I have often wanted this! I use defaultdict only occasionally, and every time I do, I start out thinking it behaves the way you describe dynamicdict. Then I read the docs again and decide I didn't actually want defaultdict to start with, and roll something different.

On Fri, Apr 10, 2020, 6:50 PM Steele Farnsworth <swfarnsworth@gmail.com> wrote:
I have implemented a class in the C code of the collections module which has similar behavior to the defaultdict class. This class, dynamicdict, supplies values for keys that have not yet been added using a default factory callable, but unlike defaultdict, the missing key itself is passed to the callable. This code can be seen here: https://github.com/swfarnsworth/cpython/blob/3.8/Modules/_collectionsmodule.c#L2234

While this does introduce a lot of redundant code, I'm not sure how it could be done without copying the implementation of the defaultdict class and adjusting how the default factory is called. For example, I don't believe that it's possible to support both behaviors within the defaultdict class without breaking backwards compatibility or adding another parameter to the constructor for the defaultdict class.

I don't know if a PEP is needed for this to be added to the standard library, or if it's something that anyone other than myself wants to see added, but I would be happy to further explain the concept, implementation, potential use cases, or anything else that might work towards the adoption of this feature.

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