It represents the 'slice' to take from the result. The first element is the starting position, the second is the end (non-inclusive) and the third is the step. An empty value before/after a colon indicates you are either starting from the beginning (s[:3]
) or extending to the end (s[3:]
). You can include actual numbers here as well, but leaving them out when possible is more idiomatic.
For instance:
In [1]: s = 'abcdefg'
Return the slice of the string that starts at the beginning and stops at index position 2:
In [2]: s[:3]
Out[2]: 'abc'
Return the slice of the string that starts at the third index position and extends to the end:
In [3]: s[3:]
Out[3]: 'defg'
Return the slice of the string that starts at the end and steps backward one element at a time:
In [4]: s[::-1]
Out[4]: 'gfedcba'
Return the slice of the string that contains every other element:
In [5]: s[::2]
Out[5]: 'aceg'
They can all be used in combination with each other as well. Here, we return the slice that returns every other element starting at index position 6 and going to index position 2 (note that s[:2:-2]
would be more idiomatic, but I picked a weird number of letters :) ):
In [6]: s[6:2:-2]
Out[6]: 'ge'
The step element determines the elements to return. In your example, the -1
indicates it will step backwards through the item, one element at a time.
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