def listify(arg):
if isinstance(arg, (set, tuple)):<EOL><INDENT>return list(arg)<EOL><DEDENT>if not isinstance(arg, list):<EOL><INDENT>return [arg]<EOL><DEDENT>return arg<EOL>
Simple utility method to ensure an argument provided is a list. If the
provider argument is not an instance of `list`, then we return [arg], else
arg is returned.

:type arg: list
:rtype: list