, this is somewhat breaking: While BaseFilter.__call__ is undocumented, people might still have Filters.existing_filter(update) somewhere within their custom filters. So maybe this is better suited for a major release.