我认为这更接近您想要的：
fin = readFile(db)
categoryDict = defaultdict(dict) # automatically create a subdict
for line in fin:
itemDict = {} # a new innermost dict for every item
itemInstance = setItemInstances(line)
itemDict[itemInstance._product_id] = itemInstance
categoryDict[itemInstance._category_id] = itemDict
602392714
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