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Learn how to properly invoke methods on a `GObject` class created in C from Python. Discover how to resolve common integration issues with `gi.repository`.
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Integrating C and Python: Calling Methods on a GObject Class

When working with a GObject class written in C, many developers face challenges when trying to call its methods from Python. This integration is crucial for leveraging existing C libraries or performance-critical functionalities while utilizing Python's flexibility. In this guide, we'll take a closer look at how to successfully call methods from a GObject class written in C using Python, and we'll also discuss a common pitfall and its solution.

The Problem

Imagine you've developed a GObject class in C named BopFooObj. You annotated the class for gobject-introspection so it could be used with Python. However, when attempting to create an instance of this class and call its methods, you encounter errors. Here's the crucial piece of code that illustrates the problem:

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Despite successfully creating the class in C and generating the necessary files (.gir and .typelib), the _class_init and _init functions are unexpectedly not called when you create an instance from Python. This leads to a frustrating dead end.

Understanding the Cause

The issue primarily arises from the way the build system interacts with g-object-introspection. The compiler is effectively unable to link the necessary libraries properly when invoking g-ir-scanner, which consequently prevents the Python runtime from recognizing the GObject structure correctly.

Here's a breakdown of the common symptoms indicating this issue:

_class_init and _init not invoked.

Methods of the class throw TypeErrors when called from Python.

Stability issues leading to segmentation faults in more complex setups.

The Solution

Fortunately, this issue can be resolved by ensuring that the g-ir-scanner correctly identifies the GObject library during the build process. Here's how to do it step by step:

Step 1: Adjust Your Build Configuration

You need to edit your meson.build file to include the proper --include directive instead of --extra-library. The inclusion of the required library explicitly informs the scanner to recognize your GObject class correctly.

Here is how you can modify your meson.build file:

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Step 2: Regenerate the Files

After making the above modification, ensure to regenerate your .gir and .typelib files. This offers your Python scripts the updated information they need to correctly instantiate and use your GObject class.

Step 3: Test the Integration

With the adjustments in place, you can now run your Python script again. This time when you create an instance of your BopFooObj and call its methods, everything should work seamlessly:

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Conclusion

Integrating C-written GObject classes with Python can be daunting, but understanding how to configure the build system is crucial for success. By following the outlined adjustments to your meson.build file and ensuring proper library inclusions, you can reliably call your methods from Python without errors or crashes. With these improvements, you're now ready to leverage the power of C from within Python, expanding the capabilities of your applications.

Feel free to share your experiences or further questions in the comments below!