[issue33453] from __future__ import annotations breaks dataclasses ClassVar and InitVar handling

[INADA Naoki]

INADA Naoki <songofacandy at gmail.com> added the comment:

> > I think we've learned lesson that we shouldn't use typing in modules
other than typing...

> This is a blanket statement that as hurtful as it is factually
incorrect.  Let me address the latter.

> 1. Dataclasses are entirely dependent on annotations *as described by the
typing module*.   While it would be entirely possible for users to pass an
arbitrary expression as an annotation to denote it's a field of the
dataclass, then it's confusing the reader of that code... what's the
point?  The fact that dataclasses isn't itself importing `typing` is an
implementation detail of dataclasses.

When I said "use typing module", I meant "using type information from
annotation value" preciously.
Using only name of the annotation is not harmful.

> 2. The problem isn't even specific to `typing` but stringified type
annotations in general.  If the magic sentinel annotation came from the
dataclasses module, it would be just as tricky to get it right as it is now
when this sentinel lives in `typing`.

Yes.  I think we shouldn't parse annotation value until we can expect
issues like this and estimate performance overhead of
massive calling of get_type_hints() on application startup.

> 3. singledispatch now also allows registering functions that are
type-annotated.  In fact, that is a nicer API than the old one.  That usage
also imports `typing`.  I don't see how this is a problem.

How about performance?
Isn't performance overhead of it is much larger than expected by people who
don't know internal of get_type_hints()?

I agree that annotation could very useful.
But I fear that utilizing annotation value may make ugly hack like
"ClassVar" prefix
or unexpected slow import time.  So I want to be very conservative about
utilizing annotation in stdlib.

Once it is happen, it's very hard to remove it from stdlib.
I think we should learn more about runtime usage of type hints in third
party library,
before using it in stdlib.

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<https://bugs.python.org/issue33453>
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