Python macros don’t shield arguments

[numberZero]

BPO	43181
Nosy	@gpshead, @vstinner, @WildCard65, @erlend-aasland, @numberZero
PRs	bpo-43181: Make PyObject_TypeCheck a static inline function #24533

^{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 2021-02-15.16:21:14.723>
created_at = <Date 2021-02-09.22:05:50.575>
labels = ['expert-C-API', 'build', '3.10']
title = 'Python macros don\xe2\x80\x99t shield arguments'
updated_at = <Date 2021-03-22.16:47:15.483>
user = 'https://github.com/numberZero'

bugs.python.org fields:

activity = <Date 2021-03-22.16:47:15.483>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2021-02-15.16:21:14.723>
closer = 'vstinner'
components = ['C API']
creation = <Date 2021-02-09.22:05:50.575>
creator = 'numberZero'
dependencies = []
files = []
hgrepos = []
issue_num = 43181
keywords = ['patch', '3.9regression']
message_count = 17.0
messages = ['386746', '386754', '386760', '386822', '386827', '386998', '387001', '387007', '387023', '387024', '387360', '388509', '388714', '388727', '388730', '388733', '389331']
nosy_count = 5.0
nosy_names = ['gregory.p.smith', 'vstinner', 'WildCard65', 'erlendaasland', 'numberZero']
pr_nums = ['24533']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue43181'
versions = ['Python 3.10']

[numberZero]

There is a lot of macros like:

 #define PyObject_TypeCheck(ob, tp) \
 (Py_IS_TYPE(ob, tp) || PyType_IsSubtype(Py_TYPE(ob), (tp)))

These work fine until an argument happen to contain a comma. That’s possible as a result of other macro’s expansion. E.g. if U(x) is defined as x,

 PyObject_TypeCheck(ob, U(f<a,b>(c)))

expands to

 (Py_IS_TYPE(ob, f<a,b>(c)) || ...)

but < and > aren’t treated as brackets by the preprocessor so Py_IS_TYPE is now invoked with 3 arguments instead of just 2, breaking module compilation.

As arguments are expected to be values, surrounding each with parentheses solves the problem. But there are many such macros so that’s not an one-line fix.

Note: the example is from PyGLM (simplified), it doesn’t compile on 3.9 due to this issue.

[WildCard65]

This feels like it's more of an issue with the C++ compiler you're using. (I can tell it's C++ because of template syntax)

[gpshead]

This bug is valid, that macro and likely others aren't up to best practices.

C macros are a PITA to get right.

[vstinner]

I'm replacing macros with static inline functions to avoid issues like this one (ex: bpo-35059). Are you interested to convert the PyObject_TypeCheck() macro to a static inline function?

"""
There is a lot of macros like:
 #define PyObject_TypeCheck(ob, tp) \
 (Py_IS_TYPE(ob, tp) || PyType_IsSubtype(Py_TYPE(ob), (tp)))
These work fine until an argument happen to contain a comma.
"""

This macro also has a common bug of macros: if one argument has a side effect, it is executed twice! For example, if an argument is a function call, the function is called twice.

See:

• https://gcc.gnu.org/onlinedocs/cpp/Macro-Pitfalls.html
• https://vstinner.github.io/split-include-directory-python38.html "Convert macros to static inline functions"

[erlend-aasland]

Are you interested to convert the PyObject_TypeCheck() macro to a static inline function?

I'd like to give it a shot if it's ok for you all.

[erlend-aasland]

Silence implies consent? I'll throw up a PR, then :)

[vstinner]

I don't think that PR 24533 should be backported to Python 3.8 and Python 3.9. I prefer to avoid any risk of regression, and so only change Python 3.10.