I'm a little confused by the mention of the "key" keyword argument. I suspect that is an internal concept to argparse, possibly passed that way to internal methods, but on the add_argument interface, it doesn't exist... instead there is "name or flags" positional arguments, from which, together with the "dest" argument, the "key" keyword argument is derived.  This is described under the explanation for the "dest" parameter.

But in the user documentation, the use of "key" is a surprise (to me, maybe I missed something).

Perhaps using "dest" rather than "key" in this documentation would be better? Or perhaps there is a precedent in the argparse documentation for what to call it?