1. enumerate()函数用于将一个可遍历的数据对象（如列表、元组或字符串）组合为一个索引序列，同时列出数据和数据下标，一般用来for循环中
例子：
>>>seasons = ['Spring', 'Summer', 'Fall', 'Winter']
>>> list(enumerate(seasons))
[(0, 'Spring'), (1, 'Summer'), (2, 'Fall'), (3, 'Winter')]
>>> list(enumerate(seasons, start=1)) # 小标从 1 开始
[(1, 'Spring'), (2, 'Summer'), (3, 'Fall'), (4, 'Winter')]
普通for循环：
>>>i = 0
>>> seq = ['one', 'two', 'three']
>>> for element in seq:
... print i, seq[i]
... i +=1
...
0 one
1 two
2 three
for循环使用enumerate：
>>>seq = ['one', 'two', 'three']
>>> for i, element in enumerate(seq):
... print i, seq[i]
...
0 one
1 two
2 three
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