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[C API] Add Py_Is(x, y) and Py_IsNone(x) functions #87919Description
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assignee = None
closed_at = <Date 2021-05-18.20:44:33.857>
created_at = <Date 2021-04-06.18:38:58.346>
labels = ['expert-C-API', '3.10']
title = '[C API] Add Py_Is(x, y) and Py_IsNone(x) functions'
updated_at = <Date 2021-09-29.23:28:18.199>
user = 'https://github.com/vstinner' bugs.python.org fields:
activity = <Date 2021-09-29.23:28:18.199>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2021-05-18.20:44:33.857>
closer = 'vstinner'
components = ['C API']
creation = <Date 2021-04-06.18:38:58.346>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 43753
keywords = ['patch']
message_count = 19.0
messages = ['390358', '390360', '390364', '390379', '390408', '390411', '390412', '390435', '390456', '390469', '390526', '390613', '390614', '390616', '390628', '390750', '390751', '393907', '402922']
nosy_count = 9.0
nosy_names = ['tim.peters', 'rhettinger', 'mark.dickinson', 'vstinner', 'Carl.Friedrich.Bolz', 'Mark.Shannon', 'pablogsal', 'shihai1991', 'erlendaasland']
pr_nums = ['25227', '28641']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue43753'
versions = ['Python 3.10'] Activityvstinner commented on Apr 6, 2021 I propose to add at least Py_Is(x, y) function to the Python C API which would be simply implemented as:
static inline int Py_Is(PyObject *x, PyObject *y) { return (x == y); }Right now, there is no benefit for CPython. The idea is to prepare the CPython code base for future optimization, like tagged pointers. It may help PyPy. It can also help to prepare C extensions to migrate to HPy, since HPy requires to use HPy_Is(ctx, x, y): "x == y" (where x and y types is HPy) fails with a compiler error with HPy.
--
CPython uses reference counting and singletons like None and False. The "x is y" operator in Python is implemented with the IS_OP opcode implemented in Python/ceval.c as:
case TARGET(IS_OP): {
PyObject *right = POP();
PyObject *left = TOP();
int res = (left == right)^oparg;
PyObject *b = res ? Py_True : Py_False;
Py_INCREF(b);
SET_TOP(b);
Py_DECREF(left);
Py_DECREF(right);
PREDICT(POP_JUMP_IF_FALSE);
PREDICT(POP_JUMP_IF_TRUE);
DISPATCH();
}In short, "x is y" in Python simply compares directly PyObject* pointer values in C: "x == y" where x and y are PyObject* pointers.
PyPy doesn't use reference counting and so implements "x is y" differently: id(x) == id(y) if I understood correctly. In PyPy, id(obj) is more complex than simply getting the object memory address. For example, id(1) is always 17 in PyPy (on Linux/x86-64 at least).
At the C API level, using "x == y" to check if y object "is" x object doesn't work well with PyPy object models. Moreover, "x == Py_None" doesn't work if None is stored as a tagged pointer in CPython.
--
Maybe we can also add functions to check if an object is a singleton:
- Py_IsNone(x): x == Py_None
- Py_IsTrue(x): x == Py_True
- Py_IsFalse(x): x == Py_False
See also bpo-39511 "[subinterpreters] Per-interpreter singletons (None, True, False, etc.)" which may need to replace Py_None singleton with Py_GetNone(). IMO it makes more sense to call Py_IS_NONE(x) function to express that code is run, rather than writing "x == Py_None" as if Py_None is and will always be a variable directly accesssible.
Py_IS_TRUE() name sounds like PyObject_IsTrue(). Can it be an issue? Can someone come with a better name? vstinner commented on Apr 6, 2021 Python has more singletons. In C:
- Py_NotImplemented: NotImplemented
- Py_Ellipsis: Ellipsis
But I don't think that "x == NotImplemented" is common enough to justify a new function. changed the title [-][C API] Add Py_IS(x, y) macro[/-] [+][C API] Add Py_IS(x, y) and Py_IsNone(x) functions[/+] on Apr 6, 2021changed the title [-][C API] Add Py_IS(x, y) macro[/-] [+][C API] Add Py_IS(x, y) and Py_IsNone(x) functions[/+] on Apr 6, 2021changed the title [-][C API] Add Py_IS(x, y) and Py_IsNone(x) functions[/-] [+][C API] Add Py_Is(x, y) and Py_IsNone(x) functions[/+] on Apr 6, 2021changed the title [-][C API] Add Py_IS(x, y) and Py_IsNone(x) functions[/-] [+][C API] Add Py_Is(x, y) and Py_IsNone(x) functions[/+] on Apr 6, 2021vstinner commented on Apr 6, 2021 rhettinger commented on Apr 6, 2021
Right now, there is no benefit for CPython.
Please don't this until we have a clear demonstrable benefit. As it stands now, this is all cost and no benefit.
Adding unnecessary abstraction layers just makes it more difficult for people to learn to be core devs. Also, it will likely result in a lot of pointless code churn where each change carries a risk of a new bug being introduced.
The notion of pointer comparison is so fundamental to C code that it is counter productive to try to abstract it away. It isn't much different than saying that every instance of "a + b" should be abstracted to "add(a, b)" and every "a[b]" should be abstracted to "index_lookup(a, b)" on the hope that maybe it might someday be helpful for PyPy or HPy even though they have never requested such a change.
From my own point of view, these need abstractions just make it harder to tell what code is actually doing. Further, it will just lead to wasting everyone's time in code reviews where the reviewer insists on the applying the new inline function and there is confusion about whether two pointers are generic pointers or python object pointers, each with their own comparison technique.
