#25389: SimpleLazyObject doesn't pickle correctly when wrapping a Model
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Reporter: benjaminjkraft | Owner: benjaminjkraft
Type: Bug | Status: new
Component: Uncategorized | Version: 1.8
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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Given a `SimpleLazyObject` wrapping an instance of a Django model, if the
`SimpleLazyObject` has not yet been initialized, pickling it will not
correctly save the `_django_version` attribute added by
[https://github.com/django/django/commit/42736ac8e8c31137131714013951249a09e6e7d4
42736ac8]. This happens because `SimpleLazyObject`'s overridden
`__reduce_ex__` clobbers that of Model, and means that unpickling the
pickled object will raise a warning. An example is given
[https://gist.github.com/benjaminjkraft/2d330e22a58681c5db5c#file-
raises_warning-py here]. This is especially relevant given that
`request.user` is a `SimpleLazyObject` wrapping a model instance, so this
will happen when pickling an untouched `request.user`.
Furthermore, pickling a `SimpleLazyObject` wrapping a class which does
certain scary things to its `__dict__` and overrides `__reduce__` can
cause an error while pickling. An abstract example is given
[https://gist.github.com/benjaminjkraft/2d330e22a58681c5db5c#file-
raises_error-py here]. (I don't think that supporting this case is a
necessity, but it was how I noticed the above issue, and the attached
patch fixes both cases.)
Currently the behavior is effectively to just initialize and then pickle
the wrapped object. I have and will attach a patch which does that in a
more direct way that works in both of the above cases. It also adds a
regression test which covers both of the above cases.
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Ticket URL: <https://code.djangoproject.com/ticket/25389>
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