feat(python): support inheritance-based interface implementation

gh-pages/content/user-guides/lib-user/language-specific/python.md

@@ -16,45 +16,48 @@ Traditionally, **Python** developers expect to be able to either *implicitly* im
 required members, or *explicitly* implement interfaces by simply adding the interface to their class' or interface's
 inheritance chain (and implementing all required members):
 
-!!! bug "Incorrect Use"
-
-    ```py hl_lines="3"
-    from jsii_dependency import IJsiiInterface
-
-    class MyNewClass(IJsiiInterface):
-        """ Traditional implementation of an interface in Python.
-
-        This will not work with interfaces defined by jsii modules, as this will
-        likely cause a metaclass conflict that the user cannot solve.
-        """
-
-        # Member implementations...
-
-        ...
-    ```
-
 The [jsii type system][jsii-type-system] however does not support *structural typing*, and interfaces must **always** be
 *explicitly* implemented. In order to correctly declare implementation of an interface from a *jsii module*, the
 following syntax is used:
 
-```py hl_lines="1 4"
-import jsii
+```py hl_lines="3"
 from jsii_dependency import IJsiiInterface
 
-@jsii.implements(IJsiiInterface)
-class MyNewClass():
-    """ A jsii-supported implementation of the `IJsiiInterface` interface
+class MyNewClass(IJsiiInterface):
+    """ Traditional implementation of an interface in Python.
 
-    This will correctly register the explicit interface implementation on the
-    type's metadata, and ensure instances get correctly serialized to and from
-    the jsii kernel.
+    This will not work with interfaces defined by jsii modules, as this will
+    likely cause a metaclass conflict that the user cannot solve.
     """
 
