[GR-73854] Document and make work adding arguments to __init__ when subclassing from Java.

PullRequest: graalpython/4293

Author: timfel

docs/user/Python-on-JVM.md

@@ -333,6 +333,10 @@ assert issubclass(PythonLevel, Level)
 assert PythonLevel.parse("INFO").getName() == "INFO"
 ```
 
+Two important caveats:
+
+  1. You cannot use other metaclasses, so inheriting from `ABC` and a Java class is not supported.
+  2. If you want to implement `__init__` to accept additional arguments, you must also override `__new__` to avoid passing the additional arguments to the Java constructor, which would not know how to handle them.
 
 ## Embedding Python into Java
 

graalpython/com.oracle.graal.python.test/src/tests/test_interop.py

@@ -675,7 +675,7 @@ class MyHandler (Handler):
             counter = 0;
             def isLoggable(self, logrecord):
                 self.counter = self.counter + 1
-                return self.__super__.isLoggable(logrecord)
+                return super().isLoggable(logrecord)
             def sayHello(self):
                 return 'Hello'
 
@@ -726,9 +726,9 @@ def test_extend_java_class_03(self):
 
         class MyLogRecord(LogRecord):
             def getLevel(self):
-                if self.__super__.getLevel() == Level.FINEST:
-                    self.__super__.setLevel(Level.WARNING)
-                return self.__super__.getLevel()
+                if super().getLevel() == Level.FINEST:
+                    super().setLevel(Level.WARNING)
+                return super().getLevel()
 
         message = "log message"
         my_lr1 = MyLogRecord(Level.WARNING, message)
@@ -1503,6 +1503,18 @@ def getName(self):
         assert pl.callStaticFromPython("INFO").getName() == "INFO"
         assert PythonLevel2.parse("INFO").getName() == "INFO"
 
+    def test_java_subclassing_with_new_arguments(self):
+        from java.util.logging import Level
+
+        class PythonLevel(Level, new_style=True):
+            def __new__(cls, misc_value):
+                return super().__new__(cls, "default name", 2)
+
+            def __init__(self, misc_value):
+                self.misc_value = misc_value
+
+        pl = PythonLevel(123)
+        assert pl.misc_value == 123
 
     def test_jython_star_import(self):
         if __graalpython__.jython_emulation_enabled:

graalpython/lib-graalpython/__graalpython__.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2020, 2025, Oracle and/or its affiliates. All rights reserved.
+# Copyright (c) 2020, 2026, Oracle and/or its affiliates. All rights reserved.
 # DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 #
 # The Universal Permissive License (UPL), Version 1.0
@@ -98,7 +98,7 @@ def import_current_as_named_module_with_delegate(module, module_name, delegate_n
 
 
 @builtin
-def build_java_class(module, ns, name, base, new_style=False):
+def build_java_class(module, ns, name, base, new_style=True):
     if new_style:
         return build_new_style_java_class(ns, name, base)
     import warnings
@@ -174,6 +174,11 @@ def __instancecheck__(cls, obj):
         def __subclasscheck__(cls, derived):
             return cls is derived or issubclass(derived, JavaClass)
 
+        def __call__(cls, *args, **kwds):
+            java_object = cls.__new__(cls, *args, **kwds)
+            java_object.this.__init__(*args, **kwds)
+            return java_object
+
         def __new__(mcls, name, bases, namespace):
             if bases:
                 new_class = None
@@ -232,7 +237,6 @@ def __new__(cls, *args, **kwds):
             delegate = object.__new__(cls)
             java_object = polyglot.__new__(JavaClass, *(args + (delegate,)))
             delegate.__this__ = java_object
-            delegate.__init__(*args, **kwds)
             return java_object
 
     return type(name, (DelegateSuperclass,), ns)