[issue39591] Functions in Python/traceback.c can take const pointer arguments

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Even if the object is not modified currently in common case, it does not 
guarantee that it cannot be modified in uncommon cases, or that it cannot be 
modified in future after introducing changes in different files.

For example, if names was created by the legacy C API like 
PyUnicode_FromUnicode(), it will be modified by calling PyUnicode_READY().

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[issue39591] Functions in Python/traceback.c can take const pointer arguments

[Andy Lester]

Andy Lester  added the comment:

> Yes, Py_INCREF and Py_DECREF change the type, and therefore constness.

Understood. The changes that I have proposed are not to objects that get sent 
through Py_INCREF/Py_DECREF.  If they did, -Wcast-qual would have caught it.  
-Wcast-qual catches if you cast, say, a const char * to a char *.

Let's let this stay closed and I'll resubmit with a clearer ticket & PR.

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[issue39591] Functions in Python/traceback.c can take const pointer arguments

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Yes, Py_INCREF and Py_DECREF change the type, and therefore constness.

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[issue39591] Functions in Python/traceback.c can take const pointer arguments

[Andy Lester]

Andy Lester  added the comment:

I'm sorry, I think my comment was misleading.

The changes I had proposed were not making the object itself const, but some of 
the arguments in the static worker functions.  For example:

-tb_displayline(PyObject *f, PyObject *filename, int lineno, PyObject *name)
+tb_displayline(PyObject *f, PyObject *filename, int lineno, const PyObject 
*name)

and

-tb_printinternal(PyTracebackObject *tb, PyObject *f, long limit)
+tb_printinternal(const PyTracebackObject *tb, PyObject *f, long limit)

I've got -Wincompatible-pointer-types-discards-qualifiers and -Wcast-qual 
turned on, and no errors occur.

Is there somewhere in the deep internals of the Python macros where constness 
can be changed but the compiler isn't reporting on it?

Thanks,
Andy

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[issue39591] Functions in Python/traceback.c can take const pointer arguments

[Andy Lester]

Change by Andy Lester :


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pull_requests:  -17798

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[issue39591] Functions in Python/traceback.c can take const pointer arguments

[Serhiy Storchaka]

Change by Serhiy Storchaka :


--
resolution:  -> rejected
stage: patch review -> resolved
status: open -> closed

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[issue39591] Functions in Python/traceback.c can take const pointer arguments

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

They cannot. PyObject cannot be const because the code that uses it can change 
its reference counter even if it does not change any other fields.

--
nosy: +serhiy.storchaka

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[issue39591] Functions in Python/traceback.c can take const pointer arguments

[Andy Lester]

Change by Andy Lester :


--
pull_requests:  -17794

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[issue39591] Functions in Python/traceback.c can take const pointer arguments

[Andy Lester]

Change by Andy Lester :


--
pull_requests: +17798
pull_request: https://github.com/python/cpython/pull/18422

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[issue39591] Functions in Python/traceback.c can take const pointer arguments

[Andy Lester]

Change by Andy Lester :


--
keywords: +patch
pull_requests: +17794
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/18420

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[issue39591] Functions in Python/traceback.c can take const pointer arguments

[Andy Lester]

New submission from Andy Lester :

The functions tb_displayline and tb_printinternal can take const pointers on 
some of their arguments.

tb_displayline(PyObject *f, PyObject *filename, int lineno, const PyObject 
*name)

tb_printinternal(const PyTracebackObject *tb, PyObject *f, long limit)

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components: Interpreter Core
messages: 361643
nosy: petdance
priority: normal
severity: normal
status: open
title: Functions in Python/traceback.c can take const pointer arguments
type: enhancement

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