argparse supports nargs parameter, which tells you how many parameters it eats.
When nargs="+" it accepts one or more parameters, so you can pass -b 1 2 3 4 and it will be assigned as a list to b argument
# args.py
import argparse
p = argparse.ArgumentParser()
# accept two lists of arguments
# like -a 1 2 3 4 -b 1 2 3
p.add_argument('-a', nargs="+", type=int)
p.add_argument('-b', nargs="+", type=int)
args = p.parse_args()
# check if input is valid
set_a = set(args.a)
set_b = set(args.b)
# check if "a" is in proper range.
if len(set_a - set(range(1, 51))) > 0: # can use also min(a)>=1 and max(a)<=50
raise Exception("set a not in range [1,50]")
# check if "b" is in "a"
if len(set_b - set_a) > 0:
raise Exception("set b not entirely in set a")
# you could even skip len(...) and leave just operations on sets
# ...
So you can run:
$ python arg.py -a 1 2 3 4 -b 2 20
Exception: set b not entirely in set a
$ python arg.py -a 1 2 3 4 60 -b 2
Exception: set a not in range [1,50]
And this is valid:
$ python arg.py -a 1 2 3 4 -b 2 3