`ast.MatchStar.name` incorrectly has class-level default value #134674

hunterhogan · 2025-05-25T21:38:53Z

Bug report

Bug description:

I apologize for my ignorance of most technical terms and the awkward explanation that flows from it.

Minimal, reproducible example

import ast
print(ast.dump(MatchStar(name=None)))

# Output
MatchStar()

But, is that really a problem?

(.venv) C:\clones\cpython>mypy -c "import ast; print(ast.dump(ast.MatchStar(name=None)))"
Success: no issues found in 1 source file

(.venv) C:\clones\cpython>mypy -c "import ast; print(ast.dump(ast.MatchStar()))"
<string>:1: error: Missing positional argument "name" in call to "MatchStar"  [call-arg]
Found 1 error in 1 file (checked 1 source file)

Ok, but is it a bug?

I'm assuming that design goals include one or more of the following:

ast.dump() produces code that does not generate errors.
Similar classes should behave similarly.

(Nevertheless, I am confident there are factors about which I am ignorant.)

Therefore, I compared ast.MatchStar to ast.Constant and ast.MatchSingleton:

# MatchStar.py
from ast import Constant, MatchSingleton, MatchStar
import ast

nodeConstructors: list[str] = [
    "ast.Constant(value=None, kind=None)",
    "ast.MatchSingleton(value=None)",
    "ast.MatchStar(name=None)",
]

for nodeAsStr in nodeConstructors:
    print(nodeAsStr, ast.dump(eval(nodeAsStr)), "", sep="\n")

# Output:
ast.Constant(value=None, kind=None)
Constant(value=None)

ast.MatchSingleton(value=None)
MatchSingleton(value=None)

ast.MatchStar(name=None)
MatchStar()

Ask mypy to lint the file above, including the output statements:

(.venv) C:\clones\cpython>mypy MatchStar.py
MatchStar.py:22: error: Missing positional argument "name" in call to "MatchStar"  [call-arg]
Found 1 error in 1 file (checked 1 source file)

Proposed changes

From

cpython/Parser/Python.asdl

Line 138 in f331d03

| MatchStar(identifier? name)

To

            | MatchStar(identifier name)

From

cpython/Lib/ast.py

Lines 154 to 162 in f331d03

    
           if ( 
        
               not show_empty 
        
               and (value is None or value == []) 
        
               # Special cases: 
        
               # `Constant(value=None)` and `MatchSingleton(value=None)` 
        
               and not isinstance(node, (Constant, MatchSingleton)) 
        
           ): 
        
               args_buffer.append(repr(value)) 
        
               continue

To

                if (
                    not show_empty
                    and (value is None or value == [])
                    # Special cases:
                    # `Constant(value=None)` and `MatchSingleton(value=None)` and `MatchStar(name=None)`
                    and not isinstance(node, (Constant, MatchSingleton, MatchStar))
                ):
                    args_buffer.append(repr(value))
                    continue

Changes tested in 3.13.3+ build

After compiling with the above changes, compare:

(.venv) C:\clones\cpython>PCbuild\amd64\python_d.exe -V && PCbuild\amd64\python_d.exe -c "import ast; print(ast.dump(ast.MatchStar(name=None)))"
Python 3.13.3+
MatchStar(name=None)

(.venv) C:\clones\cpython>py -V && py -c "import ast; print(ast.dump(ast.MatchStar(name=None)))"                        
Python 3.13.3
MatchStar()

Appendix: output from Python versions 3.9 to 3.12, and CPython main branch

(.venv) C:\clones\cpython>py -3.9 -V && py -3.9 -c "import ast; print(ast.dump(ast.MatchStar(name=None)))"              
Python 3.9.13
Traceback (most recent call last):
  File "<string>", line 1, in <module>
AttributeError: module 'ast' has no attribute 'MatchStar'

(.venv) C:\clones\cpython>py -3.10 -V && py -3.10 -c "import ast; print(ast.dump(ast.MatchStar(name=None)))" 
Python 3.10.11
MatchStar()

