Specialize syntax error for assignment expressions in invalid places

[brianschubert]

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

[brianschubert]

I tried implementing this with this patch:

diff --git a/Grammar/python.gram b/Grammar/python.gram
index 3a91d426c36..75ebf0dc7e3 100644
--- a/Grammar/python.gram
+++ b/Grammar/python.gram
@@ -1273,6 +1273,7 @@ invalid_expression:
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "f-string: lambda expressions are not allowed without parentheses") }
    | a='lambda' [lambda_params] b=':' &TSTRING_MIDDLE  {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "t-string: lambda expressions are not allowed without parentheses") }
+   | a=NAME ':=' b=expression { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "assignment expression must be parenthesized here") }
 
 invalid_if_expression:
    | disjunction 'if' b=disjunction 'else' a='*' {

This works well, but I worry it might be overly broad. There's a few invalid cases where this overrides the existing error:

lambda: x:=1
1 if True else x := 1

These currently report:

  File "<python-input-0>", line 1
    lambda: x:=1
    ^^^^^^^^^
SyntaxError: cannot use assignment expressions with lambda

  File "<python-input-1>", line 1
    1 if True else x := 1
    ^^^^^^^^^^^^^^^^
SyntaxError: cannot use assignment expressions with conditional expression

But with the above patch would report:

>>> lambda: x:=1
  File "<python-input-0>", line 1
    lambda: x:=1
            ^^^^
SyntaxError: assignment expression must be parenthesized here
>>> 1 if True else x := 1
  File "<python-input-1>", line 1
    1 if True else x := 1
                   ^^^^^^
SyntaxError: assignment expression must be parenthesized here

I think this is arguably an improvement (it's more likely that the user intended an inner assignment expression than assigning to a non-identifier LHS). But I am little hesitant, since this isn't actually representative of the first pass parser state. (Note that this only applies to sub expressions that start with an assignment expression; cases like lambda: x and y:=1 are unaffected).

This also wouldn't help in a handful of places that don't take an expression (for example, 1 if x:=1 else 2 and [a for a in x:=1]).

@pablogsal @lysnikolaou do you think the changes to the existing syntax errors shown above would be acceptable? If so I can open a PR to show what the broader impact might look like.