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GeezerVison is showing github.com — gh-149044: Implement PEP 820 – PySlot: Unified slot system for the C API by encukou · Pull Request #149055 · python/cpython

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@encukou

@encukou encukou commented Apr 27, 2026 •
edited by github-actions Bot
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  • Issue: Implement PEP 820 – PySlot: Unified slot system for the C API #149044

📚 Documentation preview 📚: https://cpython-previews--149055.org.readthedocs.build/

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encukou added 30 commits April 24, 2026 13:40
@encukou
Add header for internal slot iterator API
3b13a93
@encukou
Add public API and a generated list of slots
0396743
@encukou
Add slot iterator implementation & use it for types
6da3bb1
@encukou
Unify duplicate & NULL handling
a8e018f
@encukou
Simplify slot handling in typeobject.c
b96acd4
@encukou
Port PyType_GetSlot; remove typeslots.inc
f3a6d7c
@encukou
Port module creation
5f13d2e
@encukou
Simplify internal API
4660f2f
@encukou
Switch modexport over to new slots
6c29000
@encukou
Fix up NULL handling
dfb342c
@encukou
Move PyType_FromMetaclass to slots
110464d
@encukou
PyType_FromSlots
6fb0046
@encukou
slots.csv update
866eafc
@encukou
squash up
325f1a8
@encukou
Put slot info in generated switches
e22e3af
@encukou
Handle PyType_Spec specially
fea6e21
@encukou
Clean up the generator
a89405c
@encukou
Remove slots.csv
114a4fa
@encukou
Prettier generation
8dc5d88
@encukou
Tighten module creation
796dd75
@encukou
Fix math
b942f10
@encukou
Remove PySlot_HAS_FALLBACK
6a53b4c
@encukou
Bump version
f352558
@encukou
Better comments
bd78c06
@encukou
Limit deprecations to new-style slots
181df86
@encukou
Start on docs
644ae46
@encukou
Require some slots to be static
0433d1e
@encukou
Add to Stable ABI
b3131a3
@encukou
blurb, versionadded, whatsnew
b35230d
@encukou
Don't deprecate mixing the bases slots/args
ae0ea29
@bedevere-app bedevere-app Bot added the awaiting core review label Apr 28, 2026
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Documentation looks pretty good. I haven't looked at all of the C code yet.

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Comment thread Doc/c-api/slots.rst Resolved

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@AlexWaygood
AlexWaygood removed their request for review April 28, 2026 14:06
@brettcannon
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encukou
encukou commented Apr 30, 2026
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encukou and others added 2 commits April 30, 2026 12:07
@encukou @ZeroIntensity
Apply suggestions from code review
986b24e 
Co-authored-by: Peter Bierma <[email protected]>
@encukou
Merge branch 'main' into slots-pep820-mrg
3404bba
@read-the-docs-community

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Documentation build overview

📚 cpython-previews | 🛠️ Build #32533679 | 📁 Comparing 1d09dac against main (a386a52)

  🔍 Preview build  

65 files changed · + 2 added · ± 63 modified

+ Added

  • c-api/sentinel.html
  • c-api/slots.html

± Modified

  • glossary.html
  • c-api/concrete.html
  • c-api/extension-modules.html
  • c-api/index.html
  • c-api/lifecycle.html
  • c-api/module.html
  • c-api/perfmaps.html
  • c-api/stable.html
  • c-api/synchronization.html
  • c-api/type.html
  • and 53 more...
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@ngoldbaum ngoldbaum mentioned this pull request May 2, 2026
Supporting 3.15.0b1 and finishing off support for the 3.15 ABI PyO3/pyo3#6012
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ngoldbaum
ngoldbaum reviewed May 3, 2026
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I wrote FFI bindings for this PR for PyO3 this afternoon and spotted several minor issues.

