John Posner wrote:
On 7/31/2010 1:31 PM, John Posner wrote:
Caveat -- there's another description of defaultdict here:
http://docs.python.org/library/collections.html#collections.defaultdict
... 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.
~Ethan~
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