Inheritance from base class with property in class makes them non-instantiatable

@gvanrossum

BPO	47237
Nosy	@gvanrossum, @ericvsmith, @JelleZijlstra, @Fidget-Spinner, @AlexWaygood, @Germandrummer92

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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assignee = None
closed_at = None
created_at = <Date 2022-04-06.08:41:16.184>
labels = ['type-bug', 'library', '3.10']
title = 'Inheritance from base class with property in class makes them non-instantiatable'
updated_at = <Date 2022-04-06.15:54:34.248>
user = 'https://github.com/Germandrummer92'

bugs.python.org fields:

activity = <Date 2022-04-06.15:54:34.248>
actor = 'eric.smith'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2022-04-06.08:41:16.184>
creator = 'Germandrummer92'
dependencies = []
files = []
hgrepos = []
issue_num = 47237
keywords = []
message_count = 5.0
messages = ['416846', '416853', '416880', '416881', '416882']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'eric.smith', 'JelleZijlstra', 'kj', 'AlexWaygood', 'Germandrummer92']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue47237'
versions = ['Python 3.10']

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:

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

a: SomeProtocol = SomeDataclass()

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

So is the conclusion that this should be closed as "not a bug"?

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.

What would dataclasses do that's different from a regular class?

With CPython 3.10.7 and mypy 0.982

import abc
import dataclasses as dc
from typing import Protocol


class Proto(Protocol):
    @property
    @abc.abstractmethod
    def prop(self) -> int:
        raise NotImplementedError


@dc.dataclass
class Implicit:
    prop: int


@dc.dataclass
class Explicit(Proto):
    prop: int


x: Proto = Implicit(42)
y: Proto = Explicit(42)

The results are:

$ python test.py
Traceback (most recent call last):
  File "/private/tmp/test.py", line 24, in <module>
    y: Proto = Explicit(42)
TypeError: Can't instantiate abstract class Explicit with abstract method prop     

$ mypy test.py 
Success: no issues found in 1 source file

I see this as a perhaps surprising result of how all these things work, not as a bug. Note that Protocol isn't needed, the same error is given if Proto doesn't inherit from Protocol. A workaround would be to set a default value in the Explicit subclass (prop: int = 0). But mostly my view is "don't do that".

Another workaround that removes inheritance but preserves explicit type checking at the place of definition:

if TYPE_CHECKING:
    _: type[Proto] = Explicit

@dc.dataclass
class Explicit:
    prop: int

Would this work somehow with the ReadOnly extension proposed in PEP 767?

Yes, ReadOnly should fix this.

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Inheritance from base class with property in class makes them non-instantiatable #91393

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Inheritance from base class with property in class makes them non-instantiatable #91393

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Germandrummer92 commented on Apr 6, 2022

ericvsmith commented on Apr 6, 2022

gvanrossum commented on Apr 6, 2022

JelleZijlstra commented on Apr 6, 2022

ericvsmith commented on Apr 6, 2022

Kentzo commented on Oct 22, 2022

gvanrossum commented on Oct 22, 2022

Kentzo commented on Oct 22, 2022

vnmabus commented on Apr 2, 2025

JelleZijlstra commented on Apr 2, 2025

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