@paul.j3 That's interesting [1]. Documenting argparse.register seems crucial to me (-> reopen or file a new request?). 

After dealing more with the very sophisticated and complex functionality of argparse I'm sure that this is the only use case where such an unintuitive result may happen, so it's worth adding an explicit negative example in the docs of argparse.add_argument with a hint to the docs about "way that argparse handles types".

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[1] (and a very good example for missing capabilities of Q&A sites with regard to the relation of votes of your much better answer and the one with the highest number of votes)