Re: Unable to access referenced parameter in Python callback

Thanks for the help! Hmm, unfortunately, my callback invocation happens in a Nuget package / DLL, so the provided example is a little bit different.

I did note that the argument in the invoked callback has a different underlying type, than when creating the same object on Python side.

def Start(self, config):
    prop_test = test.Property()
    type_test = prop_test.GetType() #FINE

    type_test2 = config.GetType() # ERROR

When a Property (prop_test) is created on the Python side in Start(), its type is
<mds.Property; proxy of <Swig Object of type 'Property *' at 0x000001CC24DA3D80> >

Whereas the passed property has a type
<Swig Object of type 'Base::Property *' at 0x000001CC24CBE1E0>

I wonder whether this is expected, or this might lead to the underlying issue?

Regards,

Ben

From: William S Fulton <[email protected]>
Sent: Friday, September 17, 2021 9:08 PM
To: Ben van Basten <[email protected]>
Cc: [email protected]
Subject: Re: [Swig-user] Unable to access referenced parameter in Python callback

A few tweaks and below works for me:

%module(directors="1", allprotected="1") example
%feature("director") Base;

%inline %{
enum PropertyType{PT_REAL, PT_STRING};
class Property
{
    public:
        PropertyType GetType() const;
        int GetSize() const;
};

class Base
{
    public:
        virtual bool Start(const Property& config) = 0; // PASSED AS REFERENCE

};

void callup(Base& b, Property& p) {
  b.Start(p);
}
%}
%{
  PropertyType Property::GetType() const { return PT_REAL; }
  int Property::GetSize() const { return 1; }
%}


from example import *

class PyClient(Base):
    def Start(self, config):
        config_size = config.GetSize()
        config_type = config.GetType()
        print("Got {} {}".format(config_size, config_type))
        return True


prop = Property()
pc = PyClient()
callup(pc, prop)
William


On Fri, 17 Sept 2021 at 13:49, Ben van Basten via Swig-user <[email protected]<mailto:[email protected]>> wrote:
Hello!

In SWIG, I've got a wrapped C++ base class that is inherited in Python. The C++ side invokes callbacks on the Python derived class, which works fine. Unfortunately, one of the parameters of the callback is passed as reference. Accessing this reference yields:

Fatal unhandled exception: SWIG director method error. Error detected when calling 'Base.Start'

test.i

%module(directors="1", allprotected="1") test
%feature("director") Base;

class Base
{
    public:
        virtual bool Start(const Property& config) = 0; // PASSED AS REFERENCE

};

enum PropertyType{PT_REAL, PT_STRING};
class Property
{
    public:
        PropertyType GetType() const;
        int GetSize() const;
};

In Python, Start is correctly callback/invoked. Unfortunately, invoking GetType() or GetSize() yields the error above. Invoking functions of parameters passed as pointer is going fine.

import test

class PyClient(test.Base):
    def Start(self, config):
        config_size = config.GetSize() // YIELDS ERROR
        config_type = config.GetType() // YIELDS ERROR
        return True

I guess I need to convert the parameter Property from a reference to a pointer, but it is unclear to me how this works in SWIG.

I’m using VS2017. SWIG 4.02, Python 3.7 64bit

Any help would be really appreciated.

Ben

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