#31418: ValueError when FileBasedCache is used in Python 3.7
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Reporter: mastizada | Owner: nobody
Type: Bug | Status: closed
Component: Core (Cache system) | Version: 2.2
Severity: Release blocker | Resolution: invalid
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by Simon Charette):
Python 3.8 introduced [https://www.python.org/dev/peps/pep-0574/ pickle
protocol 5] and pointed `pickle.HIGHEST_PROTOCOL` at it. You most likely
ran your project on Python 3.8 and stored things in a cache using the
`FileBasedCache` backend and then tried running your project on Python
3.7.
If you're in process of migrating from 3.7 to 3.8 you'll likely want to
subclass `FileBasedCache` and set its `pickle_protocol` to `4` and use it
as your backend during the transition period. When you've fully migrated
to Python 3.8 you can swap back to `FileBasedCache`.
I had to do similar manipulations when moving projects from Python 2 to 3
which makes me wonder if we should add a `CACHE_PICKLE_PROTOCOL` setting
or allow to specify it via `CACHES['OPTIONS']` to avoid having to
subclassing backends when such a transition is required. This is likely to
be more of an issue as more and more people migrate to 3.8.
--
Ticket URL: <https://code.djangoproject.com/ticket/31418#comment:6>
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