jgritty jgritty - 1 year ago 238
Python Question

In Python, using argparse, allow only positive integers

The title pretty much summarizes what I'd like to have happen.

Here is what I have, and while the program doesn't blow up on a negative integer, I want the user to be informed that a negative integer is basically nonsense.

import argparse
parser = argparse.ArgumentParser()
parser.add_argument("-g", "--games", type=int, default=162,
help="The number of games to simulate")
args = parser.parse_args()

And the output:

python simulate_many.py -g 20
Setting up...
Playing games...

Output with a negative:

python simulate_many.py -g -2
Setting up...
Playing games...

Now, obviously I could just add an if to determine
if args.games
is negative, but I was curious if there was a way to trap it at the
level, so as to take advantage of the automatic usage printing.

Ideally, it would print something similar to this:

python simulate_many.py -g a
usage: simulate_many.py [-h] [-g GAMES] [-d] [-l LEAGUE]
simulate_many.py: error: argument -g/--games: invalid int value: 'a'

Like so:

python simulate_many.py -g -2
usage: simulate_many.py [-h] [-g GAMES] [-d] [-l LEAGUE]
simulate_many.py: error: argument -g/--games: invalid positive int value: '-2'

For now I'm doing this, and I guess I'm happy:

if args.games < 0:
print "-g/--games: must be positive."

Answer Source

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

def check_negative(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_negative)

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

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