Issue 22500: Argparse always stores True for positional arguments (original) (raw)

It's my understanding that giving the action="store_true" to an argument in argparse defaults to False. When using non-double-dashed/positional arguments, the argument resorts to True (even if explicitly marked default=False).

I've attached a minimal example, but, for clarity, the relevant line is as such:

parser.add_argument("meow", action="store_true", default=False)

I realize that this might strike some as an odd usage, and I always have the option of using "--meow," but I found it odd that a positional argument is always True, even if not specified in sys.argv.

A 'store_true' action takes 0 arguments. In effect nargs=0.

With an optional (flagged) argument, the default False is used if the flag is absent, and set to True when the flag is encountered (its Action __call__ function is run).

A positional is 'encountered' whenever there are enough values to meet its nargs. With nargs=0, an empty list of strings, i.e. none, is enough. Thus such a positional is always found, and its __call__ is run, setting the value to True.

As a result, action types like 'store_true', 'store_false', 'store_const' only make sense with optionals.

And I can't think of simple way of using a 'positional' to set an Namespace 'dest' to boolean values. It could be done with a custom Action, but not with the predefined ones. Or you could translate the values after parsing.