[Docs-bugs] [issue47206] pickle docs are wrong about nested classes

3 Apr 2022

      New submission from Jelle Zijlstra <jelle.zijlstra@gmail.com>:

https://docs.python.org/3.10/library/pickle.html#what-can-be-pickled-and-unp... says that only "classes that are defined at the top level of a module" can be pickled. But in fact these work fine in current Python, probably since 3.3 when __qualname__ was added (https://docs.python.org/3/library/stdtypes.html#definition.__qualname__). Similarly, the docs claim only top-level functions can be pickled, but in fact methods nested in classes work fine.

Example script demonstrating that these work:

import pickle

class X:
    class Y:
        pass

    def method(self):
        pass

print(pickle.dumps(X.Y))
print(pickle.loads(pickle.dumps(X.Y)))

print(pickle.dumps(X.Y()))
print(pickle.loads(pickle.dumps(X.Y())))

print(pickle.dumps(X.method))
print(pickle.loads(pickle.dumps(X.method)))

----------
assignee: docs@python
components: Documentation
messages: 416625
nosy: JelleZijlstra, alexandre.vassalotti, docs@python
priority: normal
severity: normal
status: open
title: pickle docs are wrong about nested classes
type: behavior
versions: Python 3.10, Python 3.11, Python 3.9

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Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue47206>
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