def contextfunction(f):
    """This decorator can be used to mark a function or method context callable.
    A context callable is passed the active :class:`Context` as first argument when
    called from the template.  This is useful if a function wants to get access
    to the context or functions provided on the context object.  For example
    a function that returns a sorted list of template variables the current
    template exports could look like this::

        @contextfunction
        def get_exported_names(context):
            return sorted(context.exported_vars)
    """
    f.contextfunction = True
    return f
