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classification
Title: Inheritance from base class with property in class makes them non-instantiatable
Type: behavior Stage:
Components: Library (Lib) Versions: Python 3.10
process
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: AlexWaygood, Germandrummer92, JelleZijlstra, eric.smith, gvanrossum, kj
Priority: normal Keywords:

Created on 2022-04-06 08:41 by Germandrummer92, last changed 2022-04-11 14:59 by admin.

Messages (5)
msg416846 - (view) Author: Daniel Draper (Germandrummer92) Date: 2022-04-06 08:41
Hi,

According to https://peps.python.org/pep-0544/#explicitly-declaring-implementation it should be possible to explicitly inherit from Protocols. This however breaks the dataclass constructor when using the @property decorator in the protocol, see this example:

```python
from typing import Protocol
from dataclasses import dataclass

class SomeProtocol(Protocol):
    @property
    def some_value(self) -> str: ...

@dataclass
class SomeDataclasss(SomeProtocol):
    some_value: str

if __name__ == '__main__':
    a = SomeDataclasss(some_value="value") # this crashes with AttributeError: can't set attribute 'some_value'
```

The pattern of @property in the protocol is one taken from the mypy docs (see https://mypy.readthedocs.io/en/stable/protocols.html#recursive-protocols for example). 

When removing the explicit inheritiance mypy also correctly typechecks the dataclass implementation when doing something like, only the explicit inheritance seems to fail in python

```python
a: SomeProtocol = SomeDataclass()
```
msg416853 - (view) Author: Eric V. Smith (eric.smith) * (Python committer) Date: 2022-04-06 10:36
Here's the error without dataclasses:

------
from typing import Protocol

class SomeProtocol(Protocol):
    @property
    def some_value(self) -> str: ...

class SomeClass(SomeProtocol):
    def __init__(self, some_value):
        self.some_value = some_value

if __name__ == '__main__':
    a = SomeClass(some_value="value")
------

Traceback (most recent call last):
  File "foo.py", line 12, in <module>
    a = SomeClass(some_value="value")
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "foo.py", line 9, in __init__
    self.some_value = some_value
    ^^^^^^^^^^^^^^^
AttributeError: property 'some_value' of 'SomeClass' object has no setter


And here it is without Protocol:
--------
class SomeProperty:
    @property
    def some_value(self) -> str: ...

class SomeClass(SomeProperty):
    def __init__(self, some_value):
        self.some_value = some_value

if __name__ == '__main__':
    a = SomeClass(some_value="value")
--------

Traceback (most recent call last):
  File "foo.py", line 10, in <module>
    a = SomeClass(some_value="value")
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "foo.py", line 7, in __init__
    self.some_value = some_value
    ^^^^^^^^^^^^^^^
AttributeError: property 'some_value' of 'SomeClass' object has no setter
msg416880 - (view) Author: Guido van Rossum (gvanrossum) * (Python committer) Date: 2022-04-06 15:43
So is the conclusion that this should be closed as "not a bug"?
msg416881 - (view) Author: Jelle Zijlstra (JelleZijlstra) * (Python committer) Date: 2022-04-06 15:46
I think the behavior with regular classes is expected (that's just how inheritance works), but a case could be made that dataclasses should handle this case specially.
msg416882 - (view) Author: Eric V. Smith (eric.smith) * (Python committer) Date: 2022-04-06 15:54
What would dataclasses do that's different from a regular class?
History
Date User Action Args
2022-04-11 14:59:58adminsetgithub: 91393
2022-04-06 15:54:34eric.smithsetmessages: + msg416882
2022-04-06 15:46:01JelleZijlstrasetmessages: + msg416881
2022-04-06 15:43:49gvanrossumsetmessages: + msg416880
2022-04-06 10:42:11eric.smithsettitle: Inheritance from Protocol with property in class makes them non-instantiatable -> Inheritance from base class with property in class makes them non-instantiatable
2022-04-06 10:36:45eric.smithsetmessages: + msg416853
title: Inheritance from Protocol with property in dataclass makes them non-instantiatable -> Inheritance from Protocol with property in class makes them non-instantiatable
2022-04-06 08:52:21AlexWaygoodsetnosy: + gvanrossum, eric.smith, JelleZijlstra, kj, AlexWaygood
2022-04-06 08:41:16Germandrummer92create