We generally don't do type checking (see discussions of "duck typing"). We generally do just let the implementation detail bubble up.  I don't think the encoding suggestion works, since we can't know what encoding the byte string is in, or even if it is a valid one.

I wonder if unquote should be polymorphic: accept both bytes and strings and produce the same type of output as its input.  I think there are other urllib methods that are, but I haven't checked.