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Inheritance from base class with property in class makes them non-instantiatable #91393
Comments
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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() |
Germandrummer92
mannequin
added
3.10
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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 setterAnd 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 |
ericvsmith
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ericvsmith
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ericvsmith
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ericvsmith
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So is the conclusion that this should be closed as "not a bug"? |
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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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What would dataclasses do that's different from a regular class? |
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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: |
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I see this as a perhaps surprising result of how all these things work, not as a bug. Note that |
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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 |
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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