SUMMARY
This post asks for some examples that will improve PEP 637, by providing examples where the existing language specification is not adequate (but PEP 637 is). The rest of this post should tell you why I want these examples, and what they should look like.

DISCLAIMER
I'm an editor of PEP 637, writing in my personal capacity. And I'm also the author of package kwkey mentioned below.

SUMMARY
PEP 637 adds to Python's syntax and semantics. It adds statements such as
>>> x[1, 2, a=3, b=4] = val
to the syntax, and adds new associated semantics.

PEP 1, which defines the PEP process, states that any PEP that changes the existing language specification should clearly explain "why the existing language specification is inadequate to address the problem that the PEP solves".

As part of addressing this question of existing Python being adequate, I wrote a new package kwey. This package provides, I believe, a class B such that
>>> B(1, 2, a=3, b=4)[x] = val
is equivalent to
>>> x[1, 2, a=3, b=4] = val
once PEP 637 is implemented. (And if not, please report a bug, and if you can provide a patch.)

GOAL
The current version of PEP 637 doesn't mention kwkey. I'd particularly like us to look for problems where using kwkey as above is not adequate but PEP 637 is adequate.

Aside. Please treat this as a semantic problem, or in other words assume that
>>> x[SOMETHING]
>>> f(SOMETHING)
impose the same constraint on a well-formed expression SOMETHING.

Here's why. PEP 637 allows syntax such as
>>> x[1:2, 3:4, a=5:6, b=7:8]
and even after PEP 637 (as it currently is)
>>> f(1:2, 3:4, a=5:6, b=7:8)
is forbidden syntax. But PEP 637 is much more than a syntax extension.

Recall that we already have literal expressions such as
>>> [1, 2, 3] # List literal
>>> (1, 2, 3) # Tuple literal
>>> {1, 2, 3} # Set literal
in Python. If current Python turns out to have adequate semantics, then perhaps adding
>>> (1:2:3) # Slice literal
might by itself be enough. This is why I want particularly semantics examples. They're worth more.

CONCLUSION
I'd like to see semantic examples of problems that can be adequately solved by PEP 637, but not by using kwkey. The PEP process says, in so many words, that such examples should be provided.

--
Jonathan