If it makes it easier, I'd definitely start with >= 3.7. That will be sufficient as a proof of concept and will provide the baseline to include in the 3.8 stdlib typing.py (if the PEP makes it through quickly enough).

But I assume people would want to use this on legacy code as well (several companies I  know who are using type checkers have large legacy code bases, for some value of legacy).

On Wed, May 22, 2019 at 11:20 AM Till <till.varoquaux@gmail.com> wrote:
Would people object to me implementing this only for `python >= 3.7`
in typing_extension? I'm new to that codebase and it seems like this
would make it easier to implement (using __class_getitems__...).
Otherwise I can aim for a first patch that adds 3.7 and work my way
down to older revisions.

--Guido van Rossum (python.org/~guido)
Pronouns: he/him/his (why is my pronoun here?)