Improve/fix filtering SyntaxWarnings by module · Issue #135801 · python/cpython · GitHub

Feature or enhancement

Proposal:

Syntax warnings during AST parsing use _PyCompile_Warn to setup their warning context automatically. Though the filename is correct, the module is inferred and often not helpful. This is especially an issue when trying to ignore specific errors in a CI environment with pytest where the module is suddenly importlib._bootstrap. In other cases with python alone it's often just sys.

AFAICT this applies mainly to the "is" with '...' literal and 'return' in 'finally' block warnings.

--
To reproduce

# test.py
class A:
    def __init__(self):
        self.var = 2

    def func(self) -> bool:
        return self.var is 2

./python.exe -W error::SyntaxWarning:sys test.py

  File "/.../test.py", line 7
    return self.var is 2
           ^^^^^^^^^^^^^
SyntaxError: "is" with 'int' literal. Did you mean "=="?

Note: The SyntaxWarning is changed to an error for module sys where test would have been the expected module.

--
The SyntaxWarning is triggered here:

cpython/Python/codegen.c

Lines 1831 to 1837 in b14986c

    
           const char *msg = (op == Is) 
        
                   ? "\"is\" with '%.200s' literal. Did you mean \"==\"?" 
        
                   : "\"is not\" with '%.200s' literal. Did you mean \"!=\"?"; 
        
           expr_ty literal = !left ? left_expr : right_expr; 
        
           return _PyCompile_Warn( 
        
               c, LOC(e), msg, infer_type(literal)->tp_name 
        
           );

cpython/Python/compile.c

Lines 1204 to 1214 in b14986c

    
           _PyCompile_Warn(compiler *c, location loc, const char *format, ...) 
        
           { 
        
               va_list vargs; 
        
               va_start(vargs, format); 
        
               PyObject *msg = PyUnicode_FromFormatV(format, vargs); 
        
               va_end(vargs); 
        
               if (msg == NULL) { 
        
                   return ERROR; 
        
               } 
        
               int ret = _PyErr_EmitSyntaxWarning(msg, c->c_filename, loc.lineno, loc.col_offset + 1, 
        
                                                  loc.end_lineno, loc.end_col_offset + 1);

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

Linked PRs

This is a difficult issue. I have spent past three days on it, and may require other three days or week. I think there will be three PRs:

Reverting PR gh-131927: Prevent emitting optimizer warnings twice in the REPL #131993 and applying an alternative fix (see Syntax warnings can be swallowed in import #139640)
Adding the module parameter in compile() and other functions and passing the module name if possible. This is the only reliable way to get the correct module name for syntax warnings in the imported module. This is a new feature, because it adds new parameters and the caller code needs to be changed to utilize this. gh-135801: Add the module parameter to compile() etc #139652
Improve heuristic for the cases when the module name is not known (and for not yet modified third-party code). The current behavior is inconvenient on Posix and not usable on Windows (module name in PYTHONWARNINGS cannot contain :). The idea is to try to match multiple names starting from different path components. This can potentially be backported to 3.14, although we will need to change the documented behavior. But the current behavior is too poor. gh-135801: Improve filtering by module in warn_explicit() without module argument #140151

pythongh-135801: Add the module parameter to compile() etc

pythongh-135801: Improve filtering by module in warn_explicit() witho…

pythongh-135801: Add the module parameter to compile() etc

pythongh-135801: Improve filtering by module in warn_explicit() witho…

#140151 improves heuristic if module is not passed in warn_explicit(), and #139652 adds the module argument in compile() and similar functions.

pythongh-135801: Add tests for filtering warnings by module

PR #140240 adds tests for filtering by module in warn_explicit(), and #140241 is a separate issue for documentation of warn_explicit().

pythongh-135801: Add tests for filtering warnings by module (pythonGH…

[3.14] gh-135801: Add tests for filtering warnings by module (GH-140240…

[3.13] gh-135801: Add tests for filtering warnings by module (GH-140240…

gh-135801: Improve filtering by module in warn_explicit() without mod…

@serhiy-storchaka Thanks for your work on this one and the ast.parse issue! It's much appreciated. I just tested the changes from #140151. The issue I described in the OP is now resolved AFACIT. Both the warnings filter with the -W flag as well as the pytest warnings filter do work as I'd expect them too now.

I saw you mentioned it in #140151 (comment) already but I do agree this would be great to have in 3.14.1 as well.

I'm not sure it fully solves all issue discussed around PEP 765 but IMO it definitely improves the situations considerably.

Thanks again!

Note that if you have modules foo.bar and bar, a filter for bar will also match foo.bar. #139652 will make filtering more accurate.

gh-135801: Add the module parameter to compile() etc (GH-139652)

Merge branch 'main' into fix-issue21914

Merge branch 'main' of github.com:python/cpython into fix-Issue18387

Merge branch 'main' of github.com:python/cpython into fix-issue24255

pythongh-135801: Add tests for filtering warnings by module (pythonGH…

pythongh-135801: Improve filtering by module in warn_explicit() witho…

pythongh-135801: Add the module parameter to compile() etc (pythonGH-…

Improve/fix filtering SyntaxWarnings by module · Issue #135801 · python/cpython · GitHub

Repository navigation

Improve/fix filtering SyntaxWarnings by module #135801

Feature or enhancement

Proposal:

Has this already been discussed elsewhere?

Links to previous discussion of this feature:

Linked PRs

2 remaining items

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

	const char *msg = (op == Is)
	? "\"is\" with '%.200s' literal. Did you mean \"==\"?"
	: "\"is not\" with '%.200s' literal. Did you mean \"!=\"?";
	expr_ty literal = !left ? left_expr : right_expr;
	return _PyCompile_Warn(
	c, LOC(e), msg, infer_type(literal)->tp_name
	);

	_PyCompile_Warn(compiler c, location loc, const char format, ...)
	{
	va_list vargs;
	va_start(vargs, format);
	PyObject *msg = PyUnicode_FromFormatV(format, vargs);
	va_end(vargs);
	if (msg == NULL) {
	return ERROR;
	}
	int ret = _PyErr_EmitSyntaxWarning(msg, c->c_filename, loc.lineno, loc.col_offset + 1,
	loc.end_lineno, loc.end_col_offset + 1);

Improve/fix filtering SyntaxWarnings by module · Issue #135801 · python/cpython · GitHub

Repository navigation

Improve/fix filtering SyntaxWarnings by module #135801

Description

Feature or enhancement

Proposal:

Has this already been discussed elsewhere?

Links to previous discussion of this feature:

Linked PRs

Activity

serhiy-storchaka commented on Oct 6, 2025

serhiy-storchaka commented on Oct 16, 2025

serhiy-storchaka commented on Oct 17, 2025

2 remaining items

cdce8p commented on Oct 30, 2025

serhiy-storchaka commented on Oct 31, 2025

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions