import json
import sys

def usage():
    print("python copy_layer.py <source layout JSON file> <destination layout JSON file> <copy from layer index> <copy to layer index>")
    sys.exit(1)

if len(sys.argv) != 5:
    usage()

try:
     first = int(sys.argv[3])
     second = int(sys.argv[4])
except ValueError:
    usage()

if first < 0 or first > 9 or second < 0 or second > 9:
    print("Layers must be between 0 and 9")
    usage()

if first == second:
    print("Layers must be unique")
    usage()

layout = json.load(open(sys.argv[1]))
maps = layout['keymaps']
maps[second] = maps[first]
json.dump(layout, open(sys.argv[2], 'w'), sort_keys=True, indent=4)