"你好".encode('utf-8')
encode
converts a unicode object to a string
object. But here you have invoked it on a string
object (because you don’t have the u). So python has to convert the string
to a unicode
object first. So it does the equivalent of
"你好".decode().encode('utf-8')
But the decode fails because the string isn’t valid ascii. That’s why you get a complaint about not being able to decode.