default behavior

Ethan Furman ethan at
Tue Aug 3 18:54:46 CEST 2010

John Posner wrote:
> On 7/31/2010 1:31 PM, John Posner wrote:
>> Caveat -- there's another description of defaultdict here:
>> ... and it's bogus. This other description claims that __missing__ is a
>> method of defaultdict, not of dict.
> Following is a possible replacement for the bogus description. Comments 
> welcome. I intend to submit a Python doc bug, and I'd like to have a 
> clean alternative to propose.
> --------------
> class collections.defaultdict([default_factory[, ...]])
> defaultdict is a dict subclass that can guarantee success on key 
> lookups: if a key does not currently exist in a defaultdict object, a 
> "default value factory" is called to provide a value for that key.  The 
> "default value factory" is a callable object (typically, a function) 
> that takes no arguments. You specify this callable as the first argument 
> to defaultdict(). Additional defaultdict() arguments are the same as for 
> dict().
> The "default value factory" callable is stored as an attribute, 
> default_factory, of the newly created defaultdict object. If you call 
> defaultdict() with no arguments, or with None as the first argument, the 
> default_factory attribute is set to None. You can reassign the 
> default_factory attribute of an existing defaultdict object to another 
> callable, or to None.
> When a lookup of a non-existent key is performed in a defaultdict 
> object, its default_factory attribute is evaluated, and the resulting 
> object is called:
> * If the call produces a value, that value is returned as the result of 
> the lookup. In addition, the key-value pair is inserted into the 
> defaultdict.
> * If the call raises an exception, it is propagated unchanged.
> * If the default_factory attribute evaluates to None, a KeyError 
> exception is raised, with the non-existent key as its argument. (The 
> defaultdict behaves exactly like a standard dict in this case.)

I think mentioning how __missing__ plays into all this would be helpful. 
  Perhaps in the first paragraph, after the colon:

if a key does not currently exist in a defaultdict object, __missing__ 
will be called with that key, which in turn will call a "default value 
factory" to provide a value for that key.


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