Question

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Good Primer for Python Slice Notation

I first ran across it last night for reversing a string "Hello"[::-1] and I can’t figure out how it actually works.

"Hello"[::-1] # returns "olleH"
[1,2,3,4,5][::-1] # returns [5,4,3,2,1]
"Hello"[1:5:1] # returns "ello"
"Hello"[1:5:2] # returns "el"

My searches for “third expression in python index” have come up empty. What is this expression and how does it work. Obviously ::-1 reverses the list but I can’t figure out what the other, positive values represent.

Answer 1

Actually the third optional argument is called step or stride.
It’s default value is 1.

>>> 'hello'[::] # here the value of stride is 1, means take a step of one between two indexes
'hello'   

>>> 'hello'[::2] #take a step of 2 so starting from h -->l --> o 
'hlo'

>>> 'abcdef'[::-1] #negative step actually means 'abcdef'[-1:-len('abcdef')-1:-1]
'fedcba'