Nice! I thought there had to be a better way. Hopefully we are using this
in SparseMatrix.
On Monday, November 27, 2017 at 3:00:06 PM UTC-6, Aaron Meurer wrote:
>
> That's a generalized version of defaultdict. For just returning the
> key, you can just use
>
> class default_to_key(dict):
> def __missing__(self, key):
> return key
>
> Worth noting is a potentially important difference between subclassing
> dict and defaultdict: when you lookup a missing entry in a
> defaultdict, it gets saved for future lookups
>
> >>> lookup = KeyAwareDefaultDict(Id)
> >>> lookup[x]
> x
> >>> lookup
> defaultdict(Lambda(_x, _x), {x: x})
>
> This is wasteful when your lookup function is fast and stateless
> (always returns the same output for a given input). If it's not fast
> or not stateless, defaultdict is better, but for Id, I think the dict
> solution might be preferred.
>
> Aaron Meurer
>
> On Mon, Nov 27, 2017 at 9:25 AM, Chris Smith <[email protected]
> <javascript:>> wrote:
> > Am I missing some obvious way to have a default dict that returns the
> key if
> > it is not present? If not, I found this by googling:
> >
> > from collections import defaultdict
> >
> > class KeyAwareDefaultDict(defaultdict):
> > def __missing__(self, key):
> > if self.default_factory is None:
> > raise KeyError(key)
> > self[key] = value = self.default_factory(key)
> > return value
> >
> >>>> lookup = KeyAwareDefaultDict(Id)
> >>>> lookup[x]
> > x
> >>>> lookup[x] = y; lookup[x]
> > y
> >
> >
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