Resolving SWIG Annotations in Python 2 and 3 Compatibility

Описание: Learn how to manage SWIG generated Python files to avoid annotations interfering with Python 2 compatibility.
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This video is based on the question https://stackoverflow.com/q/71357758/ asked by the user 'Frank Yellin' ( https://stackoverflow.com/u/6457407/ ) and on the answer https://stackoverflow.com/a/71358783/ provided by the user 'Mark Tolonen' ( https://stackoverflow.com/u/235698/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Configure SWIG for Python 2 and 3 without Annotations

When working with multiple versions of Python, such as Python 2.7 and Python 3.10, managing compatibility can become quite challenging. One common issue developers encounter is the handling of function annotations that are supported in Python 3 but not in Python 2. This guide will break down how to use SWIG effectively to support both Python versions without running into annotation-related issues.

Understanding the Problem

In your project, you are generating a SWIG extension using setuptools.setup, which produces a shareable library and a Python wrapper file (myswig.py) for both Python 2.7 and Python 3.10. While the contents of myswig.py are largely identical, the Python 3 generated file includes annotations that cause compatibility issues for users still utilizing Python 2. Here’s what you need to know:

Identical Functionality: Python 3 can readily execute code generated for Python 2, but the reverse is not true.

Annotation Issues: The annotation feature in Python 3 is leading to incompatibilities when Python 2 tries to use those files.

Your Need: Finding a way to either turn off annotations in the generated Python file for Python 3 or adding compatibility imports for Python 2.

Proposed Solution

Avoid Using -py3 Flag

The simplest solution to your problem is to omit the -py3 flag from your SWIG command when generating the interface files. Here’s why:

No Need for -py3: Using the -py3 option activates several Python 3-specific features in your generated code, including annotations. By not using this flag, you avoid unwanted features that lead to complications.

Steps to Modify Your SWIG Command

To implement this change, follow the steps below:

Open your setup script: This is where you configure how SWIG operates with your setup.

Modify the command: Instead of using the command swig -python -py3 myswig.i, change it to simply:

[[See Video to Reveal this Text or Code Snippet]]

Re-run your setup: Execute your setuptools.setup command again. This update should prevent function annotations from being included in the intervening Python 3 generated files.

Confirming Your Changes

After making the above changes, ensure that your code still executes as expected in both Python environments.

Test both versions and verify that the myswig.py generated for Python 3 now lacks annotations, maintaining compatibility with the Python 2 environment.

Conclusion

By refraining from using the -py3 flag in your SWIG command, you can generate interface files that are compatible with both Python 2.7 and Python 3.10, while avoiding the issues caused by annotations. This method balances the need for modern code with the practical constraints of a diverse user base still relying on legacy systems.

If you're still facing issues or have additional questions regarding SWIG configurations, feel free to reach out in the comments below!