Checking for NULL on pyjnius imported object

[Mike Deskevich]

Hi - I hope I'm not being dumb here, but I can't find in the docs how to check for a Java null object on the python side.

For example, I have a callback in python that the Java side populates with an object

def callback(obj):

  print(obj)

The java code may sometimes pass null in as the obj, I'd like to check on the python side for the null.  This doesn't work

def callback(obj):

  if (obj is None):

     #handle null

I presume that's because the object really does exist on the python side and that it just points to a null object on the java side.  I did see some state in the python class that did have an 0x0 in it, so I know that python knows it's a null object on Java.  I just can't figure out how to do the check correctly on the python side.

Thanks!

[Robert]

As far as I am aware  None == null

Is self missing? (this would depend on the approach used for Java calling Python)

def callback(self,obj):

  if (obj is None):

     #handle null

[Mike Deskevich]

The call back isn't existing in a class.  Here's a more realistic example.  This is they python code:

def Evaluate(sol):

  print(sol)

  print(sol is None)

sol is a Java object that's constructed on the Java side and passed into Evaluate. The first iteration sol is a valid object, on the second iteration sol is null.

<com/company/MyJavaClass at 0x22b75ad1768 jclass=com/company/MyJavaClass jself=<LocalRef obj=0x74d0cb30 at 0x22b75ac9330>>
False
<com/company/MyJavaClass at 0x22b75ad1ca8 jclass=com/company/MyJavaClass jself=<LocalRef obj=0x0 at 0x22b75ac93f0>>
False

I can see that the localref obj=0x0 in the second one means that someone knows that the object is null.

[Robert]

OK I see what you are saying. I thought Pyjnius handled that.

How are you calling Evaluate from Java?

[Elliot Garbus]

It looks like the type is not a NoneType.

>>> type(None)

<class 'NoneType'>

>>> type(0x0)

<class 'int'>

 

Print out the type of the returned object.  

type(sol)

 

If it is sufficient you could check for a value of 0 or ‘truthiness”.

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[Mike Deskevich]

Robert - It's a bit convoluted on how I get the callback executed.  I have to fit into some legacy Java code, so it's not pretty.

On the Java side I have an interface

public interface SpecialPythonEvaluator {

  void Evaluate(MyJavaClass sol);

}

And then there's a special class that inherits from the framework I'm using

public class SpecialPythonClass extends LegacyClass {

  SpecialPythonEvaluator evaluator;

  public SpecialPythonClass(SpecialPythonEvaluator evaluator) {

    this.evaluator = evaluator;

  }

  public void Evaluate(MyJavaClass sol){

    evaluator.Evaluate(sol);

  }

}

On the Python side I have the python implementation of that interface

    class PythonEvaluator(PythonJavaClass):

        __javainterfaces__ = ['com/company/SpecialPythonEvaluator']

        def __init__(self, Evaluate=None):

            self.CBEvaluate = Evaluate

        @java_method('(Lcom/company/MyJavaClass;)V')

        def Evaluate(self, sol):

            if (self.CBEvaluate is not None):

                self.CBEvaluate(sol)

Then I construct the python class that implements the Java interface and pass it into my framework so that it gets called

myEvaluator=PythonEvaluator(Evaluate)

myMainObject=SpecialPythonClass(myEvaluator)

When the original (Java) LegacyClass calls it's Evaluate method, that gets overridden by (Java) SpecialPythonClass which has a handle to an interface (Java) SpecialPythonEvaluator.

There's a (Python) implementation of Python Evaluator that then forwards the call to a first-class (Python) function Evaluate.

It's not pretty, but it does work passing the right object around.  I just need to handle a special branch for when sol is (Java) null.  I have an idea of how to do it if pyjnius doesn't give me access to that, I was just hoping to make it pretty.

ElliotG -

For both of the null and not-null cases, I get this when I print my type

  print('Type=',type(sol))

Type= <class 'jnius.reflect.com/company/MyJavaClass'>

[Elliot Garbus]

It the returned type is not NoneType, then sol is None will never be True.

Just test for zero

 

If not sol:

    # handle the case where zero is returned.

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[Robert]

It looks like you are doing the same thing as this https://github.com/Android-for-Python/Android-for-Python-Users#calling-python-from-java  which takes a convoluted brain to figure out!

But perhaps doesn't work in the argument null case?

How about move the test:

public void Evaluate(MyJavaClass sol){

    if (sol != null) {

         evaluator.Evaluate(sol);

    }

  }

[Mike Deskevich]

Yeah, I tried the "if not" test from ElliotG, that also doesn't work. 

I'll do something similar to moving the test.  I do need to have a flag because I do have to implement different behavior on the null, but I'll just catch it early and send another flag.  Thanks for your help!

[Mike Deskevich]

Just to wrap this up in case someone else has this issue.  It feels dirty to me, but it works. 

 I did this on the java end:

     public void Evaluate(MyJavaClass sol){
    if (sol == null) {
      evaluator.NullEvaluate();
    } else {
      evaluator.Evaluate(sol);
    }
  }   

and this on the python end:

        @java_method('(Lcom/company/MyJavaClass;)V')

        def Evaluate(self, sol):

            if (self.CBEvaluate is not None):

                self.CBEvaluate(sol)

        @java_method('()V')

        def NullEvaluate(self):

            if (self.CBEvaluate is not None):

                self.CBEvaluate(None)