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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def keys(self, **kwargs):
 """ DEPRECATED Iterator that returns primary keys as a sequence of dicts. """

        warnings.warn('Use of `rel.fetch.keys()` notation is deprecated. '
                      'Please use `rel.fetch("KEY")` or `rel.fetch(dj.key)` for equivalent result', stacklevel=2)
        yield from self._expression.proj().fetch(as_dict=True, **kwargs)