Steven D'Aprano added the comment:
All versions below 3.9 are in feature freeze, so they cannot get new features.
This requested feature is already supported by subclassing ``dict`` and giving
it a ``__missing__`` method. See issue #38315
--
nosy: +steven.daprano
resolution: -> reje
New submission from wang xuancong :
A very common use of defaultdict is that if the key exist, use the
corresponding mapped target, if the key does not exist, use the key itself.
However, current Python 2/3 defaultdict does not support parametric lambda
function:
>>> from collections import
