`case ...:` generates unclear `SyntaxError` message

Feature or enhancement

Proposal:

I've noticed that this simple python code sample generates unclear error message:

>>> match 1:
...    case ...: ...
  File "<stdin>", line 2
    case ...: ...
         ^^^
SyntaxError: invalid syntax

I propose making the error message clearer:

>>> match 1:
...    case ...: ...
  File "<stdin>", line 2
    case ...: ...
         ^^^
SyntaxError: cannot use case statement with ellipsis

We already have this logic for := expressions:

>>> (a[0] := 1)
  File "<stdin>", line 1
    (a[0] := 1)
     ^^^^
SyntaxError: cannot use assignment expressions with subscript

Since case requires some special grammar (not regular python expression) it would be user-friendly to improve this message.

I have a PR ready.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-138865