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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 getModel(self, modelIdentifier):
 """ Return the requested model. :param modelIdentifier: <str> model identifier :return: <object> model instance """

        if modelIdentifier in self._models:
            return self._models[modelIdentifier]
        else:
            message = "Application - getModel() - " \
                      "Model with identifier {} does not exist." \
                      .format(modelIdentifier)
            raise Exception(message)