cython

The Cython compiler for writing C extensions in the Python language.

Description

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/

3.2.4 (2026-01-04)
==================

Features added
--------------

* In preparation of Cython 3.3, a new decorator ``@collection_type(tname)`` can be used
  to advertise an extension type as being a ``'sequence'`` or ``'mapping'``.  This currently
  only has the effect of setting the ``Py_TPFLAGS_SEQUENCE`` flag on the type or not, but
  is provided for convenience to allow using the new decorator already in Cython 3.2 code.

* Several C++ exception declarations were added to ``libcpp.exceptions``.
  (Github issue https://github.com/cython/cython/issues/7389)

Bugs fixed
----------

* Pseudo-literal default values of function arguments like ``arg=str()`` could generate
  invalid C code when internally converted into a real literal.
  (Github issue https://github.com/cython/cython/issues/6192)

* The pickle serialisation of extension types using the ``auto_pickle`` feature was
  larger than necessary since 3.2.0 for types without Python object attributes.
  It is now back to the state before 3.2.0 again.
  (Github issue https://github.com/cython/cython/issues/7443)

* Constants are now only made immortal on freethreading Python if they are not shared.
  (Github issue https://github.com/cython/cython/issues/7439)

* ``PyDict_SetDefaultRef()`` is now used when available to avoid temporary borrowed references.
  (Github issue https://github.com/cython/cython/issues/7347)

* Includes all fixes as of Cython 3.1.8.

3.1.8 (2026-01-03):

* Assignment expressions used in comprehensions could look at the wrong scope,
  thus using different variables and different data.
  (Github issue https://github.com/cython/cython/issues/6547)

* Some internal C symbols were not declared as ``static``, preventing static linking
  of multiple modules.
  Patch by Yury Popov.  (Github issue https://github.com/cython/cython/issues/7310)

* Accidentally using ``except +`` in C mode did not raise a compile error but generated
  invalid C code leading to obscure error messages.
  Patch by user202729.  (Github issue https://github.com/cython/cython/issues/6560)

3.1.7 (2025-11-12):

* Unicode characters formatted from C integers with padding, as in ``f"{value:XXc}"``,
  could result in invalid Python string objects since Cython 3.1.0.
  Also, lone surrogates failed to format in this way.
  (Github issue https://github.com/cython/cython/issues/7298)

* Assigning nested structs from a list of structs (item by item) could crash Cython.
  (Github issue https://github.com/cython/cython/issues/7308)

* Cython incorrectly called ``PyList_GetItemRef()`` in PyPy and GraalPython before Py3.13.
  (Github issue https://github.com/cython/cython/issues/7269)

* Trying to instantiate internal types used by Cython is now prohibited.
  (Github issue https://github.com/cython/cython/issues/7263)

3.1.6 (2025-10-23):

* Unicode characters formatted from C integers with ``f"{value:c}"`` could result in
  invalid Python string objects since Cython 3.1.0.
  (Github issue https://github.com/cython/cython/issues/7240)

* ``cythonize`` (program and function) now uses ``concurrent.futures.ProcessPoolExecutor``
  instead of ``multiprocessing.Pool`` to fix a hang on build failures in parallel builds.
  A possible work-around is to disable parallel builds.
  Patch by Sviatoslav Sydorenko.  (Github issue https://github.com/cython/cython/issues/7183)

3.1.5 (2025-10-20):

* Conversion from C++ strings longer than ``PY_SSIZE_T_MAX`` did not validate the length.

* Some non-Limited API code was incorrectly used in generated header files.
  (Github issue https://github.com/cython/cython/issues/7157)

* Optimised unpacking of Python integers in expressions uses a slightly safer scheme.
  (Github issue https://github.com/cython/cython/issues/7134)

* Empty return statements were not always reported when tracing.
  (Github issue https://github.com/cython/cython/issues/7022)

* Value conversion errors when tracing C return statements no longer fail the trace
  but fall back to reporting ``None`` returns instead.
  (Github issue https://github.com/cython/cython/issues/6503)