[issue28336] Slicing (operation) is not symmetrical with respect to suffixes and prefixes

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

The default value of the lower bound index is 0, but the default value of the 
upper bound index is the length of the sequence. s[:a] is the same as s[0:a], 
but s[a:] is the same as s[a:len(s)].

If make s[a:0] meaning the same as s[a:len(s)], i.e. return a subsequence from 
index a to the end, while s[0:a] means the same as s[:a], i.e. returns a 
subsequence from the begin to index a, what should mean s[0:0]? There is a 
contradiction.

This change would break existing code.

--
nosy: +serhiy.storchaka
resolution:  -> rejected
stage:  -> resolved
status: open -> closed

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[issue28336] Slicing (operation) is not symmetrical with respect to suffixes and prefixes

[Juan Carlos Pujol Mainegra]

New submission from Juan Carlos Pujol Mainegra:

Let s be a string or other array-like object, a, b > 0 integers, s[0:b] returns 
a b-long prefix to s, the same as s[:b], but s[-a:0] returns empty (for len(s) 
> 0 and a > 1), while it should return the same as s[-a:], an a-long suffix (a 
> 0).

A syntax asymmetry like this shall not be imposed to those using non-literal 
slicing indexes, as it would be necessarily to introduce a control condition to 
test whether the upper bound index is a non-negative quantity, assuming the 
lower bound index is negative. Furthermore, it breaks the whole negative 
slicing idea, being that (I consider) index i always be treated as i mod 
len(s), so that constructions like s[-a:b] (for a, b > 0 or a, b < 0) could 
return s[-a:] + s[:b].

--
components: Interpreter Core
messages: 277829
nosy: jksware
priority: normal
severity: normal
status: open
title: Slicing (operation) is not symmetrical with respect to suffixes and 
prefixes
type: enhancement
versions: Python 3.6

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