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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_computation(self, message: Message, transaction_context: 'BaseTransactionContext') -> 'BaseComputation': """ Return a computation instance for the given `message` and `transaction_context` """

        if self.computation_class is None:
            raise AttributeError("No `computation_class` has been set for this State")
        else:
            computation = self.computation_class(self, message, transaction_context)
        return computation