Cython: C-Extensions for Python

Common errors

• ModuleNotFoundError: No module named 'Cython'

  cause This error occurs when the Cython package itself is not installed in the Python environment you are using, or it's installed in a different environment or Python version.
  fix Ensure Cython is installed in your active Python environment by running: `pip install Cython`

• error: Microsoft Visual C++ 14.0 or greater is required.

  cause On Windows, compiling Cython extensions (which generate C/C++ code) requires a compatible C++ compiler, typically provided by the Microsoft C++ Build Tools or a Visual Studio installation.
  fix Install the 'Desktop development with C++' workload from Visual Studio Installer, or specifically the 'Microsoft C++ Build Tools' (available at visualstudio.microsoft.com/visual-cpp-build-tools/). Ensure your environment variables are correctly set for the compiler.

• AttributeError: type object 'your_module.YourCythonClass' has no attribute '__reduce_cython__'

  cause This error typically arises when trying to pickle or copy a Cython extension type that does not have proper serialization methods (`__reduce_cython__` and `__setstate_cython__`) defined, often after upgrading Cython or Python, or when using libraries like PyInstaller.
  fix For pickling custom extension types, you need to explicitly implement `__reduce_cython__` and `__setstate_cython__` methods in your Cython class, or use the `@cython.auto_pickle(True)` decorator if applicable (though explicit implementation is generally safer for complex types).

• Segmentation fault

  cause A segmentation fault in Cython code usually indicates a low-level memory access error, such as reading or writing beyond allocated memory bounds, using uninitialized C pointers, or incorrect memory management when interacting with C libraries.
  fix Debug the Cython code to identify memory-related issues. Common steps include enabling Cython directives like `boundscheck=True` and `wraparound=True` during development, carefully managing C pointers and `malloc`/`free`, and using tools like `gdb` for C-level debugging.

Warnings

• breaking Support for Python 2.7 - 3.7 was removed in Cython 3.3.0a0, along with large chunks of legacy code.
  Fix: Upgrade to Python 3.8 or later.
• gotcha Using the pure Python syntax for static type declarations requires a recent Cython 3 release due to significant improvements over the 0.29.x releases.
  Fix: Upgrade to the latest Cython 3 release.
• breaking The script uses Cython-specific C-style parameter declarations (e.g., `int x`), which are invalid in standard Python and will result in a `SyntaxError`. This syntax is only recognized by the Cython compiler.
  Fix: Compile the script with Cython or rewrite the code using standard Python type hints (e.g., `x: int`).

Install

• pip install cython Install Cython

Imports

• cython

  import cython

  Necessary for using Cython-specific features

Quickstart

A simple example demonstrating the use of Cython's cfunc decorator to define a C function in Python.

import cython

@cython.cfunc
@cython.exceptval(-1)
def add(int x, int y):
    return x + y

print(add(2, 3))