PyArg_ParseTupleAndKeywords doesn't take const char *keywords[]

[hvenev]

BPO	21011
Nosy	@methane, @serhiy-storchaka, @MojoVampire, @hvenev

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2014-03-21.16:45:16.140>
labels = ['extension-modules', 'type-feature', '3.8']
title = "PyArg_ParseTupleAndKeywords doesn't take const char *keywords[]"
updated_at = <Date 2018-07-03.09:45:24.075>
user = 'https://github.com/hvenev'

bugs.python.org fields:

activity = <Date 2018-07-03.09:45:24.075>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Extension Modules']
creation = <Date 2014-03-21.16:45:16.140>
creator = 'h.venev'
dependencies = []
files = []
hgrepos = []
issue_num = 21011
keywords = []
message_count = 4.0
messages = ['214388', '214857', '320945', '320952']
nosy_count = 4.0
nosy_names = ['methane', 'serhiy.storchaka', 'josh.r', 'h.venev']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue21011'
versions = ['Python 3.8']

Linked PRs

• gh-65210: Add const qualifiers in PyArg_VaParseTupleAndKeywords() #105958

[hvenev]

This really annoys me. I have to store the literals in char[] and then make a char*[] from them. It would be better if a simple array of string literals could be used. It would also require less data space because string literals could be merged by the compiler.

I don't know why PyArg_ParseTupleAndKeywords would ever modify the keywords array so it makes absolutely no sense to me for the array not to be const char*[].

In all cases I have seen PyArg_ParseTupleAndKeywords being used string literals were converted to char* (dropping const) which is far far worse than inefficient code.

[MojoVampire]

This has come up before. Links to additional info:

https://mail.python.org/pipermail/python-dev/2006-February/060689.html

http://bugs.python.org/issue1772673

[methane]

I think we can't fix this until Python 4.
May I close this issue as "won't fix" for now?

Or can we add "Python 4" keyword to list backward incompatible changes we want to do in the future?

[serhiy-storchaka]

We can't change this API because this would break virtually all user code.

But we can introduce a macro like PY_SSIZE_T_CLEAN. If it is defined before including Python.h, PyArg_ParseTupleAndKeywords() will take an array of const strings. This will help for compatibility with C++.

[serhiy-storchaka]

We can fix this at least for C++, because in C++ the const char * const * declaration is compatible with arguments of type const char * const *, char * const *, const char **, and char **.

Sad, it is not so in C, but we can provide a lever to control the declaration, and may change the default in distant future.