I'm afraid I don't understand your question completely, so I'll answer what I guess you mean. Let's say you have a function in one module that expects a bytestr

I'm afraid I don't understand your question completely, so I'll answer what I guess you mean. Let's say you have a function in one module that expects a bytestring. This function is called from another module, and a unicode string is passed. What happens? Well python is a duck-typed language, so it will handle the unicode string as if it was a bytestring, and this may or may not work correctly. It's up to the modern code to interface correctly with the legacy code. This could mean manually converting between bytestring and unicode string.