default behavior

Ethan Furman ethan at
Wed Aug 4 00:48:02 CEST 2010

Christian Heimes wrote:
>> I just went and read the entry that had the bogus claim -- personally, I 
>> didn't see any confusion.  I would like to point out the __missing__ is 
>> *not* part of dicts (tested on 2.5 and 2.6 -- don't have 2.7 installed yet).
> I beg your pardon but you are wrong. __missing__ is available for all
> *subclasses* of dict since Python 2.5. See
>>>> class mydict(dict):
> ...     def __missing__(self, key):
> ...         print "__missing__", key
> ...         raise KeyError(key)
> ...
>>>> m = mydict()
>>>> m[1]
> __missing__ 1
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "<stdin>", line 4, in __missing__
> KeyError: 1

Perhaps punctuation will help clarify my intent:

__missing__ is *not* part of (dict)s, as shown by dir(dict()):

['__class__', '__cmp__', '__contains__', '__delattr__', '__delitem__',
  '__doc__', '__eq__', '__ge__', '__getattribute__', '__getitem__',
  '__gt__', '__hash__', '__init__', '__iter__', '__le__', '__len__',
  '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__',
  '__repr__', '__setattr__', '__setitem__', '__str__', 'clear', 'copy',
  'fromkeys', 'get', 'has_key', 'items', 'iteritems', 'iterkeys',
  'itervalues', 'keys', 'pop', 'popitem', 'setdefault', 'update',

And, just to state what is hopefully obvious, if you don't create 
__missing__ yourself, it still isn't in the subclass:

--> class somedict(dict):
...     "Is __missing__ defined if I don't define it?  Nope."
--> sd = somedict()
--> sd[1]
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
KeyError: 1
--> dir(sd)
['__class__', '__cmp__', '__contains__', '__delattr__', '__delitem__',
  '__dict__', '__doc__', '__eq__', '__ge__', '__getattribute__',
  '__getitem__', '__gt__', '__hash__', '__init__', '__iter__', '__le__',
  '__len__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__',
  '__reduce_ex__', '__repr__', '__setattr__', '__setitem__', '__str__',
  '__weakref__', 'clear', 'copy', 'fromkeys', 'get', 'has_key', 'items',
  'iteritems', 'iterkeys', 'itervalues', 'keys', 'pop', 'popitem',
  'setdefault', 'update', 'values']


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