'Getter for extra filter methods, lazy property so methods
are only imported/initialized when needed
@todo: document the expected signature of filter methods
@return: dict {name: callable} of known named filter methods'

@property
def extra_filter_methods(self):

    methods = self._extra_filter_methods
    if (methods is None):
        methods = {}
        self._extra_filter_methods = methods
    return methods
