Dictionaries with variable default.

Chris Angelico rosuav at gmail.com
Mon Nov 3 12:09:57 CET 2014

On Mon, Nov 3, 2014 at 10:04 PM, Antoon Pardon
<antoon.pardon at rece.vub.ac.be> wrote:
> Is it possible to have a default dictionary where the default is dependant
> on the key?
> I was hoping something like this might work:
>>>> m = defaultdict(lambda key: key+1)
> But it obviously doesn't:
>>>> m[3]
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: <lambda>() takes exactly 1 argument (0 given)

Yes, but not with the defaultdict. Instead, just implement __missing__:

class KeyPlusOne(dict):
    def __missing__(self, key): return key+1


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