def template_filter(
        self, name: str | None = None
    ) -> t.Callable[[T_template_filter], T_template_filter]:
        """A decorator that is used to register custom template filter.
        You can specify a name for the filter, otherwise the function
        name will be used. Example::

          @app.template_filter()
          def reverse(s):
              return s[::-1]

        :param name: the optional name of the filter, otherwise the
                     function name will be used.
        """

        def decorator(f: T_template_filter) -> T_template_filter:
            self.add_template_filter(f, name=name)
            return f

        return decorator
A decorator that is used to register custom template filter.
        You can specify a name for the filter, otherwise the function
        name will be used. Example::

          @app.template_filter()
          def reverse(s):
              return s[::-1]

        :param name: the optional name of the filter, otherwise the
                     function name will be used.
        
src/flask/sansio/app.py