Also, there is too much faith in functions marked as "inline" always being inlined. Compilers get to make their own choices and under some circumstances will not inline, especially for cross module calls. This risks taking code that is currently obviously fast and occasionally, invisibility slowing it down massively — from a step that is 1 cycle at most and is sometimes zero cost to a step that involves an actual function call, possibly needing to save and restore registers.
Lastly, the API changes aren't just for you or the standard library. In effect, you're telling the entire ecosystem of C extensions that they are doing it wrong. Almost certainly, some will follow this path and some won't, further fracturing the ecosystem. markshannon commented on Apr 7, 2021 For any sane design of tagged pointers, x == y (in C) will work fine.
is is not well defined except for a small set of values, so the docs for Py_Is would have to so vague as to be worthless, IMO.
vstinner commented on Apr 7, 2021
For any sane design of tagged pointers, x == y (in C) will work fine.
I wrote a proof-of-concept for using tagged pointners in CPython:
vstinner#6
"The PR is large because of the first changes which add Py_IS_NONE(), Py_IS_TRUE() and Py_IS_FALSE() macros."
For backward compatibility, I added a function to get a concrete Python object from a tagged pointer: the True as a tagged pointer becomes (PyObject *)&_Py_TrueStruct concrete object. Py_IS_TRUE() has to check for both values: tagged pointer or &_Py_TrueStruct (don't look at my exact implementation, it's wrong ;-)).
In a perfect world, there would be no backward compatibility and you would never have to create Python objects from a tagged pointer.
The other problem is that the stable ABI exposes "Py_True" as "&_Py_TrueStruct" and so C extensions built with the stable ABI uses "&_Py_TrueStruct", not the tagged pointer.
See also bpo-39511 which discuss solutions for these problems. vstinner commented on Apr 7, 2021
Also, there is too much faith in functions marked as "inline" always being inlined.
I proposed to declare it as a "static inline" function, but I'm fine with a macro as well. I mostly care about the API: call "Py_Is()", not really about the exact implementation. In practice, for now, Py_Is(x, y) will continue to be compiled as "x == y". cfbolz commented on Apr 7, 2021 Just chiming in to say that for PyPy this API would be extremely useful, because PyPy's "is" is not implementable with a pointer comparison on the C level (due to unboxing we need to compare integers, floats, etc by value). Right now, C extension code that compares pointers is subtly broken and cannot be fixed by us. Reacted by Erlend E. Aasland rhettinger commented on Apr 7, 2021
Just chiming in to say that for PyPy this API would be extremely useful
Thanks for that input. Given that there would be some value add, I withdraw my objection.
I proposed to declare it as a "static inline" function,
but I'm fine with a macro as well.
Let's use a macro then because inlining in guaranteed and we don't have to be on the lookout for failures to inline. vstinner commented on Apr 7, 2021 PR 25227: I reimplemented Py_Is() as a macro and I added unit tests.
Other added functions simply call Py_Is(), example:
#define Py_IsNone(x) Py_Is(x, Py_None) vstinner commented on Apr 8, 2021 Mark:
is is not well defined except for a small set of values, so the docs for Py_Is would have to so vague as to be worthless, IMO.
I don't propose to change the "is" operator semantic: Py_Is(x, y) is C would behave *exaclty* as "x is y" in Python.
Check my implementation: the IS_OP bytecode uses directly Py_Is(). erlend-aasland commented on Apr 9, 2021 I'd also prefer a Py_IsNotNone() because it's more explicit than !Py_IsNone(); the exclamation mark can be easily missed when reading/writing code, IMO.
Just my 2 cents. vstinner commented on Apr 9, 2021
I'd also prefer a Py_IsNotNone() because it's more explicit than !Py_IsNone()
I would prefer keep the C API small. I don't think that we need to duplicate all functions testing for something. We provide PyTuple_Check(obj) but we don't provide PyTuple_NotCheck(obj) for example.
IMO !Py_IsNone(obj) makes perfectly sense in Python.
Also, "x == Py_None" is more common than "x != Py_None". Reacted by Erlend E. Aasland erlend-aasland commented on Apr 9, 2021
I would prefer keep the C API small.
Yes, I see the value of that as well.
I tried applying this API on an extension, and I found the code to be slightly less readable for the "is not" cases. vstinner commented on Apr 9, 2021
I tried applying this API on an extension, and I found the code to be slightly less readable for the "is not" cases.
FYI you can try upgrade_pythoncapi.py on your project using the following PR to update code to use Py_IsNone/Py_IsTrue/Py_IsFalse:
python/pythoncapi-compat#8
For example, use "upgrade_pythoncapi.py -o Py_Is directory/" or "upgrade_pythoncapi.py -o Py_Is file.c". vstinner commented on Apr 10, 2021 vstinner commented on Apr 10, 2021 Carl:
Just chiming in to say that for PyPy this API would be extremely useful, because PyPy's "is" is not implementable with a pointer comparison on the C level (due to unboxing we need to compare integers, floats, etc by value). Right now, C extension code that compares pointers is subtly broken and cannot be fixed by us.
Can you come with a sentence that I can put in the documentation to explain that Py_Is() is written for interoperability with other Python implementations? vstinner commented on May 18, 2021 Well, nobody came up with a better definition, so let's go with:
"Test if the x object is the y object, the same as x is y in Python."
Thanks for the feedback and reviews! Py_Is(x, y), Py_IsNone(x), Py_IsTrue(x) and Py_IsFalse(x) are now part of Python 3.10 C API. vstinner commented on Sep 29, 2021
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