[Python-Dev] introduction of __attribute__(deprecated) ?

[Stéphane Wirtel]

Hi,

I have created the issue https://bugs.python.org/issue36347 because I
wanted to add a missing macro for the PyMemberDef.flags attribute.

In the Modules/*.c files, we can find descriptions with
PyMemberDef where the access flag has the 0 value.

Example:

static PyMemberDef members[] = {
    {"name", T_OBJECT, offsetof(MyObject, name), 0, NULL},
    {NULL},
};

So, I didn't know the meaning of this magic number (0) and after a small
search in structmember.h, this is the default value for an "READWRITE"
attribute but there is no associated macro to this magic number.

solution: add the macro for the READWRITE mode.

so, my first PR has added the new macro READWRITE, like that

#define READWRITE 0

and improve the documentation in Doc/c-api/structures.rst.

but after a review [1], Serhiy proposed to rename it and the other ones
because it did not follow the convention for names in the C API.
Use a prefix for the public names, example Py_ or PY_.

I chose PY_ because PY_WRITE_RESTRICTED already existed.

the next steps were:
* add the new macros
* keep the backward-compatibility
* updated the documentation

I haved pushed the PR for another review

so, but I don't like this definition of READONLY because there is no way
to generate a warning at the compile time when we use the READONLY macro.

#define PY_READONLY	0
#define READONLY 	PY_READONLY

after that, I have closed my PR because I was not proud with my "solution".


Today, Ronald Oussoren [2] has proposed an other solution based on an
enum and a new __attribute__, aka deprecated.

I have checked with gcc and clang and this option is interesting because
we generate a warning when we try to compile a code if we use the
deprecated READONLY [3]

In the clang documantion, the __attribute__(deprecated) is defined in
GNU and C++11 standard, like __attribute__(unused).

In the gcc documentation, this attribute also exists

Because we already use __attribute__(unused) we could add
__attribute__(deprecated).


LC_ALL=C clang test.c -o test
test.c:14:13: warning: 'READONLY' is deprecated: use PY_READONLY
[-Wdeprecated-declarations]
    int i = READONLY;
            ^
test.c:8:27: note: 'READONLY' has been explicitly marked deprecated here
    READONLY __attribute((deprecated("use PY_READONLY"))) = PY_READONLY,
                          ^
1 warning generated.

LC_ALL=C clang --version test.c -o test
clang version 7.0.1 (Fedora 7.0.1-6.fc29)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

### GCC
LC_ALL=C gcc --version test.c -o test
gcc (GCC) 8.3.1 20190223 (Red Hat 8.3.1-2)
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

LC_ALL=C gcc test.c -o test
test.c: In function 'main':
test.c:14:5: warning: 'READONLY' is deprecated: use PY_READONLY
[-Wdeprecated-declarations]
     int i = READONLY;
     ^~~
test.c:8:5: note: declared here
     READONLY __attribute((deprecated("use PY_READONLY"))) = PY_READONLY,
     ^~~~~~~~

So my question is, can we use/add __attribute__(deprecated) in our
"development" kit?

[1] https://bugs.python.org/issue36347#msg338261
[2] https://bugs.python.org/issue36347#msg338731
[3] https://bugs.python.org/issue36347#msg338745

Thank you,

Stéphane