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fix(python): user defined __jsii_proxy_class attributes are not pre…
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…served (#4625)

In #4611, we added the `_jsii_proxy_class__` attributes to the `@jsii.interface` implementations. This was required in order to comply with `typeguard` protocol checking. We didn't implement it correctly, accidentally overriding user defined proxy classes.

## Note

I have been wrecking my brain trying to understand if this bug has any runtime implications, and I couldn't find any.

#### How so?

At runtime, from what I could gather, the `__jsii_proxy_class__` attribute is only used when we try to instantiate a subclass of an abstract class:

https://github.com/aws/jsii/blob/dc77d6c7016bcb7531f6e374243410f969ea1fbf/packages/%40jsii/python-runtime/src/jsii/_reference_map.py#L65-L70

However, for abstract classes, we assign an explicit value to `__jsii_proxy_class__`:

https://github.com/aws/jsii/blob/dc77d6c7016bcb7531f6e374243410f969ea1fbf/packages/jsii-pacmak/lib/targets/python.ts#L1496-L1501

Luckily, this happens **AFTER** the `@jsii.implements` decorator has finished, thus overriding the mistake in the decorator.
Presumably, this would still be a problem for user defined abstract classes (since they don't have this assignment). However, reference resolving for user defined classes is done via native reference lookup:

https://github.com/aws/jsii/blob/dc77d6c7016bcb7531f6e374243410f969ea1fbf/packages/%40jsii/python-runtime/src/jsii/_reference_map.py#L48-L54

This is also why I couldn't come up with a real life test case, and had to resort to an artificial one. 

---

By submitting this pull request, I confirm that my contribution is made under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0
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iliapolo authored and mrgrain committed Aug 30, 2024
1 parent 3b9adc4 commit d3ec911
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2 changes: 1 addition & 1 deletion packages/@jsii/python-runtime/src/jsii/_runtime.py
Original file line number Diff line number Diff line change
Expand Up @@ -168,7 +168,7 @@ def implements(*interfaces: Type[Any]) -> Callable[[T], T]:
def deco(cls):
cls.__jsii_type__ = getattr(cls, "__jsii_type__", None)
cls.__jsii_ifaces__ = getattr(cls, "__jsii_ifaces__", []) + list(interfaces)
cls.__jsii_proxy_class__ = lambda: getattr(cls, "__jsii_proxy_class__", None)
cls.__jsii_proxy_class__ = getattr(cls, "__jsii_proxy_class__", lambda: None)

# https://github.com/agronholm/typeguard/issues/479
cls.__protocol_attrs__ = getattr(cls, "__protocol_attrs__", [])
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31 changes: 31 additions & 0 deletions packages/@jsii/python-runtime/tests/test_python.py
Original file line number Diff line number Diff line change
Expand Up @@ -27,6 +27,37 @@ def test_inheritance_maintained(self):

assert base_names == ["DerivedStruct", "MyFirstStruct"]


class TestImplementsInterface:

def test_jsii_proxy_class_defaults_to_none(self) -> None:
@jsii.implements(IBaz)
class MyBaz:
pass

klass = getattr(MyBaz, "__jsii_proxy_class__")()
assert klass == None

def test_jsii_proxy_class_preserves_user_defined_attribute(self) -> None:

class _MyBazProxy:
def baz_method(self) -> str:
return "_MyBazProxy"

@jsii.implements(IBaz)
class MyBaz:

@staticmethod
def __jsii_proxy_class__():
return _MyBazProxy

def baz_method(self) -> str:
return "MyBaz"

klass = getattr(MyBaz, "__jsii_proxy_class__")()
instance = klass()
assert instance.baz_method() == "_MyBazProxy"

def test_implements_interface(self) -> None:
"""Checks that jsii-generated classes correctly implement the relevant jsii-generated interfaces."""

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