For use with itunes podcast url's like "https://itunes.apple.com/us/podcast/grow-big-always/id1060318873"

itunesfeed.py

#! /usr/bin/env python3

import sys
from urllib.parse import urlparse
from urllib.request import urlopen
import codecs
import json


ITUNES_URL = 'https://itunes.apple.com/lookup?id='


class InvalidUrlError(Exception):
    pass


def getfeed(url):
    parsed = urlparse(url)

    if parsed.netloc != 'itunes.apple.com':
        raise InvalidUrlError
    try:
        id = parsed.path.split('/')[-1][2:]
        int(id)
    except (IndexError, ValueError):
        raise InvalidUrlError

    reader = codecs.getreader('utf-8')

    try:
        with urlopen(ITUNES_URL + id) as response:
            feed = json.load(reader(response))['results'][0]['feedUrl']
    except urllib.error.HTTPError:
        raise InvalidUrlError

    return feed


if __name__ == '__main__':
    try:
        print(getfeed(sys.argv[1]), end='')
    except InvalidUrlError:
        sys.exit('invalid url')
    except IndexError:
        sys.exit('please specify an url')