In Python, using argparse, allow only positive integers

A simpler alternative, especially if subclassing argparse.ArgumentParser, is to initiate the validation from inside the parse_args method.

Inside such a subclass:

def parse_args(self, args=None, namespace=None):
    """Parse and validate args."""
    namespace = super().parse_args(args, namespace)
    if namespace.games <= 0:
         raise self.error('The number of games must be a positive integer.')
    return namespace

This technique may not be as cool as a custom callable, but it does the job.

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About ArgumentParser.error(message):

This method prints a usage message including the message to the standard error and terminates the program with a status code of 2.

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Credit: answer by jonatan

This should be possible utilizing type. You'll still need to define an actual method that decides this for you:

def check_positive(value):
    ivalue = int(value)
    if ivalue <= 0:
        raise argparse.ArgumentTypeError("%s is an invalid positive int value" % value)
    return ivalue

parser = argparse.ArgumentParser(...)
parser.add_argument('foo', type=check_positive)

This is basically just an adapted example from the perfect_square function in the docs on argparse.

type would be the recommended option to handle conditions/checks, as in Yuushi's answer.

In your specific case, you can also use the choices parameter if your upper limit is also known:

parser.add_argument('foo', type=int, choices=xrange(5, 10))

Note: Use range instead of xrange for python 3.x

The quick and dirty way, if you have a predictable max as well as min for your arg, is use choices with a range

parser.add_argument('foo', type=int, choices=xrange(0, 1000))