Raymond Hettinger added the comment:

I'm inclined to reject this proposition. It serves very special use case.

[Silent Ghost]
Correct me if I'm wrong but this seem as a very unlikely use case

[Alec Nikolas Reiter]
Implicitly assigning the factory is a pretty good compromise, like I said 
fromkeys accepting the factory would be a nice convenience rather than 
correcting a major oversight.


After thinking about the above, I'm disinclined to make the API modification.

The current way to do it is straight-forward and reads nicely:

    >>> d = defaultdict.fromkeys('abcdefg', somedefault)
    >>> d.default_factory = somefunc

Combining those into a single step is less readable and may incorrectly imply 
some sort of interaction between the factory function and initial population of 
keys with a constant default value.   Likewise, the possibility of implicitly 
guessing and assigning the factory function is contraindicated by the 
zen-of-python in at least two places.

Given the paucity of use cases, the potential for API confusion, and the ready 
availability of reasonable alternatives, I'm closing the feature request.  
Thank for the suggestion though, it certainly seemed like something that 
deserved a little more thought.

resolution:  -> rejected
status: open -> closed

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