[Python-Dev] Pre-PEP: Allow Empty Subscript List Without Parentheses (original) (raw)

Noam Raphael noamraph at gmail.com
Sat Jun 10 21🔞26 CEST 2006

Hello,

2006/6/10, Nick Coghlan <ncoghlan at gmail.com>:

The closest parallel would be with return/yield, as those actually create real tuples the same way subscripts do, and allow the expression to be omitted entirely.

By that parallel, however, an implicit subscript (if adopted) should be None rather than (). Adapting the table from the pre-PEP to describe return statements (and yield expressions): return i, j, k <--> return (i, j, k) return i, j <--> return (i, j) return i, <--> return (i, ) return i <--> return (i) return () # (No implicit equivalent) return <--> return None With the status quo, however, subscripts are simply equivalent to the RHS of an assignment statement in requiring that the expression be non-empty: x = i, j, k <--> x = (i, j, k) x = i, j <--> x = (i, j) x = i, <--> x = (i, ) x = i <--> x = (i) x = () # (No implicit equivalent) x = None # (No implicit equivalent) The PEP doesn't make a sufficiently compelling case for introducing yet-another-variant on the implicit behaviour invoked when a particular subexpression is missing from a construct. I hope that my (hopefully) better explanation made the use case more compelling, but I want to add two points in favour of an empty tuple:

  1. If you want, you can ignore the x[(i, j, k)] equivalence completely, since it doesn't work all the times - for example, you can write "x[1:2, 3:4]", but you can't write "x[(1:2, 3:4)]". You can think of x[i, j, k] as a syntax for specifying a cell in a 3-dimensional array, resulting in a call to x.getitem with a 3-tuple describing the subscript for each dimension. In that view, "x[]", which is a syntax for specifying the cell of a 0-dimensional, should result in a getitem call with an empty tuple, as there are no subscripts to be described.

  2. My equivalencies are better than yours :-), since they are dealing with equivalencies for this specific syntax, while yours are dealing with similar properies of a syntax for doing something completely different.

I guess I could have gone with my initial instinct of -1 and saved myself some mental exercise ;)

Why? Mental exercise is a good way to keep you mental ;)

Noam

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