Make it clear in the collections Python source code that OrderedDict may be overridden

[lig]

BPO	25315
Nosy	@rhettinger, @bitdancer, @ericsnowcurrently, @zware, @Vgr255, @lig

^{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 2015-10-09.04:04:46.472>
created_at = <Date 2015-10-05.02:03:07.433>
labels = ['type-bug', 'invalid', 'docs']
title = 'Make it clear in the collections Python source code that OrderedDict may be overridden'
updated_at = <Date 2015-10-09.04:04:46.471>
user = 'https://github.com/lig'

bugs.python.org fields:

activity = <Date 2015-10-09.04:04:46.471>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2015-10-09.04:04:46.472>
closer = 'rhettinger'
components = ['Documentation']
creation = <Date 2015-10-05.02:03:07.433>
creator = 'lig'
dependencies = []
files = []
hgrepos = []
issue_num = 25315
keywords = []
message_count = 16.0
messages = ['252296', '252298', '252310', '252311', '252318', '252328', '252352', '252356', '252466', '252473', '252474', '252493', '252494', '252495', '252496', '252497']
nosy_count = 6.0
nosy_names = ['rhettinger', 'r.david.murray', 'eric.snow', 'zach.ware', 'abarry', 'lig']
pr_nums = []
priority = 'low'
resolution = 'not a bug'
stage = 'needs patch'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue25315'
versions = ['Python 3.5', 'Python 3.6']

[lig]

Consider this code in Python 3.5.0:

Python 3.5.0 (default, Sep 26 2015, 14:59:25) 
[GCC 5.1.1 20150618 (Red Hat 5.1.1-4)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from collections import OrderedDict
>>> class MyDict(OrderedDict):
...     def __setitem__(self, *args, **kwargs):
...         print(self._OrderedDict__root)
...         OrderedDict.__setitem__(self, *args, **kwargs)
... 
>>> md = MyDict({'a': 1})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in __setitem__
AttributeError: 'MyDict' object has no attribute '_OrderedDict__root'

However in Python 3.4:

Python 3.4.2 (default, Jul  9 2015, 17:24:30) 
[GCC 5.1.1 20150618 (Red Hat 5.1.1-4)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from collections import OrderedDict
>>> class MyDict(OrderedDict):
...     def __setitem__(self, *args, **kwargs):
...         print(self._OrderedDict__root)
...         OrderedDict.__setitem__(self, *args, **kwargs)
... 
>>> md = MyDict({'a': 1})
<collections._Link object at 0x7f0398a79b88>

[zware]

This is a side-effect of 3.5 having a C implementation of OrderedDict, and will not be fixed.

In the standard library, even just having a single trailing underscore is a clear indication of "if you use this, expect it to break without warning at any time." Mangled names in the standard library are more of an indication of "you *really* shouldn't use this".

[lig]

Zach, ok I got your point. But there is Python implementation in the library still which does not conform the one done in C. Such a behavior is tremendously confusing.

If there is both Python and C implementation in the library they must conform each other. And there is nothing with "you *really* shouldn't use this" when you need to especially when it is there. If it is there it must work as it written. Otherwise why Python does have this peace of code while it is completely useless as reference?

[lig]

If Python source does conform the one in C, it will be completely fine and understandable to have such a change and to rely on it in using version checks or whatever.

[Vgr255]

The same is true of the decimal and datetime modules. Names starting with an underscore are an implementation detail, and you shouldn't rely on these in production code.

[bitdancer]

A method that starts with an '_' is not part of the API unless documented to be so (as with namedtuple), and a non-special method that starts with two is purposefully mangled so that you cannot accidentally rely on it. The C implementation is an implementation of the API and the behavior when that API is used (as vetted by the tests that are run against both implementations); it is not otherwise required to "conform" to the python implementation or vice versa.

[lig]

Ok, then the OrderedDict is useless for any advanced hacking like altering the order in which __setitem__ stores items.

It is just useless Python code and so much appliances missed for this class:(

Thank you all for these completely helpful answers.

[bitdancer]

Yes, that would be correct. OrderedDict is designed to keep things in insertion order, so supporting changing that order is not part of its mission. Sorry about that, you'll have to write your own class.

Now, Raymond might find the idea of supporting other orders interesting. If so I'm sure he'll comment on the issue eventually.