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Comment thread Include/slots.h
Comment on lines +48 to +52
#define PySlot_PTR(NAME, VALUE) \
{(NAME), PySlot_INTPTR, {0}, {(void*)(VALUE)}}

#define PySlot_PTR_STATIC(NAME, VALUE) \
{(NAME), PySlot_INTPTR | PySlot_STATIC, {0}, {(void*)(VALUE)}}

@ngoldbaum ngoldbaum May 3, 2026 •
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Can you spell these last two with the explicit names of the struct fields like the ones above? Also while you're touching this code, IMO these two definitions should come before PySlot_END.

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@encukou encukou May 4, 2026

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These macros are for code compatible with C++11 and below, which doesn't support designated initializers (that is, the explicit names).

So, both style issues are intentional :) I added a comment.

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Comment thread Include/slots.h Resolved

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Comment thread Include/slots.h
#define PySlot_STATIC 0x02
#define PySlot_INTPTR 0x04

#define Py_slot_invalid 0xffff

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for consistency should this be called Py_slot_INVALID maybe?

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@encukou encukou May 4, 2026

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That would be consistent with the wrong thing. Slot IDs are named Py_tp_repr, Py_mod_exec, etc. Flags are a new thing, so they follow the current style guide: mixed prefix + uppercase.

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Comment thread Include/slots_generated.h
#define _Py_SLOT_COMPAT_VALUE(OLD, NEW) OLD
#endif

#define Py_slot_end 0

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Maybe worth adding a comment above these that they're all u16 values?

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@encukou encukou May 4, 2026

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In C, the preprocessor macros are just numbers :)
In PyType_Slot, these values are used as int.

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Comment thread Include/object.h Resolved

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Comment thread Include/slots_generated.h
#define Py_tp_finalize 80
#define Py_am_send 81
#define Py_tp_vectorcall 82
#define Py_tp_token 83

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Everything through here is duplicated with typeslots.h. Was that intentional? It probably makes sense for there to be only one source of truth for these constants.

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Thanks for the catch! I seem to have dropped the commit that removes the unused file.

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ngoldbaum
ngoldbaum reviewed May 4, 2026
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Comment thread Include/slots.h
#ifndef _Py_HAVE_SLOTS_H
#define _Py_HAVE_SLOTS_H

typedef void (*_Py_funcptr_t)(void);

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PyO3 needs to wrap this type directly so you might as well give this a public non-underscored name.

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@encukou encukou May 4, 2026

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Can PyO3 use the underlying type, or better yet, some “generic C function pointer” type?

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@ngoldbaum ngoldbaum May 4, 2026

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Unfortunately no, because Rust doesn't let you cast between function pointer types without using unsafe and std::mem::transmute. That said, in my second draft of this, I isolated the unsafety into a PySlot_FUNC rust macro that hides the wrapped function pointer, so I take back my comment from last night.

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@encukou encukou May 4, 2026

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Keep in mind this is not the correct function type, just a function type. At this level, unsafe transmuation is exactly what you need in Rust.
(A safer layer on top of the ABI would need to have separate C-specific and C++-specific variants, and presumably a Rust-specific one as well. I didn't get that into 3.15.)

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encukou added 2 commits May 4, 2026 08:43
@encukou
Merge in the main branch
e21eb07
@encukou
Address review comments
bef20e9
@encukou

encukou commented May 4, 2026

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I intend to merge this today (or early tomorrow) to get it in 3.15, as postponing to 3.16 would be very inconvenient. Beta is for fixing features (or removing, if this turns out to be a bad idea after all).
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@encukou encukou mentioned this pull request May 4, 2026
gh-149101: Implement PEP 788 #149116
Merged
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I think this is in a mergeable state. I read through all of the PEP, documentation, and code, and I couldn't find any glaring issues.

I left a number of non-blocking questions/comments; feel free to ignore them for now and address them after the beta freeze.

