gh-148669: Clarify __reduce__() module lookup behavior

[Viicos]

• Issue: Clarify pickle __reduce__() behavior when a string is returned (singleton support). #148669

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📚 Documentation preview 📚: https://cpython-previews--148670.org.readthedocs.build/

[JelleZijlstra]

Why was this removed? It's still accurate.

[Viicos]

I don't think the statement was true? It is perfectly fine to pickle an instance which lives in a separate module than the one storing the class definition right?

[JelleZijlstra]

This refers to cases where the class is no longer accessible at its assigned place. For example, if you pickle an instance of some_mod.ClassA, then do del some_mod.ClassA, you can't unpickle the instance any more.

[Viicos]

Seems like I deleted too much, we should just keep however the class definition must be importable. But the instance doesn't have to live in the same module of the class:

# module1.py

class Test: ...

# module2.py

import pickle
from module1 import Test

t = Test()
pickle.loads(pickle.dumps(t))  # fine

[JelleZijlstra]

I think this is referring to cases where the class definition is moved between the pickling and unpickling code. Not a common issue but that can be relevant if e.g. a new library (or stdlib) version moves a class from one file to another, and you try to pickle under one version and unpickle under another.

[Viicos]

Indeed that makes sense. I think I misread when -> where.

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #32630732 | 📁 Comparing 0dfae55 against main (2a07ff9)

  🔍 Preview build  

4 files changed

± library/difflib.html
± library/functions.html
± library/pickle.html
± library/tkinter.messagebox.html

[miss-islington-app]

Thanks @Viicos for the PR, and @JelleZijlstra for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14, 3.15.
🐍🍒⛏🤖

[bedevere-app]

GH-149703 is a backport of this pull request to the 3.15 branch.

[bedevere-app]

GH-149704 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-149705 is a backport of this pull request to the 3.13 branch.