Issue trying to implement class that is originally a Java interface

Orekit Python Wrapper usage
1

Hi Orekit community,

Nowadays, I am working with Orekit through the python wrapper.
I encounter a specific issue while trying to code class that directly implement Orekit interface. For instance, I tried to implement ThrustDirectionProvider interface as following:

class MySupaDupaThrustDirectionProvider(ThrustDirectionProvider):
    
    def __init__(self):
    	pass

    def computeThrustDirection(self,pvProv, date, frame):
    	... # Some interesting ccode
    	return directionVector

Then I create an instance of it:

attitudeProv = MySupaDupaThrustDirectionProvider()

However, when I try to use it for creating other object,

attitudeProvider = ThrustDirectionAndAttitudeProvider.buildFromDirectionInLOF(localFrame, attitudeProv, thrusterAxisInBody)

I obtain the following stack or errors:

orekit.JavaError: <super: <class 'JavaError'>, <JavaError object>>
    Java stacktrace:
org.orekit.errors.OrekitException: Le paramètre variableDirectionInFrame de ThrustDirectionAndAttitudeProvider-DIRECTION_IN_LOF est null
	at org.orekit.forces.maneuvers.propulsion.ThrustDirectionAndAttitudeProvider.checkParameterNotNull(ThrustDirectionAndAttitudeProvider.java:136)
	at org.orekit.forces.maneuvers.propulsion.ThrustDirectionAndAttitudeProvider.buildFromDirectionInLOF(ThrustDirectionAndAttitudeProvider.java:204)

Moreover, when I check on the object type in the console, I get

>>>> isinstance(attitudeProv,ThrustDirectionProvider)
True

but in the same time the variable point to null

>>>> attitudeProv
<attitudeProv: <null>>

while I expecting something like <MySupaDupaThrustDirectionProvider : MySupaDupaThrustDirectionProvider.MySupaDupaThrustDirectionProvider@xxxxxxxxxx>

My observation is that the object is clearly recognized as an implementation of the interface but it is not initialized.
In my understanding, It comes from the fact, that I try to implement in Python an class that is actually a Java interface. I try to implement other interfaces and the observations is quite the same.

I hope that the community can help me with issue

Best regards

Christophe

2

You’ll find that you can add some output to your computeThrustDirection method, call it as written and it will work on it’s own, but I don’t think there’s anything you can do to make it work in the attempt to buildFromDirectionInLOF

As I understand it, to subclass in python you have to work within the list of python-wrapped classes, here.

3

Hi @ShikaDing,
Thank you fo this recall that I missed.
So I followed yout proposition and I develop my own python AttitudeProvider subclass starting with the PythonAttitudeProvider to overcome my issue.
It seems to work rightnow with no error when a maneuver is built with my AttitudeProvider subclass. Thoroughful tests are now needed.

4

Hi @ShikaDing, Hi Everyone,

After few tests it seems that the workaround poposed by @ShikaDing is not suitable for my issue. But maybe something is wrong with my code.
Let start with declaring a new class that inherits from PythonAttitudeProvider:

from org.orekit.attitudes import PythonAttitudeProvider
from org.orekit.frames import FramesFactory, LOFType
from org.hipparchus.geometry.euclidean.threed import Vector3D

class MyAttitudeProvider(PythonAttitudeProvider):

    def __init__(self,params):
        self.params = params

    def getAttitude(self,pvProv, date, frame):
        # code to compute a rotation object from the input arguments
        return Attitude(date, frame, rotation.revert(), Vector3D.ZERO, Vector3D.ZERO)

Then create an instance of this class

In[1]: from MyAttitudeProvider import MyAttitudeProvider
In[2]: attProv = MyAttitudeProvider(1)

But surprisingly, the type of the instance attProv is not the one I expected

In[3]: type(attProv)
Out[3]: jcc.FinalizerProxy

At this step, I am little bit lost because of the following surprising behaviour.
In one hand, no exception is raised while constructing others objects like org.orekit.forces.maneuvers.Maneuver. On the other hand, the Maneuver.getAttitudeOverride() return a null object. Consequently, the maneuver does not provide the expected acceleration during the propagation.
Maybe @petrus.hyvonen or @ShikaDing or anyone in the community, can you help me to understand this issue and find a way to overcome this drawback.

Best regards to the community

Christophe

5

Hi I found a eror in my code right above. I forgot to initialize the superclass :face_with_hand_over_mouth:
the right code should be:

from org.orekit.attitudes import PythonAttitudeProvider
from org.orekit.frames import FramesFactory, LOFType
from org.hipparchus.geometry.euclidean.threed import Vector3D

class MyAttitudeProvider(PythonAttitudeProvider):

   def __init__(self,params):
       super().__init__()
       self.params = params

   def getAttitude(self,pvProv, date, frame):
       # code to compute a rotation object from the input arguments
       return Attitude(date, frame, rotation.revert(), Vector3D.ZERO, Vector3D.ZERO)

Doing so, my issue is solved:

attProv = MyAttitudeProvider(1)
<MyAttitudeProvider: org.orekit.attitudes.PythonAttitudeProvider@74f7d1d2>

Thanks to everyone.

Chris

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