I still disagree. You're giving the parser ambiguous input. If a parser sees "--foo --bar", and "--foo" is a valid option, but "--bar" is not, this is a legitimately ambiguous situation. Either the user really wanted "--bar", and the parser doesn't support it, or the "--bar" was meant to be the argument to the "--foo" flag. At this point, the parser must make an arbitrary decision, and argparse chooses the interpretation that the user wanted the "--bar" flag.

I understand that you have a good use case for the other interpretation. That's why I suggest you come up with a patch that allows this other interpretation to be enabled when necessary. Changing the default behavior is really a non-starter unless you can propose a sensible transition strategy (as is always necessary for changing APIs in backwards incompatible ways).