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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 fail(self, key, **kwargs):
 """A helper method that simply raises a `ValidationError`. """

        try:
            msg = self.error_messages[key]
        except KeyError:
            class_name = self.__class__.__name__
            msg = MISSING_ERROR_MESSAGE.format(class_name=class_name,
                                               key=key)
            raise AssertionError(msg)
        if isinstance(msg, str):
            msg = msg.format(**kwargs)
        raise exceptions.ValidationError(msg, self.field_name)