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gh-94673: Add Per-Interpreter tp_subclasses for Static Builtin Types #95301

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merged 23 commits into from
Aug 5, 2022
Merged

gh-94673: Add Per-Interpreter tp_subclasses for Static Builtin Types #95301

merged 23 commits into from
Aug 5, 2022
+120 −31

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ericsnowcurrently
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@ericsnowcurrently ericsnowcurrently commented Jul 26, 2022 •
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@ericsnowcurrently
Add a note about tp_cache.

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7b299d1
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Hide tp_subclasses behind functions.
4fefd9b
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Distinquish between static and heap types.
129ca1f
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Move tp_subclasses for static types to PyInterpreterState.

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Add a note about heap types and tp_subclasses.
a707c38
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Use PyDict_GetItemWithError().
5dd2474
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Add lookup___hash__().
80b1d26
@ericsnowcurrently
Handle the case of unhashable classes.
8bced84
@ericsnowcurrently
Really handle the case of unhashable classes.
533951d
@ericsnowcurrently
Stop special-casing heap types for tp_subclasses.
f9ab261
@ericsnowcurrently
Revert "Stop special-casing heap types for tp_subclasses."
ccabeb5 
This reverts commit 0046b57446a26114fdf33d11a31a0fc84e7a764a.
@ericsnowcurrently
Point to the relevant GH issue.
df204e8
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Only store builtins state on the interpreter state.
3529f0f
@ericsnowcurrently
state.subclasses -> state.tp_subclasses
47c6ded
@ericsnowcurrently
Do not set type.tp_subclasses for static builtin types.
eade37b
@ericsnowcurrently
Store the index in tp_subclasses.
5687c8e
@ericsnowcurrently ericsnowcurrently removed the request for review from markshannon July 26, 2022 23:31
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@ericsnowcurrently ericsnowcurrently added the 🔨 test-with-buildbots Test PR w/ buildbots; report in status section label Jul 27, 2022
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LGTM, I have left some questions.

FWIW, we could also get rid of the new type flag and use tp_subclasses as a tagged pointer and steal one bit to check if the tp_subclasses is per interpreter or local, I'll probably do that but for now this is fine.

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encukou commented Jul 27, 2022 •
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I'm afraid this isn't backwards compatible. The tp_subclasses docs do say it's internal only, but do currently guarantee that it's a list, so IMO it's valid for API users to treat it as a PyObject*, and expect exceptions (rather than crashes).

Consider:

  • Seeking a SC exception from Python's backcompat policy
  • Renaming the field to _tp_subclasses, so anyone currently using it gets a compiler error
  • Adding the change to What's New so people know what's up
  • Removing the field from docs
  • Getting a PEP accepted for changes like this, to find unintended consequences
  • (edit) Changing the type to void* since it's not PyObject* any more

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FWIW, tp_subclasses is already a dict and probably has been for quite a while.

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kumaraditya303 commented Jul 27, 2022 •
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but do currently guarantee that it's a list,

The current implementation uses a dictionary to store subclasses as a mapping of PyLongObject * to subclass. On the Python level it is a list but not at C level.

so IMO it's valid for API users to treat it as a PyObject*, and expect exceptions (rather than crashes).

Why do you expect it to raise an exception when you dereference a pointer which is clearly documented to be a internal implementation detail?

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encukou commented Jul 28, 2022

I'm not saying this is a bad change. I'm asking to be mindful of the possible effects on code other than CPython that might look at this field.

FWIW, tp_subclasses is already a dict and probably has been for quite a while.

Yes, the docs are wrong. I don't think that's a free pass make this an invalid PyObject* pointer, though.
Bad docs just mean that people will experiment, or read the source, to see what things do.

Why do you expect it to raise an exception when you dereference a pointer which is clearly documented to be a internal implementation detail?

Well, I do expect to be able to inspect internal details, e.g. for debugging.

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Hey, the feedback is super helpful! Thanks!

I'll go ahead and add a whatsnew entry about the special-casing of tp_subclasses for static builtin types.

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Making tp_sublcasses void * is a good idea.

@ericsnowcurrently ericsnowcurrently added the 🔨 test-with-buildbots Test PR w/ buildbots; report in status section label Jul 29, 2022
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🤖 New build scheduled with the buildbot fleet by @ericsnowcurrently for commit dace9c4 🤖

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@bedevere-bot bedevere-bot removed the 🔨 test-with-buildbots Test PR w/ buildbots; report in status section label Jul 29, 2022
@encukou encukou mentioned this pull request Jul 29, 2022
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encukou commented Jul 29, 2022

Thanks!
I left some more suggestions on the docs in ericsnowcurrently#31

encukou and others added 2 commits August 1, 2022 13:13
@encukou @ericsnowcurrently
Adjust docs for tp_subclasses
e7f580d 
- Member documentation describes the current state, for users.
- The ``versionchanged`` note describes the change.
- The What's New entry is, ideally, usable for someone stuck with
  maintaining a mysterious legacy codebase that stopped working.

Details of internals are left out -- if it isn't enough to look
at the source, they should be documented in the devguide rather
than user-facing docs.
@ericsnowcurrently
lookup_subclasses() never fails.
6b19763
@ericsnowcurrently ericsnowcurrently merged commit 87154d8 into python:main Aug 5, 2022
@ericsnowcurrently ericsnowcurrently deleted the shareable-static-types-tp_subclasses branch August 5, 2022 01:27
iritkatriel pushed a commit to iritkatriel/cpython that referenced this pull request Aug 11, 2022
@ericsnowcurrently @iritkatriel
pythongh-94673: Add Per-Interpreter tp_subclasses for Static Builtin …
d52dd19 
…Types (pythongh-95301)
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