Why is this list slice treating 0 and 1 as the same in Python? [duplicate]

Question

This question already has answers here:

Understanding slicing (36 answers)

Closed 8 years ago.

I'm a bit confused about slicing a list and the numbering here.

Python tends to start from 0 instead of 1, and it's shown below to work in that way well enough. But if we're starting from 0, and going until 3, why am I not getting exam instead of exa? After all, 0 - 3 is 4 numbers.

>>> test = "example string"
>>> print test[:3]
exa
>>> print test[0:3]
exa
>>> print test[1:3]
xa

Answer 1

This is an intentional way, last element is exclusive.

Why is it so?

Firstly, my understanding it is because test[0:len(test)] should print all the list and then test[0:len(test)-1] excludes one element. This is prettier and more readable than if the last element would be included.

Secondly, it is because if you have test[:-1] it will return all the elements, but the last, which is very intuitive. If it would include the second index, to cut the last element you'd have to do test[:-2] which is ugly and makes it harder to read...

Also, length of your resulting list is end - start, which is yet another convenient feature about it.

Answer 2

If you have

text[0:3]

it will return elements from 0 to 3 - 1.

text[0]
text[1]
text[2]

not

text[3]

In general, it will return values from start to end - 1. For further information, you could read this.