Error message for walrus etc. in PEP 695 contexts could be improved

[JelleZijlstra]

Before:

>>> type A[T: (x:=3)] = int
  File "<stdin>", line 1
SyntaxError: 'named expression' can not be used within a TypeVar bound

After:

>>> type A[T: (x:=3)] = int
  File "<stdin>", line 1
SyntaxError: named expression cannot be used within a TypeVar bound

("cannot" is one word; the quotes don't really make sense here)

Linked PRs

• gh-104572: Improve error messages for invalid constructs in PEP 695 contexts #104573

[JelleZijlstra]

It would also be nice if the error display had a ^ pointing to the precise location. Unfortunately that appears to be difficult, as the error is raised in symtable.c where we no longer have access to the original source. The same thing happens with other syntax errors raised in symtable.c:

>>> def f(a, a): pass
... 
  File "<stdin>", line 1
SyntaxError: duplicate argument 'a' in function definition

[terryjreedy]

The quotes might have come from thinking of 'named expression' as a grammar target, but then it would have to really be in the doc grammar and would need a hyphen. Error messages should save quotes for quotes from the code, as with 'a' above.