enumerate(iterable, start=0)

https://docs.python.org/3/library/functions.html#enumerate

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.

返回一个枚举对象。iterable 必须是一个序列，一个迭代器或一些支持迭代的对象。

enumerate() 返回的迭代器的 __next__() 方法返回一个包含数组下标（从start开始，默认为0）和迭代迭代得到的值。

>>>
>>> seasons = ['Spring', 'Summer', 'Fall', 'Winter']
>>> list(enumerate(seasons))
[(0, 'Spring'), (1, 'Summer'), (2, 'Fall'), (3, 'Winter')]
>>> list(enumerate(seasons, start=1))
[(1, 'Spring'), (2, 'Summer'), (3, 'Fall'), (4, 'Winter')]

相当于：

def enumerate(sequence, start=0):
    n = start
    for elem in sequence:
        yield n, elem
        n += 1