currently default_factory does not accept any parameter. This made the default value generated by it constant among keys. If default_factory takes in key as the parameter, it should be able to generate default values based on key, which provide more flexibility to the class defaultdict.

Take a look at the __missing__(key) method for the regular dict API to see if it meets your needs.  It is in the ``d[key]`` section at https://docs.python.org/3/library/stdtypes.html#dict 

>>> class UpperDict(dict):
        def __missing__(self, key):
            return key.upper()

>>> d = UpperDict()
>>> print(d['tom'])
TOM

I think this should be closed. For backwards compatibility, the defaultdict default_factory function must remain as it is. There is lots of code that uses something like `int` or `list` as the factory, and if the missing key was to be passed, the code would break.

As Raymond says, if you need the key passed to your factory, you can subclass dict and give it a __missing__ method.

Sure, thanks Raymond and Steven for your thoughts.