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/

3.2.0a0 (2025-??-??)

Features added

• Builtin exception types are now inferred. (Github issue https://github.com/cython/cython/issues/6908)

• The list of known, inferred and optimised Python builtins was updated. range is now considered a type. ascii, bin, format, hex, oct were added as functions. (Github issue https://github.com/cython/cython/issues/6931)

• Type checks on PEP-604 union types (int | None) are optimised into separate checks. (Github issue https://github.com/cython/cython/issues/6935)

• Item type inference was improved for looping over literals. (Github issue https://github.com/cython/cython/issues/6912)

• Looping over literal sequences and strings now uses efficient C array looping if possible. (Github issue https://github.com/cython/cython/issues/6926)

• Using NaN as exception return value is supported. (Github issues https://github.com/cython/cython/issues/6900, https://github.com/cython/cython/issues/6903)

• Declarations for C++ condition variables were added. (Github issue https://github.com/cython/cython/issues/6836)

Bugs fixed

• Special float exception values could generate invalid C code. (Github issues https://github.com/cython/cython/issues/6900, https://github.com/cython/cython/issues/6903)

• References to the Python bool type could generate invalid C code. (Github issue https://github.com/cython/cython/issues/6902)

• Pure mode type alias like p_int or ulong leaked into some C type contexts. (Github issues https://github.com/cython/cython/issues/6922, https://github.com/cython/cython/issues/6339)

• Vectorcalls could use needless temp assignments for self. (Github issue https://github.com/cython/cython/issues/6909)

• Includes all fixes as of Cython 3.1.2.

3.1.2 (2025-06-09):

• 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)