# Cython — Write C Extensions for Python Using Python-Like Syntax

> Cython is an optimizing static compiler that translates Python-like code into C, producing extension modules that run at native speed. It is used to build high-performance libraries and to wrap existing C/C++ code for Python access.

## Install

Save as a script file and run:

# Cython — Write C Extensions for Python Using Python-Like Syntax

## Quick Use
```bash
pip install cython

# Create a Cython file (fib.pyx)
# def fib(int n):
#     cdef int a = 0, b = 1, i
#     for i in range(n):
#         a, b = b, a + b
#     return a

# Compile and use
cythonize -i fib.pyx
python -c "from fib import fib; print(fib(40))"
```

## Introduction
Cython is both a programming language and a compiler. The language is a superset of Python that adds optional C-level type declarations. The compiler translates Cython source files into C code, which is then compiled into Python extension modules. Many core scientific Python libraries (NumPy, SciPy, pandas, scikit-learn) use Cython internally for performance-critical code paths.

## What Cython Does
- Compiles Python-like code to C for 10-100x speedup on numerical workloads
- Adds static type declarations (cdef int, cdef double) to eliminate Python overhead
- Wraps existing C and C++ libraries for seamless access from Python
- Generates Python extension modules (.so/.pyd) compatible with CPython
- Supports typed memoryviews for efficient NumPy array access without GIL contention

## Architecture Overview
Cython reads .pyx source files and translates them into C source files. Each Python-level construct is mapped to CPython C API calls, while typed variables and functions are compiled to direct C operations. The generated C file is compiled by a standard C compiler (gcc, clang, MSVC) into a shared library that CPython loads as a native extension module.

## Self-Hosting & Configuration
- Install with `pip install cython` and ensure a C compiler is available (gcc, clang, or MSVC)
- Write .pyx files with optional cdef type annotations for performance-critical variables
- Compile with `cythonize -i module.pyx` for quick builds or use a setup.py with setuptools
- Use .pxd declaration files to share typed declarations across Cython modules
- Enable the annotate flag (`cython -a module.pyx`) to generate an HTML report showing Python/C interaction hotspots

## Key Features
- Superset of Python: any valid Python file is valid Cython (gradual optimization)
- Direct C-level memory access via typed memoryviews for NumPy arrays
- `nogil` blocks release the GIL for true multi-threaded parallelism in C sections
- Wraps C/C++ libraries with `cdef extern from` declarations
- Pure Python mode allows type hints via decorators without changing .py files

## Comparison with Similar Tools
- **Numba** — JIT compiler with decorators; no compilation step but limited to numerical subset of Python
- **PyPy** — Alternative Python interpreter with tracing JIT; faster for general code but incompatible with some C extensions
- **pybind11** — C++ binding tool; better for wrapping complex C++ APIs but requires writing C++
- **CFFI** — Foreign function interface for calling C code; simpler for one-off C calls but no optimization of Python code
- **Mypyc** — Compiles type-annotated Python via mypy; easier adoption but less mature and fewer optimizations

## FAQ
**Q: Do I need to rewrite my entire codebase in Cython?**
A: No. Cython is designed for incremental optimization. Profile first, then convert only the hot functions to .pyx files with type annotations.

**Q: Does Cython work with Python 3.12+?**
A: Yes. Cython 3.0+ fully supports Python 3.12 and later, including the latest CPython internals.

**Q: How do I distribute Cython extensions to users?**
A: Include the generated C files in your sdist so users do not need Cython installed. Ship wheels with precompiled binaries for common platforms.

**Q: Can Cython compile async/await code?**
A: Yes. Cython 3.0 supports async def, await, and async for with full type optimization.

## Sources
- https://github.com/cython/cython
- https://cython.readthedocs.io/

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Source: https://tokrepo.com/en/workflows/56e9cfa5-43a5-11f1-9bc6-00163e2b0d79
Author: Script Depot