human
Complete this python program:
def ensure_timezone(func, argname, arg):
assistant
#Here is the Python code you requested.

    """Argument preprocessor that converts the input into a tzinfo object.
    Examples
    >>> from zipline.utils.preprocess import preprocess
    >>> @preprocess(tz=ensure_timezone)
    ... def foo(tz):
    ...     return tz
    >>> foo(\utc\)
    <UTC>
    """
    if isinstance(arg, tzinfo):
        return arg
    if isinstance(arg, string_types):
        return timezone(arg)
    raise TypeError("{func}()    couldn't    convert    argument    {argname}={arg!r}    to    a    timezone.".format(func=_qualified_name(func), argname=argname, arg=arg))