PySide2 / PyQT5 : Using QPluginLoader to import a QT C++ plugin

bob40

Hi

I try to import a QT C++ plugin into Python using PySide2 / PyQT5.

The dll plugin works fine when using QT C++ QPluginLoader because I can cast the QObject with qobject_cast :
ope_int = qobject_cast<Operations_Interface *>(plugin);

The dll plugin is loaded when using PySide2 or PyQT5 but I don't know how to use qobject_cast in Python to cast to a class interface.

An idea

Thanks

jazzycamel

@bob40
First, which plugin are you trying to use and could you give us a minimal example of your C++/Python code please (just so we can get a handle on what it is you're doing).

Assuming it's a C++ Qt plugin, I think you would probably have to create a SIP (for PyQt5) wrapper around the C++ interface for the plugin and then use sip.cast() to cast the QObject returned from QPluginLoader to a Python representation of the type the plugin returns.

Hope this helps :o)

bob40

Thank you for your help :

1/ The .h interface :

#include <QObject>
#include <QString>

class Operations_Interface
{
public:
    Operations_Interface() = default;
    virtual ~Operations_Interface() = default;
    virtual int addition (int a, int b) = 0;
    virtual int soustraction (int a, int b) = 0;
};


QT_BEGIN_NAMESPACE

#define Operations_Interface_iid "Chargement_Plugin.Operations_Interface"

Q_DECLARE_INTERFACE(Operations_Interface, Operations_Interface_iid)
QT_END_NAMESPACE

2/ The header of the class in the plugin

#ifndef OPERATION_PLUGIN_H
#define OPERATION_PLUGIN_H

#include <QObject>
#include <QtPlugin>
#include "operations_interface.h"

class Operations_plugin : public QObject, Operations_Interface
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID "Chargement_Plugin.Operations_Interface" FILE "Operations_plugin.json")
    Q_INTERFACES(Operations_Interface)

public:
    Operations_plugin() {};
    int addition (int a, int b) override;
    int soustraction (int a, int b) override ;
};

#endif

3/ With the .cpp

#include "Operations_plugin.h"

int Operations_plugin::addition(int a, int b)
{
    return (a + b);
}

int Operations_plugin::soustraction(int a, int b)
{
    return (a - b);
}

4/ And the Python code

from PyQt5.QtCore import QPluginLoader

def read_dll_PyQt():

    path_dll = "c:/Operations_Plugin/Release/Operations_plugin.dll"

    pluginLoader = QPluginLoader()
    pluginLoader.setFileName(path_dll)
    plugin_obj = pluginLoader.instance()

    
# TODO Code for :
# res = plugin_obj.Operations_Interface.Addition(10,5)
# ==> How to cast plugin_obj to Operations_Interface ?   

# Main
if __name__ == '__main__':
    read_dll_PyQt()