Specialize syntax error for assignment expressions in invalid places

Feature or enhancement

Proposal:

There are a few places where (unparenthesized) assignment expressions are not accepted but users might think of using them. For example:

>>> func(arg=x := foo())
  File "<python-input-0>", line 1
    func(arg=x := foo())
               ^^
SyntaxError: invalid syntax

I find this error pretty confusing. It's not clear why this is a syntax error or what users should do it fix it (i.e. add parentheses).

I propose adding a specialized syntax error to point users in the right direction, similar to what's currently done for assert:

>>> assert x := 1
  File "<python-input-12>", line 1
    assert x := 1
           ^^^^^^
SyntaxError: cannot use named expression without parentheses here

Refs #138716, #146260

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

Commencez par reproduire les exemples de l’issue et lire la gestion des erreurs de syntaxe de CPython pour les assignment expressions, en la comparant au diagnostic spécialisé existant pour assert. Le travail sera considéré comme terminé lorsque les assignment expressions non valides et non parenthésées recevront un message clair orientant les utilisateurs vers les parenthèses, tandis que les formes valides conserveront leur comportement actuel.