[New-bugs-announce] [issue44922] isinstance breaks on imported dataclasses

[Oleg Baskakov]

New submission from Oleg Baskakov <baskakov at google.com>:

Hey I was trying to import dataclasses from another file and somehow isinstance doesn't work anymore:
main.py:
```
import codegen
from dataclasses import dataclass

@dataclass
class AtomX:
    my_symbol: str
    quantity: str = ""

codegen.inheritance_map(AtomX("qwerty"))

```
codegen.py:
```
from main import AtomX

def inheritance_map(candidate):
    assert isinstance(candidate, AtomX)
```

PS the same code with `assert candidate.__class__.__name__ == "AtomX"` works fine

----
Python 3.9.6 (v3.9.6:db3ff76da1, Jun 28 2021, 11:49:53) 
[Clang 6.0 (clang-600.0.57)] on darwin
I'm running inside of PyCharm

----------
messages: 399628
nosy: baskakov
priority: normal
severity: normal
status: open
title: isinstance breaks on imported dataclasses
type: behavior
versions: Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue44922>
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