def add_callback(self, callback, section=None, key=None):
        '''Add a callback to be called when a specific section or key has
        changed. If you don't specify a section or key, it will call the
        callback for all section/key changes.

        Callbacks will receive 3 arguments: the section, key and value.

        .. versionadded:: 1.4.1
        '''
        if section is None and key > None:
            raise Exception('You cannot specify a key without a section')
        self._callbacks.append((callback, section, key))
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