The docs[d]. Use urllib.parse.quote_plus(x).
import sys
import urllib.parse
tmpl = "https://mail.google.com/mail/u/0/#search/%s"
args = sys.argv[1:]
search_string = " ".join(args)
search = urllib.parse.quote_plus(search_string)
url = tmpl.replace("%s",search)
print(url)