Why does overriding __contains__ break OrderedDict.keys?

python, python-2.7

Solution

When `Foo.__contains__` is not defined:

a['bar']

calls `Foo.__getitem__`, which executes

    self[key] = data

This calls `OrderedDict.__setitem__`, which is defined this way:

def __setitem__(self, key, value, PREV=0, NEXT=1, dict_setitem=dict.__setitem__):
    'od.__setitem__(i, y) <==> od[i]=y'
    # Setting a new item creates a new link at the end of the linked list,
    # and the inherited dictionary is updated with the new key/value pair.
    if key not in self:
        root = self.__root
        last = root[PREV]
        last[NEXT] = root[PREV] = self.__map[key] = [last, root, key]
    dict_setitem(self, key, value)

Since `Foo.__contains__` is not defined,

    if key not in self:

is True. So the key is properly added to `self.__root` and `self.__map`.

When `Foo.__contains__` is defined,

    if key not in self:

if False. So the key is not properly added to `self.__root` and `self.__map`. `Foo.__contains__` effective fools `OrderedDict.__setitem__` into thinking that the `'bar'` key has already been added.

I found it helpful to play with the following code (adding print statements in `__setitem__` and `__iter__`):

from collections import OrderedDict

dictclass = OrderedDict

class Foo(dictclass):
    def __getitem__(self,key):
        try:
            return dictclass.__getitem__(self,key)
        except KeyError:
            pass

        data = key*2
        self[key] = data
        return data

    def __contains__(self,whatever):
        print('contains: {}'.format(whatever))
        return dictclass.__contains__(self,whatever) or 'bar' in whatever

    def __setitem__(self, key, value, PREV=0, NEXT=1, dict_setitem=dict.__setitem__):
        'od.__setitem__(i, y) <==> od[i]=y'
        # Setting a new item creates a new link at the end of the linked list,
        # and the inherited dictionary is updated with the new key/value pair.
        print('key not in self: {}'.format(key not in self))
        if key not in self:
            root = self._OrderedDict__root
            last = root[PREV]
            last[NEXT] = root[PREV] = self._OrderedDict__map[key] = [last, root, key]
        dict_setitem(self, key, value)

    def __iter__(self):
        'od.__iter__() <==> iter(od)'
        # Traverse the linked list in order.
        NEXT, KEY = 1, 2

        root = self._OrderedDict__root
        curr = root[NEXT]
        print('curr: {}'.format(curr))
        print('root: {}'.format(root)) 
        print('curr is not root: {}'.format(curr is not root))

        while curr is not root:
            yield curr[KEY]
            curr = curr[NEXT]

a = Foo()
print a['bar']
# barbar

print a.keys()
# ['bar']

Notice that you can avoid this problem by making `Foo` a subclass of `collections.MutableMapping` and delegating most of its behavior to a `OrderedDict` attribute:

import collections
dictclass = collections.OrderedDict

class Foo(collections.MutableMapping):
    def __init__(self, *args, **kwargs):
        self._data = dictclass(*args, **kwargs)
    def __setitem__(self, key, value):
        self._data[key] = value
    def __delitem__(self, key):
        del self._data[key]
    def __iter__(self):
        return iter(self._data)
    def __len__(self):
        return len(self._data)

    def __getitem__(self,key):
        try:
            return self._data[key]
        except KeyError:
            pass

        data = key*2
        self[key] = data
        return data

    def __contains__(self,whatever):
        return dictclass.__contains__(self,whatever) or 'bar' in whatever

which yields

a = Foo()
print a['bar']
# barbar

print a.keys()
# ['bar']

even with `__contains__` defined.

Problem

Original source