code: def add_template_filter(self, func: Callable, name: Optional[str]=None) -> None:
          
          self.jinja_env.filters[name or func.__name__] = func
docs: Add a template filter.
  
          This is designed to be used on the application directly. An
          example usage,
  
          .. code-block:: python
  
              def to_upper(value):
                  return value.upper()
  
              app.add_template_filter(to_upper)
  
          Arguments:
              func: The function that is the filter.
              name: The filter name (defaults to function name).