Eric V. Smith <e...@trueblade.com> added the comment:
The more I think about this, the more I think Łukasz is correct that just
checking for strings starting with "ClassVar", "typing.ClassVar", or
"t.ClassVar" is the correct thing to do. This is the change he made in
https://github.com/python-attrs/attrs/pull/367.
In some private email I had discussed extracting the module name, if any, from
the string annotation, and looking it up in sys.modules. This doesn't buy you
much because you have to know how the caller imported typing. If they used
"import typing as t", then you can't look up "t" in sys.modules. You could do
some horrible frame trick to find out what the caller knew as "t", but that
still won't work in plenty of cases. I don't want to use a complex solution
that still doesn't work in all cases, and would indeed work in fewer places
than just examining the string. The only name we could reliably detect is
"typing.ClassVar", all others wouldn't be in sys.modules correctly.
So, that leaves us with just looking at the string and guessing what the caller
meant. I think the three strings that Łukasz suggested are good enough.
This will need to be well documented. The documentation should note that things
like this will break:
from __future__ import annotations
from typing import ClassVar as CV
@dataclass
class C:
x: CV
x will not be a class variable here, since @dataclass will see the annotation
as "CV" and not know that it means typing.ClassVar.
InitVar has a similar problem. What strings to use there? "InitVar" and
"dataclasses.InitVar", of course. But maybe "dc.InitVar"? It's hard to say with
no in-the-field usage examples to search for. I'll start with just the first
two strings.
I really don't want to use regexes here. But a refinement would be to not just
use .startswith, and instead use a word boundary. After all, "ClassVarOfMine"
should probably not be considered a ClassVar. I'll leave that issue for another
day, though.
I'll have a patch ready before the sprints are over.
----------
assignee: -> eric.smith
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue33453>
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