[Patches] [ python-Patches-1062353 ] set pickling problems

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Patches item #1062353, was opened at 2004-11-08 11:07
Message generated for change (Comment added) made by ddorfman
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Category: Modules
Group: None
Status: Closed
Resolution: Wont Fix
Priority: 5
Submitted By: Dima Dorfman (ddorfman)
Assigned to: Nobody/Anonymous (nobody)
Summary: set pickling problems

Initial Comment:
As with deque (SF #1062279), two problems with set.__reduce__:

  1. Recursive sets (which can be constructed with the aid
     of a hashable mutable object) aren't pickled correctly
     because __reduce__ wants a reference to itself in the
     call to its constructor. Fix by moving the keys to the
     state argument and resurrecting them in __setstate__
     (test_pickling_recursive).

  2. Without the standard reduce, we have to take care of
     the instance dictionary ourselves. The test for this is
     in a new TestSubclassOps class that is mixed in to
     TestSetSubclass and TestFrozenSetSubclass. I'm not sure
     if such a mixin is the best way to distribute that test.

The biggest drawback to this patch is that __setstate__
makes it possible to mutate a frozenset. This implementation
clears the cached hash after such a mutation, but even then
it can be used to cause havoc in dicts. Such havoc isn't
fatal (this doesn't do anything that a regular class can't
do), but it can be confusing. Not being able to do this was
a desirable property of frozenset, but it's unlikely to
happen on accident, and sets.ImmutableSet has surived
without it. Unless one of the pickle gurus provides a better
alternative to SF #1062277, this might be the best option.

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>Comment By: Dima Dorfman (ddorfman)
Date: 2004-11-09 09:03

Message:
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Fair enough. If sets weren't meant to be recursive then 1. 31 is sufficient.

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Comment By: Raymond Hettinger (rhettinger)
Date: 2004-11-09 07:25

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Am adding the dict argument so that subclass dictionaries
are handled without extra coding.  See Objects/setobject.c 1.31

Sets were designed to be non-recursive.  While you can
create shennanigans to introduce hashable mutable objects to
be stored recursively, I have no interest in building
support for them.  Certainly, it is not worth introducing
other anomalies or worth compilcating the code.

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