Issue47237
Created on 2022-04-06 08:41 by Germandrummer92, last changed 2022-04-11 14:59 by admin.
| Messages (5) | |||
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| 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() ``` |
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| msg416853 - (view) | Author: Eric V. Smith (eric.smith) * ![]() |
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
|
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| msg416880 - (view) | Author: Guido van Rossum (gvanrossum) * ![]() |
Date: 2022-04-06 15:43 | |
So is the conclusion that this should be closed as "not a bug"? |
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| msg416881 - (view) | Author: Jelle Zijlstra (JelleZijlstra) * ![]() |
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. |
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| msg416882 - (view) | Author: Eric V. Smith (eric.smith) * ![]() |
Date: 2022-04-06 15:54 | |
What would dataclasses do that's different from a regular class? |
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| History | |||
|---|---|---|---|
| Date | User | Action | Args |
| 2022-04-11 14:59:58 | admin | set | github: 91393 |
| 2022-04-06 15:54:34 | eric.smith | set | messages: + msg416882 |
| 2022-04-06 15:46:01 | JelleZijlstra | set | messages: + msg416881 |
| 2022-04-06 15:43:49 | gvanrossum | set | messages: + msg416880 |
| 2022-04-06 10:42:11 | eric.smith | set | title: 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 10:36:45 | eric.smith | set | messages:
+ 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 08:52:21 | AlexWaygood | set | nosy:
+ gvanrossum, eric.smith, JelleZijlstra, kj, AlexWaygood |
| 2022-04-06 08:41:16 | Germandrummer92 | create | |