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Comment thread Doc/c-api/slots.rst Resolved

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Comment thread Include/internal/pycore_slots.h Resolved

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Comment thread Lib/test/test_capi/test_misc.py
Comment on lines 925 to 929
self.assertRaisesRegex(
SystemError,
"Py_tp_bases is not a tuple",
TypeError,
"metaclass conflict",
_testcapi.create_heapctype_with_none_bases_slot
)

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Hmm, this seems like a downgrade in terms of the error message. I don't see a change to create_heapctype_with_none_bases_slot in this PR, so is there a reason the error changed?

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@encukou encukou May 4, 2026

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Yeah, that's a side effect from merging Py_tp_base & Py_tp_bases -- the latter strictly required a tuple of types, so it had this message.
This is a silly restriction. I'll document lifting it.

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@encukou encukou May 4, 2026

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(And I'll get SC permission to update the PEP, or revert this.)

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@encukou encukou Jun 10, 2026

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Turns out the PEP already specifies merging Py_tp_base & Py_tp_bases. Fix in #151252

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Comment thread Objects/moduleobject.c Resolved

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Comment thread Objects/moduleobject.c
Comment on lines +794 to 796
}
break;
}

@ZeroIntensity ZeroIntensity May 4, 2026

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What happens if there's a slot other than Py_slot_invalid or Py_mod_exec here? Does _PySlotIterator_Next somehow handle that ahead-of-time?

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@encukou encukou May 4, 2026

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This is PyModule_ExecDef; it ignores slots other than Py_mod_exec.

Py_slot_invalid is the error indicator here (i.e. an exception was set in _PySlotIterator_Next) -- not the standard error API but nicer than the alternatives I tried.

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Comment thread Objects/typeobject.c
Comment on lines +5403 to 5405
memcpy(tp_doc, it.current.sl_ptr, len);
}
break;

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Same question here -- do we want a default: case for debugging?

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@encukou encukou May 4, 2026

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This is the first pass, most slots (especially the function pointers) are handled in the second pass.

_PySlotIterator_Next handles unknown slots, and ones that don't apply to types -- it sets an exception and reports Py_slot_invalid.

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Comment thread Tools/c-analyzer/c_common/scriptutil.py Resolved

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Comment thread Include/internal/pycore_slots.h Outdated
_PySlot_KIND slot_struct_kind;
} _PySlotIterator_state;

#define SEEN_ENTRY_BITS (8 * sizeof(unsigned int))

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It doesn't seem like this is used outside slots.c. I think that it would make more sense to move it there, but if you'd really like to keep it in the header, let's be consistent and give this a _Py prefix.

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@bedevere-app bedevere-app Bot added awaiting merge and removed awaiting core review labels May 4, 2026
encukou and others added 5 commits May 4, 2026 22:58
@encukou @ZeroIntensity
Apply suggestions from code review
018ce8b 
Co-authored-by: Peter Bierma <[email protected]>
Co-authored-by: Petr Viktorin <[email protected]>
@encukou
Prefix a private constant
6c9726a
@encukou
Merge in the main branch
ca86080
@encukou
Doc change in Py_tp_bases
a67116e
@encukou
Fix badly formatted suggestion
1d09dac
@encukou encukou added the 🔨 test-with-buildbots Test PR w/ buildbots; report in status section label May 4, 2026
@encukou
Merge in the main branch
06b2c3d
@encukou
encukou merged commit 508b498 into python:main May 5, 2026
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@bedevere-app bedevere-app Bot removed the awaiting merge label May 5, 2026
@encukou
encukou deleted the slots-pep820-mrg branch May 5, 2026 07:18
@chris-eibl chris-eibl mentioned this pull request May 7, 2026
gh-148690: Build Windows freethreaded binaries into separate directory and include python3t.dll on GIL-enabled #149218
Merged
@encukou encukou mentioned this pull request Jun 10, 2026
gh-149044: Improve Py_tp_base[s] docs & error message for non-type bases #151252
Merged
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@ngoldbaum ngoldbaum ngoldbaum left review comments
@ZeroIntensity ZeroIntensity ZeroIntensity approved these changes
@markshannon markshannon Awaiting requested review from markshannon markshannon is a code owner
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