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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_contextualize_items_by_step(self, default=None):
 """Get a dictionary of the contextualize_items grouped by the step or the default value"""

        if self.items:
            res = {}
            for elem in self.items.values():
                if elem.num in res:
                    res[elem.num].append(elem)
                else:
                    res[elem.num] = [elem]
            return res
        else:
            return default