[Python-Dev] dict.addlist()

[Phillip J. Eby]

At 09:56 AM 1/20/04 -0500, Phillip J. Eby wrote:
>try:
>     return self._somemapping[key]
>except:
>     self._somemapping[key] = value = somethingExpensive(key)
>     return value

Oops, that should've been except KeyError.  Which actually points up 
another reason to want to use a setdefault mechanism, although admittedly 
not for the builtin dictionary type.  Using exceptions for control flow can 
mask actual errors occuring within a component being used.

Thus, when I create mapping-like objects, I try as much as possible to push 
"defaulting" into the lower-level mappings, to avoid needing to trap errors 
and turn them into defaults.  Sometimes, however, that default is expensive 
to create, so I actually use a factory argument, and it's often named 
'factory'.  Hence, my enthusiasm for the suggestion.