On 5/1/22 15:44, Paul Bryan wrote:
Can someone state what's currently unpalatable about 649? It seemed to
address the forward-referencing issues, certainly all of the cases I
was expecting to encounter.
Carl's talk was excellent here; it would be lovely if he would chime in
and reply. Here is my almost-certainly-faulty recollection of what he said.
* PEP 649 doesn't work for code bases that deliberately using
un-evaluatable expressions but still examine them at runtime. Some
code bases would have a major circular import problem if they had to
import every module they use in annotations. By only importing
those in "if TYPE_CHECKING" blocks, mypy (which inspects each module
in isolation) can resolve the references, so it works fine at static
analysis time. Occasionally they /also/ need to examine the
annotation at runtime, but this is only a rudimentary check, so a
string works fine for them. So 563 works, but 649 throws e.g. a
NameError. Carl proposes a mitigation strategy here: run the
co_annotations code object with a special globals dict set that
creates fake objects instead of failing on lookups.
* PEP 649 is a pain point for libraries using annotations for
documentation purposes. The annotation as written may be very
readable, but evaluating it may turn into a very complicated object,
and the repr() or str() of that object may be a complicated and
obscure the original intent. Carl proposes using much the same
strategy here; also it /might/ work to use ast.unparse to pull the
original expression out of the source code, though this seems like
it would be less reliable.
That's everything I remember... but I was operating on two hours' sleep
that day.
You might also consult Brett's thread about finding edge cases in PEPs
484, 563, and 649:
https://discuss.python.org/t/finding-edge-cases-for-peps-484-563-and-649-type-annotations/14314/18
Cheers,
//arry/
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