Argument is URL or path

Depends on what the program must do. If it just prints whether it got a URL, sys.argv[1].startswith('http://') might do. If you must actually use the URL for something useful, do

from urllib2 import urlopen

try:
    f = urlopen(sys.argv[1])
except ValueError:  # invalid URL
    f = open(sys.argv[1])

import urlparse

def is_url(url):
    return urlparse.urlparse(url).scheme != ""
is_url(sys.argv[1])