The result of calling dict.* methods on OrderedDict is undefined.

[ericsnowcurrently]

BPO	24721
Nosy	@rhettinger, @markshannon, @ericsnowcurrently

^{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 2015-07-25.20:33:30.953>
created_at = <Date 2015-07-25.17:10:58.383>
labels = ['type-bug', 'invalid', 'docs']
title = 'The result of calling dict.* methods on OrderedDict is undefined.'
updated_at = <Date 2015-07-26.10:41:18.347>
user = 'https://github.com/ericsnowcurrently'

bugs.python.org fields:

activity = <Date 2015-07-26.10:41:18.347>
actor = 'Mark.Shannon'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2015-07-25.20:33:30.953>
closer = 'rhettinger'
components = ['Documentation']
creation = <Date 2015-07-25.17:10:58.383>
creator = 'eric.snow'
dependencies = []
files = []
hgrepos = []
issue_num = 24721
keywords = []
message_count = 5.0
messages = ['247354', '247358', '247360', '247361', '247423']
nosy_count = 4.0
nosy_names = ['rhettinger', 'docs@python', 'Mark.Shannon', 'eric.snow']
pr_nums = []
priority = 'low'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue24721'
versions = ['Python 2.7', 'Python 3.4', 'Python 3.5', 'Python 3.6']

[ericsnowcurrently]

(see bpo-24667)

collections.OrderedDict subclasses dict so calling dict's methods on an OrderedDict works. However, neither the pure Python nor the C implementation of OrderedDict was written to support doing so. In fact, both of them currently enter an inconsistent state when this happens. For example:

    # Python 3.4 (pure Python implementation)
    >>> from collections import OrderedDict
    >>> od = OrderedDict([('spam', 1), ('eggs', 2)])
    >>> dict.__delitem__(od, 'spam')
    >>> str(od)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/usr/lib/python3.4/reprlib.py", line 24, in wrapper
        result = user_function(self)
      File "/usr/lib/python3.4/collections/__init__.py", line 198, in __repr__
        return '%s(%r)' % (self.__class__.__name__, list(self.items()))
      File "/usr/lib/python3.4/_collections_abc.py", line 485, in __iter__
        yield (key, self._mapping[key])
    KeyError: 'spam'

    # Python 3.5 (C implementation)
    >>> from collections import OrderedDict
    >>> od = OrderedDict([('spam', 1), ('eggs', 2)])
    >>> dict.__delitem__(od, 'spam')
    >>> str(od)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    KeyError: 'spam'

This is a consequence of subclassing a builtin type, which typically do not have good support for subclassing (e.g. bpo-10977).

It probably isn't worth making any changes to the code of either OrderedDict implementations. At most I'd recommend a note in the OrderedDict documentation indicating that the results of passing an OrderedDict object to dict.* methods are undefined.

[rhettinger]

This is a consequence of subclassing a builtin type

Not really. This is how subclassing works in general. Any time you a user calls a parent class directly on an instance of subclass, they are bypassing whatever the subclass needs to do to maintain its invariants.

class A:
    def __init__(self):
        self.data = []
    def add(self, x):
        self.data.append(x)

class B(A):
    'Track the number of odds'
    def __init__(self):
        A.__init__(self)
        self.odds = 0
    def add(self, x):
        A.add(self, x)
        self.odds += (x % 2)

b = B()
b.add(1)
b.add(2)
b.add(3)
b.add(4)
A.add(b, 5)
assert b.odds == sum(x%1 for x in b.data), 'OMG, B is broken!'

There is nothing special about OrderedDicts in this regard. Perhaps there should be a FAQ entry regarding the "facts of life" in the world of object oriented programming.

[ericsnowcurrently]

Ah, you're right. I was hung up on bpo-10977. :)

[ericsnowcurrently]

Feel free to close this, Raymond.

[markshannon]

I think this is a bug. This is not a normal case of subclassing as the interpreter calls the C API PyDict_XXX() in many cases where a dictionary subclass is passed in.

For example:
class C: pass
c = C()
# Liskov substitution principle says this is OK.
c.__dict__ = OrderedDict()
c.a = 1

All access to the ordered dict is via the dict.__setitem__ method.

I think this should be documented.