[New-bugs-announce] [issue38431] dataclasses.InitVar breaks with typing.Optional

[Samuel Colvin]

New submission from Samuel Colvin <samcolvin at gmail.com>:

The following code works fine with python 3.7 but breaks with 3.8:

```
import dataclasses
from typing import Optional

@dataclasses.dataclass
class TestingDataclass:
    base_path: dataclasses.InitVar[Optional[str]] = None
```

Exception traceback:

```
Traceback (most recent call last):
  File "test.py", line 6, in <module>
    class TestingDataclass:
  File "/usr/local/lib/python3.8/dataclasses.py", line 995, in dataclass
    return wrap(cls)
  File "/usr/local/lib/python3.8/dataclasses.py", line 987, in wrap
    return _process_class(cls, init, repr, eq, order, unsafe_hash, frozen)
  File "/usr/local/lib/python3.8/dataclasses.py", line 967, in _process_class
    str(inspect.signature(cls)).replace(' -> None', ''))
  File "/usr/local/lib/python3.8/inspect.py", line 3050, in __str__
    formatted = str(param)
  File "/usr/local/lib/python3.8/inspect.py", line 2568, in __str__
    formatannotation(self._annotation))
  File "/usr/local/lib/python3.8/inspect.py", line 1202, in formatannotation
    return repr(annotation)
  File "/usr/local/lib/python3.8/dataclasses.py", line 213, in __repr__
    return f'dataclasses.InitVar[{self.type.__name__}]'
  File "/usr/local/lib/python3.8/typing.py", line 757, in __getattr__
    raise AttributeError(attr)
AttributeError: __name__
```

The code runs fine with `str` instead of `Optional[str]`.

Tested locally with `Python 3.8.0rc1 (tags/v3.8.0rc1:34214de6ab, Oct 10 2019, 16:15:14)`. The same error can be seen (in a more involved context) on travis [here](https://travis-ci.org/samuelcolvin/pydantic/jobs/596131963)

----------
components: Library (Lib)
messages: 354388
nosy: samuelcolvin
priority: normal
severity: normal
status: open
title: dataclasses.InitVar breaks with typing.Optional
type: behavior
versions: Python 3.8

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38431>
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