举个例子

prices = {'apple': 0.40, 'banana': 0.50}
my_purchase = {
　　'apple': 1,
　　'banana': 6}
grocery_bill = sum(prices[fruit] * my_purchase[fruit]
　　　　　　　　　 for fruit in my_purchase)
print 'I owe the grocer $%.2f' % grocery_bill

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