Ken Jin <[email protected]> added the comment:
Wow, thank you for the excellent bug report! Surprisingly, issue42904 's patch
fixes this problem as well. I've spent a few hours trying to debug this, and
here's some findings:
TLDR:
3.9.1 - 3.10 added ForwardRef recursive evaluation. A guess is that since
get_type_hints has always been passing the wrong locals() for classes as
mentioned in issue42904, that may cause nested ForwardRefs to be resolved
incorrectly.
Observations:
a.py:
```
class Root:
a: List["Person"]
```
On first call of get_type_hints(Root), _eval_type evals in the following order:
```
# These print the repr, globals['__name__'], locals['__name__'], and code
object (only for ForwardRef __forward_code__)
ForwardRef: ForwardRef("List['Person']"), a, None, <code object <module> at
0x0503A7A8, file "<string>", line 1>
GenericAlias: typing.List[ForwardRef('Person')], a, a
ForwardRef: ForwardRef('Person'), a, a, <code object <module> at 0x0503AAF0,
file "<string>", line 1>
NA: <class 'a.Person'>
```
Then on a second repeated call of get_type_hints(Root), _eval_type does:
```
ForwardRef: ForwardRef("List['Person']"), a, None, <code object <module> at
0x0503A7A8, file "<string>", line 1>
GenericAlias: typing.List[ForwardRef('Person')], a, a
ForwardRef: ForwardRef('Person'), a, a, <code object <module> at 0x0503AAF0,
file "<string>", line 1>
```
It's skipping the last evaluation of <class 'a.Person'>. This also occurs if I
call it with RootB after Root.
I managed to reproduce this in 3.9.1-2 as well. The only requirement is to
change the example to
```
class Root:
a: 'List["Person"]'
```
----------
nosy: +gvanrossum, kj, levkivskyi
versions: +Python 3.9
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<https://bugs.python.org/issue43646>
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