Lots of people mention strings, but for a concreteish example: you have some code which reads bytes from the network, say "cookie", and uses that as a key into

Lots of people mention strings, but for a concreteish example: you have some code which reads bytes from the network, say "cookie", and uses that as a key into a dict. Later you try to access the stored value with headers["cookie"]. This works in python 2. It silently fails in python 3 because b"cookie" and "cookie" are different keys. Not really related to whether the original code supported Unicode or not.