I commend the Steering Council for its wise decision. I'm sure that
once the Python community spends more time considering this issue, and
innovating new solutions, we can come up with a path forward that'll be
good news for everybody.
Best wishes,
//arry/
On 4/20/21 11:57 AM, Thomas Wouters wrote:
(Starting a new thread so as not to derail any of the ongoing
discussions.)
Thanks, everyone, for your thoughts on Python 3.10 and the impact of
PEP 563 (postponed evaluation of annotations) becoming the default.
The Steering Council has considered the issue carefully, along with
many of the proposed alternatives and solutions, and we’ve decided
that at this point, we simply can’t risk the compatibility breakage of
PEP 563. We need to roll back the change that made stringified
annotations the default, at least for 3.10. (Pablo is already working
on this.)
To be clear, we are not reverting PEP 563 itself. The future import
will keep working like it did since Python 3.7. We’re delaying making
PEP 563 string-based annotations the default until Python 3.11. This
will give us time to find a solution that works for everyone (or to
find a feasible upgrade path for users who currently rely on evaluated
annotations). Some considerations that led us to this decision:
- PEP 563’s default change is clearly too disruptive to downstream
users and third-party libraries to happen right now. We can’t risk
breaking even a small subset of the FastAPI/pydantic users, not to
mention other uses of evaluated type annotations that we’re not aware
of yet.
- PEP 563 provides no warning to users of the feature it’s disabling.
Without that, we can’t expect users to be aware of the upcoming
breakage. The lack of a warning was by design, and made sense in a
world where type annotations were only consumed by static type
checkers --- but that’s not actually the situation we’re in. There
are clearly existing real-world, run-time uses of type annotations
that would be adversely affected by this change.
- Originally, PEP 563 was scheduled to take effect in Python 4, and
this changed recently (after the discussion in the Language Summit of
2020). It's possible that third-party libraries and users didn’t plan
to react in the current time frame as they were not aware of this
change in timing.
- There isn’t enough time to properly discuss PEP 649 or any of the
alternatives before the beta 1 deadline, and we really need to make
sure we don’t compound errors here. We need to look for a long term
solution, which isn’t possible while still maintaining the release
deadlines of Python 3.10. That means we’re also deferring PEP 649 to
Python 3.11.
In the Steering Council’s unanimous opinion, rolling back the default
flip for stringified annotations in Python 3.10 is the least
disruptive of all the options.
We need to continue discussing the issue and potential solutions,
since this merely postpones the problem until 3.11. (For the record,
postponing the change further is not off the table, either, for
example if the final decision is to treat evaluated annotations as a
deprecated feature, with warnings on use.)
For what it’s worth, the SC is also considering what we can do to
reduce the odds of something like this happening again, but that’s a
separate consideration, and a multi-faceted one at that.
For the Steering Council,
Thomas.
--
Thomas Wouters <tho...@python.org <mailto:tho...@python.org>>
Hi! I'm an email virus! Think twice before sending your email to help
me spread!
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