(.venv) C:\clones\cpython>py -3.11 -V && py -3.11 -c "import ast; print(ast.dump(ast.MatchStar(name=None)))" 
Python 3.11.9
MatchStar()

(.venv) C:\clones\cpython>py -3.12 -V && py -3.12 -c "import ast; print(ast.dump(ast.MatchStar(name=None)))" 
Python 3.12.10
MatchStar()

(.venv) c:\clones\cpython>PCbuild\amd64\python_d.exe -V && PCbuild\amd64\python_d.exe -c "import ast; print(ast.dump(ast.MatchStar(name=None)))"
Python 3.15.0a0
MatchStar()

CPython versions tested on:

3.13, 3.9, 3.10, 3.11, 3.12, CPython main branch

Operating systems tested on:

Windows

Linked PRs

The text was updated successfully, but these errors were encountered:

bedevere-app · 2025-05-25T23:57:39Z

GH-134681 is a backport of this pull request to the 3.13 branch.

…hon#134681

serhiy-storchaka · 2025-05-26T07:04:20Z

identifier can only be a string. MatchStar(None) is invalid if you change the declaration to MatchStar(identifier name).

serhiy-storchaka · 2025-05-26T07:09:43Z

It looks to me that this is a MyPy issue.

JelleZijlstra · 2025-05-26T14:31:42Z

I think MatchStar is behaving correctly here. ast.MatchStar(name=None) is different from ast.MatchSingleton(value=None) because in the former case the None indicates the absence of value (it corresponds to *_ in a sequence pattern); in the latter case the None indicates a value that is specifically None.

However, reading this did lead me to a real opportunity for improvement: #134718.

hunterhogan added the type-bug An unexpected behavior, bug, or error label May 25, 2025

hunterhogan mentioned this issue May 25, 2025

gh-134674: fix: ast.MatchStar.name incorrectly having class-level default value #134676

Closed

picnixz added interpreter-core (Objects, Python, Grammar, and Parser dirs) topic-parser labels May 25, 2025

hunterhogan mentioned this issue May 25, 2025

[3.13] fix: remove ast.MatchStar.name class-level default value (gh-134674) #134681

Closed

hunterhogan added a commit to hunterhogan/cpython that referenced this issue May 26, 2025

Add automatically generated file "Python-ast.c" (pythongh-134674) pyt…

cf29fd4

…hon#134681

hunterhogan added a commit to hunterhogan/cpython that referenced this issue May 26, 2025

pythongh-134674: Fix MatchStar name field handling and ast.dump display

041ab30

hunterhogan mentioned this issue May 26, 2025

gh-134674: remove ast.MatchStar.name class-level default value #134683

Closed

hunterhogan mentioned this issue May 26, 2025

ast.dump( show_empty = True) does not show most None values. #134733

Open

serhiy-storchaka closed this as not planned Won't fix, can't repro, duplicate, stale May 27, 2025

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`ast.MatchStar.name` incorrectly has class-level default value #134674

`ast.MatchStar.name` incorrectly has class-level default value #134674

hunterhogan commented May 25, 2025 •

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ast.MatchStar.name incorrectly has class-level default value #134674

ast.MatchStar.name incorrectly has class-level default value #134674

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Uh oh!

Bug report

Bug description:

Minimal, reproducible example

But, is that really a problem?

Ok, but is it a bug?

Proposed changes

From

To

From

To

Changes tested in 3.13.3+ build

Appendix: output from Python versions 3.9 to 3.12, and CPython main branch

CPython versions tested on:

Operating systems tested on:

Linked PRs

bedevere-app bot commented May 25, 2025

Uh oh!

serhiy-storchaka commented May 26, 2025

Uh oh!

serhiy-storchaka commented May 26, 2025

Uh oh!

JelleZijlstra commented May 26, 2025

Uh oh!

`ast.MatchStar.name` incorrectly has class-level default value #134674

`ast.MatchStar.name` incorrectly has class-level default value #134674

hunterhogan commented May 25, 2025 •

edited by bedevere-app bot

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