Python implementation of pickle calls persistent_id() for a persistent id

[serhiy-storchaka]

Bug report

Pickle allows to define the persistent_id() method in the Pickler class. If it is defined, it is called for every pickled object, and its result will be pickled instead if it is not None. In the Python implementation persistent_id() is also called for the result of persistent_id(), but the result of the second call is ignored. The C implementation only calls persistent_id() once. Calling it twice is just a waste of time in best case, and can have other negative impact (for example if reference for every object for which it was called is stored), so I consider it a bug.

It was found during work on #125752.

Linked PRs

• gh-126489: Do not call persistent_id() for a persistent id in Python pickle #126490
• [3.13] gh-126489: Do not call persistent_id() for a persistent id in Python pickle (GH-126490) #126514
• [3.12] gh-126489: Do not call persistent_id() for a persistent id in Python pickle (GH-126490) #126516