'Return an initialized filter. Only initialize filters once.
Args:
filter_name: The name of the filter, as a string.
Returns:
an initialized instance of the filter.
Raises:
DefinitionError if the type of filter has not been defined.'

@classmethod
def GetFilter(cls, filter_name):

    try:
        filt_cls = cls.GetPlugin(filter_name)
    except KeyError:
        raise DefinitionError(('Filter    %s    does    not    exist.' % filter_name))
    return filt_cls()
