Circumvent the Non-Standard Behaviour of __getattr__ in a Cython Extension Class

Learn how to properly implement `__getattr__` in a Cython class to ensure expected behavior with attribute access.
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This video is based on the question https://stackoverflow.com/q/77053304/ asked by the user 'Szymon Łopaciuk' ( https://stackoverflow.com/u/7471530/ ) and on the answer https://stackoverflow.com/a/77064875/ provided by the user 'Szymon Łopaciuk' ( https://stackoverflow.com/u/7471530/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Circumventing the Non-Standard Behaviour of __getattr__ in Cython

When developing in Cython, one may encounter unexpected behaviors, especially when it comes to attribute access in cdef classes. A common problem involves the method __getattr__, which can behave differently in Cython compared to how it works in standard Python. Recently, a user faced this challenge while trying to override the __getattr__ method in a Cython class. This post will delve into the problem and present a solution.

The Problem: Unwanted __getattribute__ Behavior

In standard Python, the __getattr__ method is called when an attribute is not found in the instance's dictionary. However, in Cython, the __getattr__ method sometimes functions like __getattribute__, leading to unexpected outcomes. The user described a situation where they wanted their Cython class to return a specific value for a defined attribute and another value for undefined attributes:

Example Code

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

In this example:

Accessing Test().field should return 24.

However, accessing Test().anything_else is expected to return 42.

Unfortunately, in Cython, Test().field incorrectly returns 42, indicating that __getattr__ is incorrectly being treated as __getattribute__.

The Solution: Correct Declaration of Attributes

The user initially struggled with this behavior until they identified a key change that needed to be made concerning how they declared their attributes using cython.declare. The correct syntax should use = rather than :.

Correct Implementation

Here’s the updated code that resolves the issue:

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

By declaring the class attribute correctly, the previously problematic attribute access is resolved.

Conclusion

This simple fix enables the __getattr__ method to work as expected in Cython, preserving the functionality that developers rely on from Python. Instead of explicitly checking each attribute in __getattr__, you can confidently declare your fields correctly and maintain the desired behavior without tedious workarounds.

Key Takeaway

When defining attributes in a Cython class, always ensure that the declaration uses = instead of :. This makes all the difference in how attribute access is managed and helps in circumventing the non-standard behavior of __getattr__.

By applying this solution, you can enjoy a smooth development experience in both Cython and native Python environments, ensuring consistent and predictable attribute access throughout your code.

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