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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 depth(self):
 """ The number of hierarchy levels in this category graph. Returns 0 if it contains no categories. """

        categories = self._categories
        if not categories:
            return 0
        first_depth = categories[0].depth
        for category in categories[1:]:
            if category.depth != first_depth:
                raise ValueError('category depth not uniform')
        return first_depth