On Thu, Dec 12, 2019 at 11:01:54AM -0800, Andrew Barnert via Python-ideas wrote:
> I don’t see any problem with a[] being the same as a[()]. We already
> have a[1,] is the same as a[(1,)] rather than a[1]
A comma is not an "index argument seperator". a[1,] is not the same as
a(1,) where the comma seperates arguments to the function call, and it
is permitted to end the call with a seperator. It is equivalent to:
temp = 1, # A tuple.
a[temp]
In the same way, a[2,4,8,16] is the same as
temp = 2,4,8,16 # A tuple.
a[temp]
So a[] would be:
temp = # A syntax error.
a[temp]
not an empty tuple.
The status quo is that subscripting syntax `a[ ... ]` requires that the
subscript (index, key or slice) inside the square brackets be explicitly
given. Even if you give `__getitem__` a default value for the subscript,
when using subscript syntax the caller must still provide it explicitly.
Your suggestion roughly corresponds to hard-coding into the interpreter
the rule that the empty subscript be changed to a default of `()`
regardless of what default `__getitem__` is given, or whether it is
given a default at all.
But why have the empty index default to () rather than None or 0 or -1
or slice(None, None, None) or some other value? That's an arbitrary
and not a good one.
And why overrule the locally defined default? I think it would be
confusing to define:
def __getitem__(self, index=0): ...
and then have `self[]` pass index=() instead of 0.
Even if subscripting honoured the locally defined default, it is still
an arbitrary choice with no real justification.
> There are presumably historical reasons why it turned out this way,
> but if you were designing a new language that had tuple and slice and
> ellipsis indexing like current Python, would you expect [] to be
> anything other than [()], or find it confusing?
Yes, I would expect it to be a syntax error, because syntactically, a
blank token (nothing at all) is not how we create empty tuples.
--
Steven
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