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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 set_observer(self, observer):
 """ Validates and sets the color's observer angle. .. note:: This only changes the observer angle value. It does no conversion of the color's coordinates. :param str observer: One of '2' or '10'. """

        observer = str(observer)
        if observer not in color_constants.OBSERVERS:
            raise InvalidObserverError(self)
        self.observer = observer