翻历史贴，偶然发现一个帖子，自己不明白了
求教该如何理解

>>> def f(a, l = []):
...     l.append(a)
...     return l
...
>>> f(1)
[1]
>>> f(2)
[1, 2]
>>> f(1)
[1, 2, 1]
>>> f(1,[])
[1]
>>> f(2)
[1, 2, 1, 2]
>>> f(3,[])
[3]
>>> f(3)
[1, 2, 1, 2, 3]
>>>
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