[Python-ideas] Type hints for text/binary data in Python 2+3 code

[Andrew Barnert]

On Mar 22, 2016, at 10:58, Guido van Rossum <guido at python.org> wrote:
> 
> One thing I can do without committing to too much is to add a Text
> type to typing.py. It would have no (defined) behavior but it could be
> imported and used in annotations. Then mypy and other checkers could
> start using it and we could even experiment with different proposals
> without having to make more changes to typing.py (which we've found
> are hard to push out, because it's in the 3.5 stdlib -- it's
> provisional so we can change it, but we can't easily change what's
> already in 3.5.0 or 3.5.1).
> 
> Should it be possible to subclass Text, and what should it mean?
> 
> Or perhaps at runtime (i.e. in typing.py) Text would just be an alias
> for str in Python 3 and an alias for unicode in Python 2? That's
> easiest.

It seems like the worry is that you may need to change it again (e.g., to some "virtual type" that's like unicode in 2.7 except that it doesn't have a constructor from str or methods that accept str), and changing something that's in the stdlib (even in a provisional module) is hard?

If so, could you define it as an alias for str in Python 3.5+, and leave it up to backports to define appropriately? Of course you're writing the 2.7 backport, and you'll define it as an alias for unicode--but if you later decide that was too inflexible, you can change the backport without having any effect on the PEP, 3.5 docs, or 3.5 stdlib.