random.Random implements __getstate__() and __reduce__()

[vterron]

BPO	17489
Nosy	@rhettinger, @mdickinson
Files	issue17489_example.py

^{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/rhettinger'
closed_at = <Date 2013-03-23.13:49:35.561>
created_at = <Date 2013-03-20.01:49:12.667>
labels = ['type-feature', 'library']
title = 'random.Random implements __getstate__() and __reduce__()'
updated_at = <Date 2013-04-02.17:13:29.904>
user = 'https://bugs.python.org/vterron'

bugs.python.org fields:

activity = <Date 2013-04-02.17:13:29.904>
actor = 'python-dev'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2013-03-23.13:49:35.561>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2013-03-20.01:49:12.667>
creator = 'vterron'
dependencies = []
files = ['29513']
hgrepos = []
issue_num = 17489
keywords = []
message_count = 5.0
messages = ['184721', '184746', '184800', '184867', '185850']
nosy_count = 5.0
nosy_names = ['rhettinger', 'mark.dickinson', 'python-dev', 'Ramchandra Apte', 'vterron']
pr_nums = []
priority = 'low'
resolution = 'wont fix'
stage = None
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue17489'
versions = ['Python 3.4']

[vterron]

The random.Random class implements both the __getstate__() and __reduce__() methods. My knowledge of the pickle module is very limited, but apparently this causes __getstate__() never to get called. Maybe this is a remnant from earlier code, or are both methods actually needed?

[RamchandraApte]

__getstate__ is for pickling instances [0], __reduce__ is for pickling the object itself [1]

^0 http://docs.python.org/2/library/pickle.html#object.\_\_getstate__
^1 http://docs.python.org/2/library/pickle.html#object.\_\_reduce__

[vterron]

Ramchandra, that is not the behavior that I am able to reproduce. Please see the attached example: in the presence of __reduce__(), __getstate__() is not being called whether the class or an instance its pickled. This is also in consonance with what is stated in the docs, which warn that "Although powerful, implementing __reduce__() directly in your classes is error prone. For this reason, class designers should use the high-level interface (i.e., __getnewargs__(), __getstate__() and __setstate__()) whenever possible".

[rhettinger]

Possibly __getstate__ could be removed. That said, I would feel a sense of risk in taking it out -- that method has been present for a *very* long time who knows what might rely on it being there.

[python-dev]

New changeset 68ed5b2ca867 by R David Murray in branch 'default':
bpo-17489: Add explanatory comment that __getstate__ is not called.
http://hg.python.org/cpython/rev/68ed5b2ca867