python - 为什么输出会是11032？ Python3.6

Question

d = {10:"x", 1:"wx", 2:"yz"}
a = d.setdefault(1)
b = d.setdefault(3)
s = "{}" * len(d)
print(s.format(*d))

为什么输出会是11032？

Answer 1

2次setdefault调用后，

d = {10: "x", 1: "wx", 2: "yz"}
d.setdefault(1)  # does not change the dictionary because there's already 1
d.setdefault(3)  # add 3 with value None (default if not speicfied)

d 变为:

>>> d
{10: 'x', 1: 'wx', 2: 'yz', 3: None}

迭代字典产生字典键:10, 1, 2, 3。 (由 *d 执行迭代以解压参数 d):

>>> for key in d:
...     print(key)
... 
10
1
2
3

因此，s.format(*d) 相当于 '{}{}{}{}'.format(10, 1, 2, 3) .