The Python library will not compile with a C++2020 compiler because the code uses the reserved “module” keyword

[aCuria]

BPO	39355
Nosy	@vstinner, @encukou, @serhiy-storchaka, @corona10, @erlend-aasland, @aCuria, @AliyevH
PRs	bpo-39355: Renamed module argument to pyModule in header files #28359 bpo-39355: Renamed module argument to pyModule in header files #28373 bpo-39355: making Python.h compatible with C++20 compilers #31282 gh-91321: Add _testcppext C++ extension #32175

^{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 = <Date 2022-03-29.13:39:30.927>
created_at = <Date 2020-01-16.09:25:21.719>
labels = ['easy', '3.9', '3.10', '3.11', 'expert-C-API', 'type-feature', 'invalid']
title = 'The Python library will not compile with a C++2020 compiler because the code uses the reserved \xe2\x80\x9cmodule\xe2\x80\x9d keyword'
updated_at = <Date 2022-03-30.14:14:56.748>
user = 'https://github.com/aCuria'

bugs.python.org fields:

activity = <Date 2022-03-30.14:14:56.748>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2022-03-29.13:39:30.927>
closer = 'vstinner'
components = ['C API']
creation = <Date 2020-01-16.09:25:21.719>
creator = 'aCuria'
dependencies = []
files = []
hgrepos = []
issue_num = 39355
keywords = ['patch', 'easy (C)']
message_count = 19.0
messages = ['360103', '360106', '360110', '360115', '398915', '399073', '399975', '401109', '401778', '413056', '414646', '414654', '416253', '416254', '416255', '416256', '416257', '416259', '416360']
nosy_count = 7.0
nosy_names = ['vstinner', 'petr.viktorin', 'serhiy.storchaka', 'corona10', 'erlendaasland', 'aCuria', 'AliyevH']
pr_nums = ['28359', '28373', '31282', '32175']
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue39355'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[aCuria]

The Python library will not compile with a C++2020 compiler because the code uses the reserved “module” keyword
 
For example, in warnings.h, we have the following code:
 
#ifndef Py_LIMITED_API
PyAPI_FUNC(int) PyErr_WarnExplicitObject(
    PyObject *category,
    PyObject *message,
    PyObject *filename,
    int lineno,
    PyObject *module,
PyObject *registry);
 
 
In modsupport.h we have the following code:
PyAPI_FUNC(int) PyModule_ExecDef(PyObject *module, PyModuleDef *def);
 
We can fix this by using a different identifier, for example “pyModule” instead of “module”

[serhiy-storchaka]

Names of arguments can be just removed from function declarations in header files.

[vstinner]

Qt has a similar issue with "slots": bpo-1086854 and bpo-38007.

[serhiy-storchaka]

Qt has different issue. "slots" is not a keyword, and the issue can be avoided by including Python.h before Qt.h or undefining the "slots" macro.

It could be a harder issue if "module" would be a field name of a public structure. But names of arguments are not part of the API.

[AliyevH]

If nobody works, i would like to solve this

[vstinner]

Please go ahead. Anyone is free to propose a fix.

[AliyevH]

We have tested with cxx-modules that issue.
module is just a specifier for export (only export is a compiler-based keyword in >= C++20)
That's why we can use module as argument name and there's no need to rename or delete *module arguments from header files.

What do you recommend to do?

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1103r3.pdf

[aCuria]

the word "module" should be treated as a reserved keyword.

Any use of "module" as an argument name should be changed to something else
throughout the code base.

On Fri, Aug 20, 2021 at 11:28 PM Hasan <report@bugs.python.org> wrote:

Hasan <hasan.aleeyev@gmail.com> added the comment:

We have tested with cxx-modules that issue.
module is just a specifier for export (only export is a compiler-based
keyword in >= C++20)
That's why we can use module as argument name and there's no need to
rename or delete *module arguments from header files.

What do you recommend to do?

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1103r3.pdf

----------

---

Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue39355\>

---

[AliyevH]

Okey. There will be huge changes for this issue as this keyword has been used in a lot of places.

That's why i will try to send pull requests module by module to keep it clean and simple for review.

[erlend-aasland]

There will be huge changes for this issue as this keyword has been used in a lot of places.

For external extension modules, it should be sufficient to change only the exposed headers; that is Include/.h and Include/cpython/.h. We should also change the AC tool, since that may be used by external projects.

[vstinner]

The Python library will not compile with a C++2020 compiler because the code uses the reserved “module” keyword

What is the error message? How can the error be reproduced?

[aCuria]

Compile with a compiler supporting the C++20 core feature (Modules)

https://en.cppreference.com/w/cpp/compiler_support

In visual studio, use C/C++ > Language > CPP Language Standard > C++20 or
higher

On Mon, Mar 7, 2022 at 5:32 PM STINNER Victor <report@bugs.python.org>
wrote:

STINNER Victor <vstinner@python.org> added the comment:

> The Python library will not compile with a C++2020 compiler because the
code uses the reserved “module” keyword

What is the error message? How can the error be reproduced?

