Python Tips

Some Tips in Python

• (Reverse)Slice

  As we know, slice is very useful in python. Normally we use a[x:y:d] to make a slice from a list. This is quiet easy to understand, but when it comes to assigning a negative value to “d”, things become confusing.

  For example: Let’s create a list: a = [1,2,3,4,5,6]

  With input: a[::-1] Obviously we got: [6,5,4,3,2,1]

  With: a[:3:-1] Output is: [6,5]

  With: a[3::-1] Output is: [4,3,2,1]

  With: a[4:2:-1] Output is: > [5,4]

  With: a[2:4:-1] Output is: > []

  Summary:

  When the step parameter becomes a negative value, the indexes of elements won’t change, but the start and the end position are switched!!!

  Besides, if there is a direction conflict, it will always return an empty list []

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