ast.parse creates only Constant(value=0) nodes for a `case +0:` pattern, which is inconsistent.

[15r10nk]

Bug report

Bug description:

I found the following asymmetry in the generated ast while I was working with the new cpython 3.15.

c="""
match 0:
 case +0:
 pass
 case -0:
 pass
+0
-0
"""

import ast

print(ast.dump(ast.parse(c),indent=2))

output (Python 3.15.0b3+dev):

Module(
 body=[
 Match(
 subject=Constant(value=0),
 cases=[
 match_case(
 pattern=MatchValue(
 value=Constant(value=0)),
 body=[
 Pass()]),
 match_case(
 pattern=MatchValue(
 value=UnaryOp(
 op=USub(),
 operand=Constant(value=0))),
 body=[
 Pass()])]),
 Expr(
 value=UnaryOp(
 op=UAdd(),
 operand=Constant(value=0))),
 Expr(
 value=UnaryOp(
 op=USub(),
 operand=Constant(value=0)))])

+0 is parsed to Constant(value=0) instead of UnaryOp(op=UAdd(),operand=Constant(value=0)) when it is used as a match-expression.

This was really unexpected for me and can maybe produce some problems with ast.parse/unparse roundtripps or code which works with the ast and require workarounds (like in my case)

Is it possible to implement this in a way that it generates UnaryOp(...)?

@johnslavik you implemented this #148566, maybe you can help me here.

CPython versions tested on:

3.15

Operating systems tested on:

No response

Linked PRs

• gh-152708: Fix AST generation asymmetry for unary positive in match statements #152712

[Twix1288]

I would love to look at this bug to fix