fix(python): user defined __jsii_proxy_class attributes are not preserved

[iliapolo]

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:

jsii/packages/@jsii/python-runtime/src/jsii/_reference_map.py

Lines 65 to 70 in dc77d6c

	if inspect.isabstract(klass):
	klass = klass.__jsii_proxy_class__()
	# Create our instance, bypassing __init__ by directly calling __new__, and
	# then assign our reference to __jsii_ref__
	inst = klass.__new__(klass)

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

jsii/packages/jsii-pacmak/lib/targets/python.ts

Lines 1496 to 1501 in dc77d6c

	code.line(
	'# Adding a "__jsii_proxy_class__(): typing.Type" function to the abstract class',
	);
	code.line(
	`typing.cast(typing.Any, ${this.pythonName}).__jsii_proxy_class__ = lambda : ${this.proxyClassName}`,
	);

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:

jsii/packages/@jsii/python-runtime/src/jsii/_reference_map.py

Lines 48 to 54 in dc77d6c

	# First we need to check our reference map to see if we have any instance that
	# already matches this reference.
	try:
	# TODO: Handle discovery of possible new interfaces on the ObjRef
	return self._refs[ref.ref]
	except KeyError:
	pass

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

---

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[mergify]

Thank you for contributing! ❤️ I will now look into making sure the PR is up-to-date, then proceed to try and merge it!

[mergify]

Merging (with squash)...