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 would be happy to further explain the concept, implementation, potential use cases, or anything else that might work towards the adoption of this feature.

You probably want to bring this up on the python-ideas mailing list for discussion. Features like this typically get discussed there first.

Thank you, I have done so.

This feature is already provided by just supplying a `__missing__` dunder:

py> class MyDict(dict):
...     def __missing__(self, key):
...             return "The key is {}".format(key)
...
py> d = MyDict()
py> d[1234]
'The key is 1234'


I'm closing this ticket, but if the discussion on Python-Ideas reaches consensus that your class is necessary, feel free to re-open it.