     # Member implementations...
 
     ...
 ```
 
+!!! info Legacy Style
+    When using libraries generated by `jsii-pacmak` version `1.52.1` and earlier, using the inheritance chain style
+    above may result in metaclass conflicts when performing multiple inheritance. In such cases, the implementation can
+    be performed using the `@jsii.implements` decorator instead.
+
+    ```py hl_lines="1 4"
+    import jsii
+    from jsii_dependency import IJsiiInterface
+
+    @jsii.implements(IJsiiInterface)
+    class MyNewClass():
+        """ A jsii-supported implementation of the `IJsiiInterface` interface
+
+        This will correctly register the explicit interface implementation on the
+        type's metadata, and ensure instances get correctly serialized to and from
+        the jsii kernel.
+        """
+
+        # Member implementations...
+
+        ...
+    ```
+
 [jsii-type-system]: ../../../specification/2-type-system.md
 
 ## Property Overrides

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

@@ -3,13 +3,12 @@
 
 import attr
 
-from typing import cast, Any, Callable, ClassVar, List, Optional, Mapping, Type, TypeVar
+from typing import cast, Any, Callable, List, Optional, Mapping, Type, TypeVar
 
 from . import _reference_map
 from ._compat import importlib_resources
 from ._kernel import Kernel
 from .python import _ClassPropertyMeta
-from ._kernel.types import ObjRef
 
 
 # Yea, a global here is kind of gross, however, there's not really a better way of
@@ -144,6 +143,8 @@ def deco(cls):
 def interface(*, jsii_type: str) -> Callable[[T], T]:
     def deco(iface):
         iface.__jsii_type__ = jsii_type
+        # This interface "implements itself" - this is a trick to ease up implementation discovery.
+        iface.__jsii_ifaces__ = [iface]
         _reference_map.register_interface(iface)
         return iface
 

packages/@jsii/python-runtime/tests/test_compliance.py

@@ -169,6 +169,22 @@ def next(self):
         return next_
 
 
+class SubclassNativeFriendlyRandom_Inheritance(
+    Number, IFriendly, IRandomNumberGenerator
+):
+    def __init__(self):
+        super().__init__(908)
+        self.next_number = 100
+
+    def hello(self):
+        return "SubclassNativeFriendlyRandom"
+
+    def next(self):
+        next_ = self.next_number
+        self.next_number += 100
+        return next_
+
+
 @jsii.implements(IFriendlyRandomGenerator)
 class PureNativeFriendlyRandom:
     """
@@ -753,6 +769,10 @@ def test_testInterfaces():
         poly.say_hello(SubclassNativeFriendlyRandom())
         == "oh, SubclassNativeFriendlyRandom"
     )
+    assert (
+        poly.say_hello(SubclassNativeFriendlyRandom_Inheritance())
+        == "oh, SubclassNativeFriendlyRandom"
+    )
     assert poly.say_hello(PureNativeFriendlyRandom()) == "oh, I am a native!"
 
 
@@ -777,6 +797,20 @@ def test_testNativeObjectsWithInterfaces():
     assert generator_bound_to_pure_native.next_times100() == 100_000
     assert generator_bound_to_pure_native.next_times100() == 200_000
 
+    ###
+    # One more time, but this time implementing the interface via "classic" inheritance.
+    ###
+    subclassed_native = SubclassNativeFriendlyRandom_Inheritance()
+    generator_bound_to_p_subclassed_object = NumberGenerator(subclassed_native)
+
+    assert generator_bound_to_p_subclassed_object.generator is subclassed_native
+    generator_bound_to_p_subclassed_object.is_same_generator(subclassed_native)
+    assert generator_bound_to_p_subclassed_object.next_times100() == 10000
+
+    # When we invoke nextTimes100 again, it will use the objref and call into the same
+    # object.
+    assert generator_bound_to_p_subclassed_object.next_times100() == 20000
+
 
 def test_testLiteralInterface():
     obj = JSObjectLiteralForInterface()

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

@@ -508,10 +508,8 @@ abstract class BaseMethod implements PythonBase {
   public emit(
     code: CodeMaker,
     context: EmitContext,
-    opts?: BaseMethodEmitOpts,
+    { renderAbstract = true, forceEmitBody = false }: BaseMethodEmitOpts = {},
   ) {
-    const { renderAbstract = true, forceEmitBody = false } = opts ?? {};
-
     const returnType: string = toTypeName(this.returns).pythonType(context);
 
     // We cannot (currently?) blindly use the names given to us by the JSII for
@@ -874,9 +872,8 @@ abstract class BaseProperty implements PythonBase {
   public emit(
     code: CodeMaker,
     context: EmitContext,
-    opts?: BasePropertyEmitOpts,
+    { renderAbstract = true, forceEmitBody = false }: BasePropertyEmitOpts = {},
   ) {
-    const { renderAbstract = true, forceEmitBody = false } = opts ?? {};
     const pythonType = toTypeName(this.type).pythonType(context);
 
     // "# type: ignore[misc]" is needed because mypy cannot check decorated things
@@ -973,7 +970,10 @@ class Interface extends BasePythonClassType {
         if (this.separateMembers) {
           code.line();
         }
-        member.emit(code, context, { forceEmitBody: true });
+        member.emit(code, context, {
+          forceEmitBody: true,
+          renderAbstract: false,
+        });
       }
     } else {
       code.line('pass');
@@ -998,7 +998,8 @@ class Interface extends BasePythonClassType {
       toTypeName(b).pythonType({ ...context, typeAnnotation: false }),
     );
 
-    params.push('typing_extensions.Protocol');
+    // Interfaces are kind of like abstract classes. We type them the same way.
+    params.push('metaclass = jsii.JSIIAbstractClass');
 
     return params;
   }
@@ -1009,12 +1010,16 @@ class Interface extends BasePythonClassType {
 }
 
 class InterfaceMethod extends BaseMethod {
+  // Interface members are always abstract
+  public readonly abstract = true;
   protected readonly implicitParameter: string = 'self';
   protected readonly jsiiMethod: string = 'invoke';
   protected readonly shouldEmitBody: boolean = false;
 }
 
 class InterfaceProperty extends BaseProperty {
+  // Interface members are always abstract
+  public readonly abstract = true;
   protected readonly decorator: string = 'builtins.property';
   protected readonly implicitParameter: string = 'self';
   protected readonly jsiiGetMethod: string = 'get';