[Python-Dev] Pre-PEP: Allow Empty Subscript List Without Parentheses

[Noam Raphael]

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