Improve error message for `except a as b.c:` case

[sobolevn]

Feature or enhancement

Right now the syntax error is not very clear:

I propose to instead use something like:

@JelleZijlstra suggested to use similar error messages to := case, where we also only expect a name:

>>> (a.b := 3)
  File "<unknown>", line 1
    (a.b := 3)
     ^^^
SyntaxError: cannot use assignment expressions with attribute
>>> (a[0] := 3)
  File "<unknown>", line 1
    (a[0] := 3)
     ^^^^
SyntaxError: cannot use assignment expressions with subscript
>>> ((a, b) := 3)
  File "<unknown>", line 1
    ((a, b) := 3)
     ^^^^^^
SyntaxError: cannot use assignment expressions with tuple

I am working on this right now :)

Linked PRs

• gh-123440: Improve error message for except as used with not a name #123442

[sobolevn]

In the next PRs I will also address:

>>> import a as x[0]
  File "<python-input-2>", line 1
    import a as x[0]
                 ^
SyntaxError: invalid syntax

>>> match 1:
...    case x as y[0]: pass
...    
  File "<python-input-3>", line 2
    case x as y[0]: pass
               ^
SyntaxError: invalid syntax