----------

---

Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue39355\>

---

[vstinner]

If I build a C++ extension with -std=c++20, I get a compiler error on PyModuleDef_HEAD_INIT:

Modules/_testcppext.cpp:419:5: error: either all initializer clauses should be designated or none of them should be
419 | .m_name = "_testcppext",
| ^

Code:
---

static struct PyModuleDef module = {
    PyModuleDef_HEAD_INIT,
    .m_name = "_testcppext",
    .m_doc = module_doc,
    ...
};

---

Macro defined as (simplified code):
---

#define PyObject_HEAD_INIT(type) \
    { 1, type },

#define PyModuleDef_HEAD_INIT { \
    PyObject_HEAD_INIT(NULL)    \
    NULL, /* m_init */          \
    0,    /* m_index */         \
    NULL, /* m_copy */          \
  }

---

[vstinner]

When I wrote a PR to use the C header file pythoncapi_compat.h in the datatable C++ project, I got multiple C++ compiler warnings in static inline functions and in some macros:
h2oai/datatable#3231 (comment)

• Usage of NULL
• Usage of "old-style cast" like (ssize_t)1 or (PyObject*)obj

I solved this issue in pythoncapi_compat.h by using reinterpret_cast and nullptr:

• python/pythoncapi-compat@3477463
• Add C++ support pythoncapi-compat#18
• Use reinterpret_cast and nullptr in pythoncapi_compat.h h2oai/datatable#3237

By the way, pythoncapi_compat.h no longer uses "module":
python/pythoncapi-compat#22

The Python C API has similar issues, but warnings about NULL and old-style cast depend on the C++ compiler flags:

• -Wzero-as-null-pointer-constant
• -Wold-style-cast

[vstinner]

STINNER Victor:

What is the error message? How can the error be reproduced?

Keith (aCuria):

Compile with a compiler supporting the C++20 core feature (Modules)
https://en.cppreference.com/w/cpp/compiler_support

I built a C++ extension which calls PyModule_AddType(): I get no warning. I tested GCC and LLVM clang.

Commands:

clang -Wno-unused-result -g -Og -Wall -O0 -fPIC -I/home/vstinner/python/main/Include -I/home/vstinner/python/main -c Modules/_testcppext.cpp -o build/temp.linux-x86_64-3.11-pydebug/Modules/_testcppext.o -I/home/vstinner/python -Werror -Wall -Wextra -Wconversion -Wno-typedef-redefinition -std=c++20 -Wzero-as-null-pointer-constant -Wold-style-cast

gcc -Wno-unused-result -g -Og -Wall -O0 -fPIC -I/home/vstinner/python/main/Include -I/home/vstinner/python/main -c Modules/_testcppext.cpp -o build/temp.linux-x86_64-3.11-pydebug/Modules/_testcppext.o -I/home/vstinner/python -Werror -Wall -Wextra -Wconversion -Wno-typedef-redefinition -std=c++20 -Wzero-as-null-pointer-constant -Wold-style-cast

Reformatted commands:

['clang',
'-Wno-unused-result',
'-g',
'-Og',
'-Wall',
'-O0',
'-fPIC',
'-I/home/vstinner/python/main/Include',
'-I/home/vstinner/python/main',
'-c', 'Modules/_testcppext.cpp',
'-o', 'build/temp.linux-x86_64-3.11-pydebug/Modules/_testcppext.o',
'-I/home/vstinner/python',
'-Werror',
'-Wall',
'-Wextra',
'-Wconversion',
'-Wno-typedef-redefinition',
'-std=c++20',
'-Wzero-as-null-pointer-constant',
'-Wold-style-cast']

['gcc',
'-Wno-unused-result',
'-g',
'-Og',
'-Wall',
'-O0',
'-fPIC',
'-I/home/vstinner/python/main/Include',
'-I/home/vstinner/python/main',
'-c', 'Modules/_testcppext.cpp',
'-o', 'build/temp.linux-x86_64-3.11-pydebug/Modules/_testcppext.o',
'-I/home/vstinner/python',
'-Werror',
'-Wall',
'-Wextra',
'-Wconversion',
'-Wno-typedef-redefinition',
'-std=c++20',
'-Wzero-as-null-pointer-constant',
'-Wold-style-cast']

[vstinner]

I wrote the draft PR #76356 to test https://bugs.python.org/issue39355 and #75463. Problem: I don't get any compiler warning or error about the "module" C++20 keyword. I tested GCC 11.2.1 and clang 13.0.0 of Fedora 35.

[vstinner]

The C++20 "module" keyword is "contextual keyword". It's only a keyword if the first line if a file contains "module".

• https://en.cppreference.com/w/cpp/language/modules#Module_declarations
• https://en.cppreference.com/w/cpp/keyword/module

It's not the case in any .h file of the Python C API, so Python doesn't need to be changed. I close the issue.

[erlend-aasland]

The C++20 "module" keyword is "contextual keyword". It's only a keyword if the first line if a file contains "module".

Great! No changes needed. Thanks for investigating.

[vstinner]

Follow-up issue: bpo-47165 "[C API] Test that the Python C API is compatible with C++".