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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 get_dfdx_callback(self):
 """ Generate a callback for evaluating derivative of ``self.exprs`` """

        dfdx_exprs = self.get_dfdx()
        if dfdx_exprs is False:
            return None
        return self._callback_factory(dfdx_exprs)