def prefixed_key(f):
    """
    A method decorator that prefixes return values.

    Prefixes any string that the decorated method `f` returns with the value of
    the `prefix` attribute on the owner object `self`.
    """
    def prefixed_method(self, *args, **kwargs):
        key = f(self, *args, **kwargs)
        return f"{self.prefix}:{key}"

    return prefixed_method