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Python默认的for ... in ...循环是不带索引序号的，例如：

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presidents = ["Washington", "Adams", "Jefferson", "Madison", "Monroe", "Adams", "Jackson"]
for name in presidents:
    print("President : {}".format(name))

执行结果如下：

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President : Washington
President : Adams
President : Jefferson
President : Madison
President : Monroe
President : Adams
President : Jackson

在这个例子中，我们可以打印出总统的名字，但不能打印出该总统是第几任总统。那么，如何打印出是第几任总统呢？用enumerate()函数。改成如下形式：

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presidents = ["Washington", "Adams", "Jefferson", "Madison", "Monroe", "Adams", "Jackson"]
for num, name in enumerate(presidents, start=1):
    print("President {}: {}".format(num, name))

执行结果如下：

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President 1: Washington
President 2: Adams
President 3: Jefferson
President 4: Madison
President 5: Monroe
President 6: Adams
President 7: Jackson

搞定！