The Cython language makes writing C extensions for the Python language as
easy as Python itself. Cython is a source code translator based on Pyrex_,
but supports more cutting edge functionality and optimizations.

The Cython language is a superset of the Python language (almost all Python
code is also valid Cython code), but Cython additionally supports optional
static typing to natively call C functions, operate with C++ classes and
declare fast C types on variables and class attributes. This allows the
compiler to generate very efficient C code from Cython code.

This makes Cython the ideal language for writing glue code for external
C/C++ libraries, and for fast C modules that speed up the execution of
Python code.

The newest Cython release can always be downloaded from https://cython.org/.
Unpack the tarball or zip file, enter the directory, and then run::

pip install .

Note that for one-time builds, e.g. for CI/testing, on platforms that are not
covered by one of the wheel packages provided on PyPI and the pure Python wheel
that we provide is not used, it is substantially faster than a full source build
to install an uncompiled (slower) version of Cython with::

NO_CYTHON_COMPILE=true pip install .

.. _Pyrex: https://www.cosc.canterbury.ac.nz/greg.ewing/python/Pyrex/

3.1.2 (2025-06-09)

Bugs fixed

Attribute lookups failed on the bool builtin type.
(Github issue https://github.com/cython/cython/issues/6905)
Type checks on or-ed union types could incorrectly return false.
(Github issue https://github.com/cython/cython/issues/6420)
Negative list indexing could accidentally wrap around twice in PyPy and the Limited API.
Iterating over literal sequences with starred (unpacked) items could infer a wrong
type for the loop variable and fail to assign the values.
(Github issue https://github.com/cython/cython/issues/6924)
Calls to C functions taking exception types failed to check for a None argument.
(Github issue https://github.com/cython/cython/issues/6420)
Fused functions had an incorrect __module__ attribute.
(Github issue https://github.com/cython/cython/issues/6897)
The type of Cython implemented functions had an incorrect __module__ attribute.
(Github issue https://github.com/cython/cython/issues/6841)
Errors while indexing into bytearray or str in nogil sections could crash.
(Github issue https://github.com/cython/cython/issues/6947)
bytearray.append() could silently accept some invalid character numbers.
The C++11 <type_traits> header was included regardless of the C++ version.
(Github issue https://github.com/cython/cython/issues/6896)
PyDict_GetItemStringRef() was accidentally used in older Limited API versions.
(Github issue https://github.com/cython/cython/issues/6914)
abort() was used but not always available in the Limited API.
(Github issue https://github.com/cython/cython/issues/6918)
Some dependencies were missing from the depfile.
(Github issue https://github.com/cython/cython/issues/6938)
Embedded function signatures were not always separated from the existing docstring.
(Github issue https://github.com/cython/cython/issues/6904)
numpy.math was missing from Cython/Includes/ and could not be cimported.
(Github issue https://github.com/cython/cython/issues/6859)
Some tests were adapted for NumPy 2.x.
(Github issue https://github.com/cython/cython/issues/6898)
Some C compiler warnings were fixed.
(Github issue https://github.com/cython/cython/issues/6870)
Cython.Build was not officially exposing the cythonize function.
(Github issue https://github.com/cython/cython/issues/6934)

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