Behavior of assigning to __dict__ is not documented

[sorcio]

BPO	15360
Nosy	@amauryfa, @pitrou, @ezio-melotti, @merwok, @sorcio, @mhsmith

^{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 = None
closed_at = None
created_at = <Date 2012-07-15.17:25:06.099>
labels = ['interpreter-core', 'type-feature', 'docs']
title = 'Behavior of assigning to __dict__ is not documented'
updated_at = <Date 2017-07-13.23:10:20.703>
user = 'https://github.com/sorcio'

bugs.python.org fields:

activity = <Date 2017-07-13.23:10:20.703>
actor = 'Malcolm Smith'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Interpreter Core']
creation = <Date 2012-07-15.17:25:06.099>
creator = 'davide.rizzo'
dependencies = []
files = []
hgrepos = []
issue_num = 15360
keywords = []
message_count = 4.0
messages = ['165538', '165772', '165983', '166492']
nosy_count = 9.0
nosy_names = ['amaury.forgeotdarc', 'pitrou', 'ezio.melotti', 'eric.araujo', 'Arfrever', 'cvrebert', 'docs@python', 'davide.rizzo', 'Malcolm Smith']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue15360'
versions = ['Python 2.7', 'Python 3.3', 'Python 3.4']

[sorcio]

The documentation (at least the obvious places, see Doc/reference/datamodel.rst) says classes and class instances have the '__dict__' attribute, but nothing is said about what happens when assigning to it (like obj.__dict__ = something).

As far as I understand that's undefined behavior and other implementations may not work the same as CPython (and CPython itself behaves differently between versions).

I'd submit a documentation patch if I knew how to specify this matter. Maybe just say the behavior is not defined?

[amauryfa]

What kind of behavior are you referring to?
Are there really differences between versions?

[sorcio]

Amaury, I don't honestly know, I would have proposed something otherwise.

I have been advised on #python (Freenode) not to assign to obj.__dict__ because its behaviour changes between versions and implementations, but I wouldn't know what has changed between CPython version.

One obscure thing is what can be assigned to __dict__. For class dicts no assignment is allowed. Object dicts can be assigned dict objects. If you try to assign a non-dict mapping it will complain.

However you can assign a dict-derived object, something like:

class SillyDict(dict):
    def __getitem__(self, key):
        return "hello"

obj.__dict__ = SillyDict()

Unfortunately accessing the attributes of obj will still use PyDict_* functions, thus ignoring any magic methods.

PyPy's behavior is slightly different. A class __dict__ attribute still is read only, but you can assign any mapping object to an object __dict__.

[pitrou]

I think assigning to __dict__ is an implementation detail; we shouldn't document it before it is clearly agreed what it should do (which probably deserves asking on python-dev).