We are discussing a proposal to extend Python's syntax to allow
d[1, 2, a=3, b=4]
We are also discussing the associated semantics. At present
d[1, 2]
d[(1, 2)]
are semantically equivalent.
There is a proposal, that
d[1, 2, a=3, b=4]
d[(1, 2), a=3, b=4]
be semantically equivalent.
I find this troubling, for example because
fn(1, 2, a=3, b=4)
fn((1, 2), a=3, b=4)
are semantically different.
Here's another example. If we are allowed to write
d[*argv, a=3, b=4]
then the proposal makes this equivalent to
d[argv, a=3, b=4]
when type(argv) is tuple.
Consider now
>>> def fn(*argv): print(argv)
>>> argv = 'key'
>>> fn(*argv)
('k', 'e', 'y')
I think it would be a trap for the unwary, that the equivalence of
d[argv, a=3, b=4]
d[*argv, a=3, b=4]
depends on the type of argv.
The root of the proposal that
d[1, 2, a=3, b=4]
d[(1, 2), a=3, b=4]
be semantically equivalent is this: At present
d[1, 2]
d[(1, 2)]
are semantically equivalent.
Why not instead , as part of the proposed semantics, make
d[1, 2]
d[(1, 2)]
semantically different, but only for those classes that ask for it. (This
would automatically preserve backwards compatibility.)
I believe this is possible and straightforward in the future, and also in
the present via the 'o' and K mechanism. I'm happy to implement this in
kwkeys, when I have time.
An aside: I also strongly believe that writing and studying examples that
use the new syntax, via the 'o' and K mechanism, is essential to making
good choices regarding the semantics, the writing and approval of the PEP,
and the success of the extension to Python (should the PEP be accepted).
I hope this helps us come to a shared understanding.
--
Jonathan
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