Python, argparse: how to have nargs = 2 with type = str and type = int

Question

I spent several times on argparse documentation, but I'm still struggling with this module for one option in my program:

parser.add_argument("-r", "--rmsd", dest="rmsd", nargs=2, help="extract the poses that are close from a ref according RMSD", metavar=("ref","rmsd")) 

I would like the first argument to be a string (type str ) and be required, while the second argument must be of type int , and if no value is specified, then have a default value (for example, default=50 ). I know how to do this when only one argument is expected, but I have no idea what to do when nargs = 2 ... Is this even possible?

Answer 1

You can do the following. The required keyword sets the required field and default=50 sets the default value of option 50 if not specified:

 import argparse parser = argparse.ArgumentParser() parser.add_argument("-s", "--string", type=str, required=True) parser.add_argument("-i", "--integer", type=int, default=50) args = parser.parse_args() print args.string print args.integer 

Output:

 $ python arg_parser.py -s test_string test_string 50 $ python arg_parser.py -s test_string -i 100 test_string 100 $ python arg_parser.py -i 100 usage: arg_parser.py [-h] -s STRING [-i INTEGER] arg_parser.py: error: argument -s/--string is required 

Answer 2

I tend to agree with Mike's decision, but here is another way. This is not ideal since the use / help line tells the user to use 1 or more arguments.

 import argparse def string_integer(int_default): """Action for argparse that allows a mandatory and optional argument, a string and integer, with a default for the integer. This factory function returns an Action subclass that is configured with the integer default. """ class StringInteger(argparse.Action): """Action to assign a string and optional integer""" def __call__(self, parser, namespace, values, option_string=None): message = '' if len(values) not in [1, 2]: message = 'argument "{}" requires 1 or 2 arguments'.format( self.dest) if len(values) == 2: try: values[1] = int(values[1]) except ValueError: message = ('second argument to "{}" requires ' 'an integer'.format(self.dest)) else: values.append(int_default) if message: raise argparse.ArgumentError(self, message) setattr(namespace, self.dest, values) return StringInteger 

And with this you get:

 >>> import argparse >>> parser = argparse.ArgumentParser(description="") parser.add_argument('-r', '--rmsd', dest='rmsd', nargs='+', ... action=string_integer(50), ... help="extract the poses that are close from a ref " ... "according RMSD") >>> parser.parse_args('-r reference'.split()) Namespace(rmsd=['reference', 50]) >>> parser.parse_args('-r reference 30'.split()) Namespace(rmsd=['reference', 30]) >>> parser.parse_args('-r reference 30 3'.split()) usage: [-h] [-r RMSD [RMSD ...]] : error: argument -r/--rmsd: argument "rmsd" requires 1 or 2 arguments >>> parser.parse_args('-r reference 30.3'.split()) usage: [-h] [-r RMSD [RMSD ...]] : error: argument -r/--rmsd: second argument to "rmsd" requires an integer 

Answer 3

I would recommend using two arguments:

 import argparse parser = argparse.ArgumentParser(description='Example with to arguments.') parser.add_argument('-r', '--ref', dest='reference', required=True, help='be helpful') parser.add_argument('-m', '--rmsd', type=int, dest='reference_msd', default=50, help='be helpful') args = parser.parse_args() print args.reference print args.reference_msd 

Answer 4

Sorry for the late jump. I would use a function to call a type.

 def two_args_str_int(x): try: return int(x) except: return x parser.add_argument("-r", "--rmsd", dest="rmsd", nargs=2, type=two_args_str_int help="extract the poses that are close from a ref according RMSD", metavar=("ref","rmsd"))