It's well known that you should make the "correct" choice the default. So unicode should have been opt-out (from __past__ import legacy_strings). Result: Everyone batch-adds unicode opt-out to their whole codebase, but new files use the modern way. Gradually people remove the opt outs as they go over old code.
This couldn’t work. For starters, the names of classes, attributes and functions are str in both versions, which is to say, bytestrings in Python 2 and Unicode strings in Python 3. This cannot be handwaved away with a pragma.
That ignores how python scoping works. Built-ins are actually the last thing python checks.
You can override the built-ins by re-defining the symbol in any of: local functions, enclosing functions, or the global scope and it will impact the entirety of that scope.
You missed the point. It’s not about names themselves. What happens if I want to lookup bytestring-named attributes on a class defined in a Unicode-string module or vice versa?
You missed the point. It’s not about names themselves. What happens if I want to lookup bytestring-named attributes on a class defined in a Unicode-string module or vice versa?
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.
The correct way should be the default, but in the case where you already chose the wrong default, you shouldn't introduce the correct way of handling things and make it the default all at once.
Upgrading to a new major version should never require changes to your code if you fixed all deprecation warnings in the previous major version. Otherwise, you make incremental upgrades impossible.
The new alternative should always be introduced as opt-in with the old default deprecated, and then made default in the next major version.
