Overriding constructors for classes inheriting from Java

[msalman-abid]

As of the latest version (23.1.0), GraalPy does not support overriding constructors for Python classes that inherit from Java.

The example below throws an error when we try to create a custom Exception wrapper in Python:

package my.pack;

import org.graalvm.polyglot.Value;
import org.graalvm.polyglot.Context;

public class Main {

    static Context context = Context.newBuilder().allowAllAccess(true).build();
    static Value clz;

    public static void main(String[] args) {
        context.eval("python", """
            import java
            class ParseException(java.lang.Exception):
                def __init__(self, message):
                    self.__super__(message)
            """);

        clz = context.getBindings("python").getMember("ParseException");
        Value obj = clz.newInstance("Hello from Java");
    }
}

Result

// error
Exception in thread "main" TypeError: ParseException.__init__() missing 1 required positional argument: 'message'
        at org.graalvm.sdk/org.graalvm.polyglot.Value.newInstance(Value.java:933)
        at my.pack.Main.main(Main.java:18)

[msimacek]

Yes, it's a known limitation, because we need to have already created the Java object by the time we call __init__, so we can pass it as self. Java constructors are not like __init__, you can't call them on an already existing object. We should mention it in the documentation. If all you need is to have a python exception subclass, then don't implement __init__ at all. The arguments get passed to the Java constructor implicitly. Example:

import java
class ParseException(java.lang.Exception):
  pass
print(ParseException("my message").getMessage())

[msalman-abid]

I see, thanks for the explanation. Is this a hard limitation, or will this be addressed in the a future release?
If it will remain as-is, an update in the docs will be helpful

[msimacek]

Calling the constructor from __init__ is a hard limitation, Java just doesn't allow calling a constructor on an existing instance. However, what we could do is to allow overriding __new__. Something like this:

class MyException(java.lang.Exception):
  def __new__(cls):
    return cls.__constructor__("my message")

This currently doesn't work, I made up the __constructor__ method. But I think we could make it work.

[msalman-abid]

Will this then be addressed in some upcoming release? We can keep this issue open if required for tracking purposes, and not close it even if #368 is merged

[msimacek]

Let's keep it open, I want to do the __new__ overriding I mentioned earlier.