argparse: Emulating GCC's "-fno-<option>" semantics

Question

One nice feature of GCC's command line parsing is that most flags of the form "-fmy-option" have a negative version called "-fno-my-option". The rightmost occurrence takes precedence, so you can just append "-fno-my-option" to your CFLAGS or similar in a Makefile to disable an option without clobbering the other flags.

I'd like to support something similar in a tool whose wrapper script is Python and uses argparse. The obvious hack of just defining both versions of the argument with an action of store_true doesn't work, because that won't let me ask for the rightmost occurrence.

Obviously, it's easy to support a syntax like --my-option=yes / --my-option=no, but it would be nice for the user not to have to pass the parameter.

Is there a way to get argparse to have an on/off switch for a boolean flag like this?

hpaulj · Accepted Answer · 2016-10-19 16:48:28Z

2

Without any fancy foot work I can setup a pair of arguments that write to the same dest, and take advantage of the fact that the last write is the one that sticks:

In [765]: parser=argparse.ArgumentParser()
In [766]: a1=parser.add_argument('-y',action='store_true')
In [767]: a2=parser.add_argument('-n',action='store_false')

Without a dest parameter these use a name deterived from the option strings. But I can give a dest, or change that value after creation:

In [768]: a1.dest
Out[768]: 'y'
In [769]: a2.dest
Out[769]: 'n'
In [770]: a1.dest='switch'
In [771]: a2.dest='switch'

Now use of either will set the switch attribute.

In [772]: parser.parse_args([])
Out[772]: Namespace(switch=False)

The default comes from the first defined argument. That's a function of how defaults are set at the start of parsing. For all other inputs, it's the last argument that sets the value

In [773]: parser.parse_args(['-y'])
Out[773]: Namespace(switch=True)
In [774]: parser.parse_args(['-n'])
Out[774]: Namespace(switch=False)
In [775]: parser.parse_args(['-n','-y','-n','-y'])
Out[775]: Namespace(switch=True)
In [776]: parser.parse_args(['-n','-y','-n'])
Out[776]: Namespace(switch=False)

The default could also be set with a separate command:

parser.set_defaults(switch='foo')

If you wanted to use this sort of feature a lot you could write a little utility function that creates the pair of arguments with any flags and dest you want. There's even a bug/issues request for such an enhancement, but I doubt if it will be implemented.

edited Oct 19, 2016 at 16:48

answered Oct 19, 2016 at 16:30

hpaulj

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1 Comment

Rupert Swarbrick Over a year ago

That's great, thank you for explaining. Incidentally, you can also set the dest as part of the add_argument call, so you get something like: p.add_argument('--with-x', action='store_true', dest='x') and p.add_argument('--without-x', action='store_false', dest='x') or whatever.

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