python是很友好的语言，但是有时也有一些小烦恼，
s = 'spam'
s[:0] # => ''
s[0][0][0] # => 's'
[[]] # => [[]]
n = [1,2,3,4,5]
n[3:1] = ['?'] # => [1, 2, 3, '?', 4, 5]
#当赋值非序列给分片时
n[2:3] = 1
TypeError: can only assign an iterable
d = {}
d[(1,2,3)] = 1
d # => {(1,2,3):1}
