Python "Cython.Compiler.Errors.CompileError" Building a Wheel in CI

A package builds C from .pyx sources via Cython, and that cythonization failed - Cython is missing, too old for your Python, or the generated C does not compile on this interpreter.

What this error means

A wheel build fails with Cython.Compiler.Errors.CompileError, or "Cython is required to compile" / "No module named 'Cython'" during the build step. It often appears on a brand-new Python before the package ships a wheel.

Error compiling Cython file:
------------------------------------------------------------
...
^
------------------------------------------------------------
mypkg/_speedups.pyx:42:0: Compiler crash in ...
Cython.Compiler.Errors.CompileError: mypkg/_speedups.pyx

Common causes

Cython missing or too old

The isolated build env lacks Cython, or has a version too old to target your Python, so cythonization fails before the C compile.

Generated C incompatible with this Python

On a very new Python, an old package’s .pyx/generated C uses removed CPython internals, so the compile crashes.

How to fix it

Provide a recent Cython for the build

Add Cython to the project’s build requirements, or pre-install it and disable isolation.

python -m pip install --upgrade pip setuptools wheel "cython>=3"
pip install --no-build-isolation <package>

Prefer a wheel or a newer release

A prebuilt wheel skips Cython entirely; a newer package release usually supports your Python.

pip install --only-binary :all: <package>
# or pin a release with wheels for your Python
pip install "<package>>=<version-with-wheels>"

How to prevent it

• Declare cython in [build-system] requires for packages that need it.
• Prefer wheels and recent releases in CI.
• Pin Python to a version the package’s Cython sources support.

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