str = ‘abcdefg‘
str[首：尾：步长】 步长为负数的话是倒着取
1 str = ‘abcdef‘ 2 a1 = str[0:4:2] 3 print(a1) #ac 4 a2 = str[-1:1:-2] 5 print(a2) #fd
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原文：https://www.cnblogs.com/yifengs/p/10546559.html
