def register_filter(self, filter_name, filter_ref, force=False):
if not force and (filter_name in self.filters_list()):<EOL><INDENT>self.log_warning("<STR_LIT>" % ext_in)<EOL>return<EOL><DEDENT>self.__jinja2_environment.filters[filter_name] = filter_ref<EOL>
Add/register one filter.

Args:
    filter_name (str): Filter name used inside :program:`Jinja2` tags.
    filter_ref: Reference to the filter itself, i.e. the corresponding :program:`Python` function.
    force (bool): If set to ``True``, forces the registration of a filter no matter if it already exists or not.

Note:
    The list of user added/registered filters can be retrieve with :mth:`registered_filters_list`