gh-132493: Avoid eager import of annotationlib in typing (again)

[JelleZijlstra]

gh-132494 made typing.py eagerly import annotationlib again because
typing contains several protocols. Avoid this by using annotationlib only if .__annotations__ fails.

An earlier version of this PR instead made it so we compute the protocol attrs only when needed (generally on the first isinstance() call). That would make startup faster, but has some compatibility implications in cases where attributes get added to the class later, so I think this version is safer.

• Issue: Avoid eagerly evaluating annotations #132493

[AlexWaygood]

If I understand correctly, this will significantly slow down the firstisinstance() call against a runtime-checkable protocol, but following isinstance() calls will be unaffected. I think that's a good tradeoff, but it might be worth giving the folks who closely watch the benchmarks a heads-up. It might do "interesting" things to this benchmark: https://github.com/python/pyperformance/blob/main/pyperformance/data-files/benchmarks/bm_typing_runtime_protocols/run_benchmark.py

Since it's a micro-benchmark anyway, we could possibly even adjust that benchmark so that it does an isinstance() call to warm the __protocol_attrs_cache__ cache before the benchmark starts timing things?

[JelleZijlstra]

I think it might already do some warmup runs? That makes sense for the specializing interpreter anyway. I'll post on Discord.

[AlexWaygood]

It's a shame we can't use functools.cached_property for properties on metaclasses :-( it would simplify the caching quite nicely

[JelleZijlstra]

Michael confirmed that it does a warmup run by default.

[bedevere-app]

When you're done making the requested changes, leave the comment: I have made the requested changes; please review again.

[JelleZijlstra]

I have made the requested changes; please review again

[bedevere-app]

Thanks for making the requested changes!

@AlexWaygood: please review the changes made to this pull request.

[AlexWaygood]

Nice, this looks pretty good

[JelleZijlstra]

I realize this also impacts the note in the docs (https://docs.python.org/3.14/library/typing.html#typing.runtime_checkable) "The members of a runtime-checkable protocol are now considered “frozen” at runtime as soon as the class has been created."

This no longer quite holds; they are now frozen as of the first isinstance() check. Not a great compatibility story; I may want to think more about how to handle this.

[AlexWaygood]

This change does make sense to me. I agree that the "Protocol classes are now frozen at class-creation time" note in the docs no longer holds true, but I think the message to users from that note is "don't monkeypatch attributes onto Protocol classes". If users are following that advice, I don't think they would be broken by this change...?

We might be able to just edit the existing "Changed in Python 3.12" note to make it a bit more vague about when protocols are frozen, rather than adding a "Changed in Python 3.14" note? Not sure.

[JelleZijlstra]

An alternative that keeps us from importing annotationlib too early is to do something like

try:
annotations = cls.__annotations__
except Exception:
annotations = get_annotations(cls, format=Format.FORWARDREF)

So we only import annotationlib in the case where we actually need deferred annotations. I think I prefer that for compatibility.

[AlexWaygood]

Ah that sounds like an appealing alternative. Let's keep the new regression test you added as well, though!

[AlexWaygood]

probably need to change the PR title ;)

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot aarch64 Fedora Stable Clang 3.x (tier-2) has failed when building commit 5707837.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/234/builds/7415) and take a look at the build logs.
4. Check if the failure is related to this commit (5707837) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/234/builds/7415

Failed tests:

• test_interpreters

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-aarch64.clang/build/Lib/threading.py", line 1079, in _bootstrap_innerself._context.run(self.run)
~~~~~~~~~~~~~~~~~^^^^^^^^^^
File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-aarch64.clang/build/Lib/threading.py", line 1021, in runself._target(*self._args, **self._kwargs)
~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-aarch64.clang/build/Lib/test/test_interpreters/test_stress.py", line 30, in task
interp = interpreters.create()
File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-aarch64.clang/build/Lib/test/support/interpreters/__init__.py", line 76, in createid= _interpreters.create(reqrefs=True)
interpreters.InterpreterError: interpreter creation failed
k
Traceback (most recent call last):
File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-aarch64.clang/build/Lib/threading.py", line 1079, in _bootstrap_innerself._context.run(self.run)
~~~~~~~~~~~~~~~~~^^^^^^^^^^
File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-aarch64.clang/build/Lib/threading.py", line 1021, in runself._target(*self._args, **self._kwargs)
~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-aarch64.clang/build/Lib/test/test_interpreters/test_stress.py", line 47, in run
interp = interpreters.create()
File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-aarch64.clang/build/Lib/test/support/interpreters/__init__.py", line 76, in createid= _interpreters.create(reqrefs=True)
interpreters.InterpreterError: interpreter creation failed
k

