How to read from stdin or from a file if no data is piped in Python.

stdin.py

# echo '{"_payload":{"navigationResult":{"list":[{"order_id":"2","date":"date"}]}}}' | python parser.py
# python parser.py --input input.json
# see: http://stackoverflow.com/questions/2264991/how-to-read-from-stdin-or-from-a-file-if-no-data-is-piped-in-python

import argparse
import json

def parse(json_data):       
    data = json.load(json_data)
    resultlist = data['_payload']['navigationResult']['list']
    for order in resultlist:
        print "[%s] - [%s]"  %(order['order_id'],order['date'])
    
    json_data.close()

def init():
    parser = argparse.ArgumentParser('Combined orders json parser')
    parser.add_argument('--input', type = argparse.FileType('r'), default = '-')
    args = parser.parse_args()
    parse(args.input)

if __name__ == '__main__':
	init()