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/
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)
The f-string syntax was extended according to PEP-701. (Github issue https://github.com/cython/cython/issues/5452)
The runtime Python dispatch for fused functions is substantially faster. (Github issues https://github.com/cython/cython/issues/1385, https://github.com/cython/cython/issues/6996)
Freelists (via cdef class decorator and for internally used types such as async)
are now also used in the Limited API and with extension type specs enabled.
(Github issue https://github.com/cython/cython/issues/7151)
Module imports now quickly check for an already imported module to speed up reimports. Patch by Lysandros Nikolaou. (Github issue https://github.com/cython/cython/issues/7035)
Type checks on PEP-604 union types (int | None) are optimised into separate checks.
(Github issue https://github.com/cython/cython/issues/6935)
Assignments to the PEP-604 union type float | None allow any suitable Python number as input
and convert it to a Python float automatically.
(Github issue https://github.com/cython/cython/issues/5750)
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)
Releasing the GIL from an unknown lock state is more efficient. (Github issue https://github.com/cython/cython/issues/6847)
cython.critical_section(PyMutex) now works, as with Python objects.
(Github issue https://github.com/cython/cython/issues/6847)
bool(c_int/float/ptr) avoid passing through Python objects.
(Github issue https://github.com/cython/cython/issues/7015)
Unused exceptions in except clauses are detected in some more cases to avoid their normalisation.
(Github issue https://github.com/cython/cython/issues/7021)
Some object constants are now marked as immortal to speed up their reference counting
in recent CPython versions. This can be configured with the CYTHON_IMMORTAL_CONSTANTS C macro.
(Github issue https://github.com/cython/cython/issues/7118)
Several improvements were made in freethreaded Python code. (Github issues https://github.com/cython/cython/issues/6936, https://github.com/cython/cython/issues/6939, https://github.com/cython/cython/issues/6949, https://github.com/cython/cython/issues/6984, https://github.com/cython/cython/issues/7011, https://github.com/cython/cython/issues/7114)
Several improvements were made for the Limited API. (Github issues https://github.com/cython/cython/issues/6959, https://github.com/cython/cython/issues/6991)
Some GIL-safe C-API wrappers were added to the libc/libcpp declarations. (Github issue https://github.com/cython/cython/issues/6829)
String and number constants use less storage space in the module. (Github issues https://github.com/cython/cython/issues/6971, https://github.com/cython/cython/issues/6978, https://github.com/cython/cython/issues/6986)
The managed dict/weakref support in CPython 3.12+ is used for internal types to benefit from future CPython optimisations here. (Github issue https://github.com/cython/cython/issues/6891)
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)
Several improvements were made for the GraalPython support. Patch by Michael Šimáček. (Github issue https://github.com/cython/cython/issues/7074)
The annotated source HTML page shows alternating +/− markers to open/close lines. Patch by Kamil Monicz. (Github issue https://github.com/cython/cython/issues/7099)
Unicode 16.0.0 is used to parse identifiers. (Github issue https://github.com/cython/cython/issues/6836)
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)
Vectorcalls could use needless temp assignments for self.
(Github issue https://github.com/cython/cython/issues/6909)
Iterating over temporary memoryviews could crash Cython. (Github issue https://github.com/cython/cython/issues/7005)
Type casts on constants as part of numeric expressions could crash Cython. (Github issue https://github.com/cython/cython/issues/6779)
Long code bodies in with statements could crash Cython.
(Github issue https://github.com/cython/cython/issues/7122)
Indexing bytes failed to optimise in some cases.
(Github issue https://github.com/cython/cython/issues/6997)
The type objects of heap types were not always correctly decrefed on deallocation. (Github issue https://github.com/cython/cython/issues/7145)
Pure mode type aliases like p_int or ulong leaked into some C type contexts,
risking to conflict with user declarations.
(Github issues https://github.com/cython/cython/issues/6922, https://github.com/cython/cython/issues/6339)
Boolean (emptyness) tests on builtin containers could fail to handle (unlikely) errors. (Github issue https://github.com/cython/cython/issues/7090)
The return type of bytes.join() was sometimes referred as plain object.
(Github issue https://github.com/cython/cython/issues/6987)
Type inference could fail to understand prange() arguments.
(Github issue https://github.com/cython/cython/issues/6974)
embedsignatures failed if lambda was used in function default arguments.
(Github issue https://github.com/cython/cython/issues/6880)
An internal C function was not marked as static and leaked a linker symbol.
(Github issue https://github.com/cython/cython/issues/6957)
Cython's tools and frontend scripts now use python3 instead of just python
in their shebang line.
Patch by Matti Picus. (Github issue https://github.com/cython/cython/issues/7053)
Includes all fixes as of Cython 3.1.4.
3.1.4 (2025-09-16):
The monitoring code could crash on tracing. (Github issue https://github.com/cython/cython/issues/7050)
Initialising the monitoring code could fail with a CPython exception. See https://github.com/nedbat/coveragepy/issues/1790#issuecomment-3257410149
Optimised integer shifting triggered undefined behaviour in C. (Github issue https://github.com/cython/cython/issues/7089)
Deallocating objects that inherit from external types defined in pxd files could run into an infinite loop. (Github issue https://github.com/cython/cython/issues/7143)
A reference to metaclasses could be leaked on instantiation. (Github issue https://github.com/cython/cython/issues/7130)
(Unlikely) error handling during empty builtin container tests was ineffective. (Github issue https://github.com/cython/cython/issues/7190)
Generated *_api.h files used potentially unknown Cython configuration macros.
(Github issue https://github.com/cython/cython/issues/7108)
cythonize() avoids parallel compiler runs on systems using spawn() in multiprocessing.
Patch by Marcel Bargull. (Github issue https://github.com/cython/cython/issues/3262)
The @cython.ufunc decorator was missing in type checker stubs.
Patch by jayClean. (Github issue https://github.com/cython/cython/issues/7109)