英文文档：

enumerate(iterable, start=0)

Return an enumerate object. iterable must be a sequence, an iterator, or some other object which supports iteration. The __next__() method of the iterator returned by enumerate() returns a tuple containing a count (from start which defaults to 0) and the values obtained from iterating over iterable.

Equivalent to:

def enumerate(sequence, start=0):

    n = start

    for elem in sequence:

        yield n, elem

        n += 1

说明：

　　1. 接受一个可迭代对象（序列或者迭代器），返回一个可枚举对象（同时返回索引和值，其中索引可以指定起始值）。

>>> seasons = ['Spring', 'Summer', 'Fall', 'Winter']

>>> list(enumerate(seasons))

[(0, 'Spring'), (1, 'Summer'), (2, 'Fall'), (3, 'Winter')]

>>> list(enumerate(seasons, start=1)) #指定起始值

[(1, 'Spring'), (2, 'Summer'), (3, 'Fall'), (4, 'Winter')]