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Windows11 Bigmem 3.x (tier-1) has failed when building commit 5707837.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1079/builds/6274) and take a look at the build logs.
4. Check if the failure is related to this commit (5707837) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1079/builds/6274

Summary of the results of the build (if available):

Click to see traceback logs

remote: Enumerating objects: 6, done. remote: Counting objects: 20% (1/5) remote: Counting objects: 40% (2/5) remote: Counting objects: 60% (3/5) remote: Counting objects: 80% (4/5) remote: Counting objects: 100% (5/5) remote: Counting objects: 100% (5/5), done. remote: Compressing objects: 20% (1/5) remote: Compressing objects: 40% (2/5) remote: Compressing objects: 60% (3/5) remote: Compressing objects: 80% (4/5) remote: Compressing objects: 100% (5/5) remote: Compressing objects: 100% (5/5), done. remote: Total 6 (delta 0), reused 0 (delta 0), pack-reused 1 (from 1) 
From https://github.com/python/cpython
* branch main -> FETCH_HEAD
Note: switching to '5707837049e8eade92e29f20f055112821b75cd0'.
You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by switching back to a branch.
If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -c with the switch command. Example:
git switch -c <new-branch-name>
Or undo this operation with:
git switch -
Turn off this advice by setting config variable advice.detachedHead to false
HEAD is now at 5707837049e gh-132493: Avoid eager import of annotationlib in typing (again) (#132596)
Switched to and reset branch 'main'
Traceback (most recent call last):
File "R:\buildarea\3.x.ambv-bb-win11.bigmem\build\Tools\buildbot\..\..\PCbuild\get_external.py", line 77, in <module>
main()
~~~~^^
File "R:\buildarea\3.x.ambv-bb-win11.bigmem\build\Tools\buildbot\..\..\PCbuild\get_external.py", line 50, in main
zip_path = fetch_zip(
args.tag,
...<3 lines>...
verbose=args.verbose,
)
File "R:\buildarea\3.x.ambv-bb-win11.bigmem\build\Tools\buildbot\..\..\PCbuild\get_external.py", line 19, in fetch_zip
filename, headers = urlretrieve(
~~~~~~~~~~~^
url,
^^^^
zip_dir /f'{commit_hash}.zip',
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
reporthook=reporthook,
^^^^^^^^^^^^^^^^^^^^^^
)
^
File "R:\buildarea\3.x.ambv-bb-win11.bigmem\build\externals\pythonx86\tools\Lib\urllib\request.py", line 242, in urlretrievewhile block := fp.read(bs):
~~~~~~~^^^^
File "R:\buildarea\3.x.ambv-bb-win11.bigmem\build\externals\pythonx86\tools\Lib\http\client.py", line 479, in read
s =self.fp.read(amt)
File "R:\buildarea\3.x.ambv-bb-win11.bigmem\build\externals\pythonx86\tools\Lib\socket.py", line 719, in readintoreturnself._sock.recv_into(b)
~~~~~~~~~~~~~~~~~~~~^^^
File "R:\buildarea\3.x.ambv-bb-win11.bigmem\build\externals\pythonx86\tools\Lib\ssl.py", line 1304, in recv_intoreturnself.read(nbytes, buffer)
~~~~~~~~~^^^^^^^^^^^^^^^^
File "R:\buildarea\3.x.ambv-bb-win11.bigmem\build\externals\pythonx86\tools\Lib\ssl.py", line 1138, in readreturnself._sslobj.read(len, buffer)
~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^ConnectionResetError: [WinError 10054] An existing connection was forcibly closed by the remote host
Could Not Find R:\buildarea\3.x.ambv-bb-win11.bigmem\build\Lib\*.pyc
The system cannot find the file specified.
Could Not Find R:\buildarea\3.x.ambv-bb-win11.bigmem\build\PCbuild\python*.zip

[JelleZijlstra]

I don't think that's this PR