This is another expression of the bigger problem of handling arguments that look like flags.  In optparse the 'nargs' (or equivalent, it doesn't handle positionals), control how many arguments an Option takes, regardless of their form.  In argparse, the distinction between a option (flag) and argument has priority.  So it is difficult to treat strings like '--def' as a plain argument.  The default behavior is to treat it as an optional flag.

https://bugs.python.org/issue9334