Incorrect SyntaxError location for walrus operator in multi-line conditional expression

[zrekryu]

Bug report

Bug description:

When using the walrus operator (:=) inside a multi-line conditional expression, Python raises a SyntaxError with the error position pointing to the wrong location.
The caret (^) is shown under the expression body instead of the if clause where the syntax issue actually occurs, making debugging misleading.
The error indicator should instead highlight the if portion (or walrus usage context), not the preceding expression line.

Code

(
    1
    if x := True
    else 2
)

Traceback

File "[REDACTED]", line 2
    1
    ^
SyntaxError: expected 'else' after 'if' expression

It locates error to 1 but it should have located to if x := True (line no. 3)

It does not errors when condition is parenthesized:

(
    1
    if (x := True)
    else 2
)  # output: 1

CPython versions tested on:

3.14

Operating systems tested on:

Linux

[gvanrossum]

But that's allowed in if statements. Why not in if expressions?

[brianschubert]

Likewise in the else expression:

(
    1
    if 2
    else x := 3
)

which gives:

  File "<python-input-1>", line 2
    1
    ^
SyntaxError: cannot use assignment expressions with conditional expression

(which makes sense given that this is parsed as (1 if 2 else x) := 3, but it could be clearer if the error highlighted :=)

[picnixz]

So you suggest highlighting the walrus misusage here and reuse the same error message as with the else case?

[brianschubert]

I think the case I brought up is actually not very related and should probably be discussed separately (it's also less clear to me that changing it would be an improvement).

For the OP's case, I find the error somewhat confusing even when the whole expression is on a single line:

>>> 1 if x := True else 2
  File "<python-input-5>", line 1
    1 if x := True else 2
    ^^^^^^
SyntaxError: expected 'else' after 'if' expression

so I think this could definitely benefit from a specialized error message.

Maybe something like "assignment expression must be parenthesized inside conditional expression", which highlights the inner assignment expression? (not a parser expert, so I don't really know how doable that is)

[picnixz]

I can't remember the exact rule being involved here so let's ask @pablogsal

[brianschubert]

The current error is generated by this alternative, so I think we could add a specialized error for this case by adding an alternative just before it. This seems to do the trick locally:

diff --git a/Grammar/python.gram b/Grammar/python.gram
index 3a91d426c36..9ee350ae3d4 100644
--- a/Grammar/python.gram
+++ b/Grammar/python.gram
@@ -1264,6 +1264,8 @@ invalid_expression:
     # Soft keywords need to also be ignored because they can be parsed as NAME NAME
    | !(NAME STRING | SOFT_KEYWORD) a=disjunction b=expression_without_invalid {
         _PyPegen_raise_error_for_missing_comma(p, a, b) }
+   | disjunction 'if' a=disjunction ':=' b=disjunction {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "assignment expression must be parenthesized inside conditional expression" ) }
    | a=disjunction 'if' b=disjunction !('else'|':') { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "expected 'else' after 'if' expression") }
    | a=disjunction 'if' b=disjunction 'else' !expression {
         RAISE_SYNTAX_ERROR_ON_NEXT_TOKEN("expected expression after 'else', but statement is given") }

but I'd like to hear a parser expert's thoughts on if this is the right direction to go.

[gvanrossum]

Something's really weird with the relative priorities of walrus and if-else here.