list切片(know how to slice sequences)

刚读了Effective Python

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a = ['a','b','c','d','e','f','g','h'] print('first four: ',a[:4])#['a','b','c','d'] print('Last four: ',a[-4:])#['e','f','g','h'] print('Middle two: ',a[3:-3])#['d','e']

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assert a[:5] == a[0:5] assert a[5:] == a[5:len(a)]

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a[:20] # ['a','b','c','d','e','f','g','h'] a[-20:] # ['a','b','c','d','e','f','g','h'] a[20] # IndexError: list index out of range

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a = ['a','b','c','d','e','f','g','h'] a[2:7] = [99,22,14] print(a) #['a','b',99,22,14,'h'] b = a[:] assert b == a and b is not a b = a assert a is b # still the same list object

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avoid using start,end,and stride in a single slice [ start: end: stride]

a = ['red','orange','yellow','green','blue','purple'] odds = a[::2] # ['red','yellow','blue'] evens = a[1::2] # ['orange','green','purple'] x = b'mongoose'# type(x) <class 'bytes'>(in python3) y = x[::-1] print(y)# b'esoognom'

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#specifying start, end, and stride in a slice can be extremely confusing #prefer using positive stride values in slice widthout start or end indexs.Avoid negative stride values if possible