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bpo-39573: Porting to Python 3.10: Py_SET_SIZE() macro #20610

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vstinner merged 2 commits into from
Jun 4, 2020
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bpo-39573: Porting to Python 3.10: Py_SET_SIZE() macro #20610

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bpo-39573: Porting to Python 3.10: Py_SET_SIZE() macro
vstinner Jun 3, 2020
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Rewrite macros
vstinner Jun 4, 2020
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Rewrite macros
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@vstinner
vstinner committed Jun 4, 2020
commit 3feadc84f702f16620710ecc549d1c8375d78d1f
6 changes: 3 additions & 3 deletions Doc/whatsnew/3.10.rst
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Expand Up @@ -139,7 +139,7 @@ Porting to Python 3.10
compatibility, this macro can be used::

#if PY_VERSION_HEX < 0x030900A4
# define Py_SET_TYPE(obj, type) do { Py_TYPE(obj) = (type); } while (0)
# define Py_SET_TYPE(obj, type) ((Py_TYPE(obj) = (type)), (void)0)
#endif
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@eric-wieser eric-wieser Jun 4, 2020 •
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This macro doesn't behave the same way as the function in 3.9. I think a more accurate macro would be:

Suggested change
#if PY_VERSION_HEX < 0x030900A4
# define Py_SET_TYPE(obj, type) do { Py_TYPE(obj) = (type); } while (0)
#endif
#if PY_VERSION_HEX < 0x030900A4
# define Py_SET_TYPE(obj, type) ((Py_TYPE(obj)) = (type), (void)0)
#endif

which can be used as part of an expression like the function version of Py_SET_TYPE can:

return Py_SET_TYPE(my_obj, some_type), my_obj;

is legal in 3.9 and with my macro, but illegal with this PR.

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@vstinner vstinner Jun 4, 2020

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Oh wow, the C language is so surprising sometimes :-) I designed Py_SET_SIZE() as a static inline function to ensure that the result is void and to prevent strange usage. But I didn't know that return Py_SET_TYPE(my_obj, some_type), (...); was legit.

I updated macros to use , (void)0. According to https://stackoverflow.com/a/25021889 this syntax is endorsed by the C standard, since NDEBUG uses #define assert(ignore) ((void)0).

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@eric-wieser eric-wieser Jun 4, 2020 •
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Thanks for finding some precedent. I've seen "always use parens and do { } while(0);" given as macro advice forever, but I don't remember ever seeing this case before.

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@vstinner vstinner Jun 4, 2020

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In the meanwhile, I created the backport compatiblity header file :-) It defines a static inline function which returns void:
https://github.com/pythoncapi/pythoncapi_compat/blob/219cdd858595fb5d478117b0ef4758fedc3d2365/pythoncapi_compat.h#L29

Maybe tomorrow when it will be more mature (than 1 hour old), numpy might want to use it.


(Contributed by Dong-hee Na in :issue:`39573`.)
Expand All @@ -150,7 +150,7 @@ Porting to Python 3.10
compatibility, this macro can be used::

#if PY_VERSION_HEX < 0x030900A4
# define Py_SET_REFCNT(obj, refcnt) do { Py_REFCNT(obj) = (refcnt); } while (0)
# define Py_SET_REFCNT(obj, refcnt) ((Py_REFCNT(obj) = (refcnt)), (void)0)
#endif

(Contributed by Victor Stinner in :issue:`39573`.)
Expand All @@ -161,7 +161,7 @@ Porting to Python 3.10
compatibility, this macro can be used::

#if PY_VERSION_HEX < 0x030900A4
# define Py_SET_SIZE(obj, size) do { Py_SIZE(obj) = (size); } while (0)
# define Py_SET_SIZE(obj, size) ((Py_SIZE(obj) = (size)), (void)0)
#endif

(Contributed by Victor Stinner in :issue:`39573`.)
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