How to use the shyaml.MissingKeyError function in shyaml

def traverse(contents, path, default=None):
    try:
        try:
            value = mget(contents, path)
        except (IndexOutOfRange, MissingKeyError):
            if default is None:
                raise
            value = default
    except (IndexOutOfRange, MissingKeyError,
            NonDictLikeTypeError, IndexNotIntegerError) as exc:
        msg = str(exc)
        raise InvalidPath(
            "invalid path %r, %s"
            % (path, msg.replace('list', 'sequence').replace('dict', 'struct')))
    return value

except ValueError:
            raise IndexNotIntegerError(
                "non-integer index %r provided on a list."
                % head)
        try:
            value = dct[idx]
        except IndexError:
            raise IndexOutOfRange(
                "index %d is out of range (%d elements in list)."
                % (idx, len(dct)))
    else:
        try:
            value = dct[head]
        except KeyError:
            ## Replace with a more informative KeyError
            raise MissingKeyError(
                "missing key %r in dict."
                % (head, ))
        except Exception:
            raise NonDictLikeTypeError(
                "can't query subvalue %r of a leaf%s."
                % (head,
                   (" (leaf value is %r)" % dct)
                   if len(repr(dct)) < 15 else ""))
    return aget(value, key)