argparse -- mutually exclusive sets of arguments?

[Roy Smith]

My command either takes two positional arguments (in which case, both
are required):

$ command foo bar

or the name of a config file (in which case, the positional arguments
are forbidden):

$ command --config file

How can I represent this with argparse; add_mutually_exclusive_group()
isn't quite the right thing. It could specify that foo and --config are
mutually exclusive, but not (as far as I can see) the more complicated
logic described above.

 

[Terry Reedy]

My command either takes two positional arguments (in which case, both
are required):

$ command foo bar

or the name of a config file (in which case, the positional arguments
are forbidden):

$ command --config file

How can I represent this with argparse; add_mutually_exclusive_group()
isn't quite the right thing. It could specify that foo and --config are
mutually exclusive, but not (as far as I can see) the more complicated
logic described above.

Make the two positional arguments be one duple?
Or tell argparse that all three are optional and handle the 'more
complicated logic' in your own code after argparse returns.

 

[Ian Kelly]

My command either takes two positional arguments (in which case, both
are required):

$ command foo bar

or the name of a config file (in which case, the positional arguments
are forbidden):

$ command --config file

How can I represent this with argparse; add_mutually_exclusive_group()
isn't quite the right thing. It could specify that foo and --config are
mutually exclusive, but not (as far as I can see) the more complicated
logic described above.

I don't think you could even do the former. An argument must be
optional in order to be mutually exclusive with anything. This works,
however:

parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group(required=True)
group.add_argument('--config', type=file)
group.add_argument('--foobar', nargs=2, metavar=('FOO', 'BAR'))
print parser.parse_args()

Downsides are that the resulting interface is a little more formal and
a little less friendly, and unless you customize the action you'll
wind up with a 2-element 'foobar' arg instead of separate 'foo' and
'bar' args.