Required arguments in argparse: at least one of a group

[Marco]

Is there the possibility using the argparse module to group two or more 
arguments in order to have at least one of them required? For instance, 
I would like to have not an error only in the following cases:

   python finder.py --file myfile --dir mydir
   python finder.py --pattern mypattern --dir mydir
   python finder.py --file myfile --pattern mypattern --dir mydir

where --dir is required, and --file _or_ --parser have to be specified. 
In other words, I want the parser prints an error message just in this case:

   python finder.py --dir mydir

Thanks in advance, Marco
-- 
Marco