3.0 pickle docs -- what about old-style classes?

[birkenfeld]

BPO	2572
Nosy	@birkenfeld, @rhettinger, @avassalotti, @benjaminp, @ashemedai

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/avassalotti'
closed_at = <Date 2009-04-27.16:22:13.794>
created_at = <Date 2008-04-07.19:18:19.566>
labels = ['docs']
title = '3.0 pickle docs -- what about old-style classes?'
updated_at = <Date 2009-04-27.16:22:13.793>
user = 'https://github.com/birkenfeld'

bugs.python.org fields:

activity = <Date 2009-04-27.16:22:13.793>
actor = 'georg.brandl'
assignee = 'alexandre.vassalotti'
closed = True
closed_date = <Date 2009-04-27.16:22:13.794>
closer = 'georg.brandl'
components = ['Documentation']
creation = <Date 2008-04-07.19:18:19.566>
creator = 'georg.brandl'
dependencies = []
files = []
hgrepos = []
issue_num = 2572
keywords = []
message_count = 7.0
messages = ['65098', '65126', '67354', '86493', '86521', '86604', '86674']
nosy_count = 5.0
nosy_names = ['georg.brandl', 'rhettinger', 'alexandre.vassalotti', 'benjamin.peterson', 'asmodai']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue2572'
versions = ['Python 3.0']

[birkenfeld]

Can 3.0 unpickle pickled old-style classes? Which pickling methods are
supported? Etc.

[avassalotti]

Python 3.0 shouldn't have any problem unpickling old-style classes.
However, they will be unpickled as new-style classes. Therefore, there
might be a few corner-cases that might cause some problems. For example,
if an old-style class contains a __slots__:

Python 2.5.1 (r251:54863, Mar  7 2008, 04:10:12) 
>>> import pickle
>>> class A:
...   __slots__ = []
... 
>>> pickle.dumps(A())
'(i__main__\nA\np0\n(dp1\nb.'

Python 3.0a3+ (py3k:62050M, Mar 30 2008, 17:29:33) 
>>> class A:
...   __slots__ = []
... 
>>> pickle.loads(b'(i__main__\nA\np0\n(dp1\nb.')
Traceback (most recent call last):
  ...
TypeError: __class__ assignment: '_EmptyClass' deallocator differs from 'A'

[benjaminp]

Alexandre, would you mind fixing up the docs?

[ashemedai]

We want to document that old-style classes get converted into new-style
classes and that in general this goes without problems, except for some
corner cases such as containing a __slots__ directive? Or did I miss
anything else?

[birkenfeld]

Sounds like this is the best that we without intimate pickle knowledge
can do :)

[rhettinger]

Recommend that this be closed with no action taken. It is an important
step for Py3.x that old style classes be left behind and that no mention
of them occur in the 3.x docs. Let's not carry 2.x with us into the 3.x
world. The whole point was to leave the old world behind.

Also, the issue is not new in the sense that any cross-version pickling
may exercise functions or classes that did not exist in other versions
or whose meaning changed over time. This is the nature of cross-version
pickling.

If someone wants to make pickling notes on the Python wiki, that would
be fine. The pickling docs are already somewhat lengthy and complex,
yet will never cover every issue someone could think up.

[birkenfeld]

Okay, as the original submitter I withdraw this request.