pypi / cython

cython

What changed in cython 3, from its official release notes, upgrade guides and advisories. Sources checked 2026-11-13.

2 breaking1 security6 new APIs and options5 performance11 behavior and defaults1 tooling12 fixes and other

See which of these apply to your code, ranked, in your agent:

npx @unstale/cli init

cython 338 changes

Unpickling object pickle data with instance dict state into an object that has no __dict__ is now an error.
3.2.0release notes
Instantiating internal types used by Cython is now prohibited.
3.2.0release notes

Expressions use a slightly safer scheme when unpacking Python integers.
3.1.5release notes

Declarations for C++ condition variables were added.
3.2.0release notes
The new --embed-modules option lists extension modules to statically link into the generated extension module so they are initialised on application start.
3.2.0release notes
The C macro configures whether some object constants are marked immortal to speed reference counting in recent CPython versions.
3.2.0release notes
Some GIL-safe C-API wrappers were added to the libc and libcpp declarations.
3.2.0release notes
Cython's setup.py now accepts the desired Limited API version through --cython-limited-api.
3.2.0release notes
T-strings are implemented according to PEP-750, backporting the template classes while preferring separately installed backports.
3.2.0release notes

bool(c_int), bool(c_float) and bool(c_ptr) avoid passing through Python objects.
3.2.0release notes
Runtime Python dispatch for fused functions is substantially faster.
3.2.0release notes
Looping over literal sequences and strings now uses efficient C array looping when possible.
3.2.0release notes
Imports quickly check whether a module is already imported, speeding up reimports.
3.2.0release notes
Type checks on union types such as int | None are optimised into separate checks.
3.2.0release notes

Some lesser-used platforms now use Python 3.9 abi3 wheels instead of Python-version-specific wheels; an abi3 wheel was also added for armv7l.
3.2.0release notes
Builtin exception types are now inferred.
3.2.0release notes
Mixed operations use less code and are more streamlined for Python 3.12 and later.
3.2.0release notes
F-string syntax was extended according to PEP-701.
3.2.0release notes
Assignments accept any suitable Python number and automatically convert it to a Python float.
3.2.0release notes
Freelists are now used with the Limited API and extension type specs, including freelists declared with a cdef class decorator and internally used types.
3.2.0release notes
NaN is now supported as an exception return value.
3.2.0release notes
Variables assigned inside prange loops can now be initialised outside the loop.
3.2.0release notes
The known, inferred and optimised builtin list changed: range is now considered a type, and ascii, bin, format, hex and oct are added as functions.
3.2.0release notes
Unknown return type annotations using -> now produce warnings instead of being rejected; -> None is also allowed.
3.2.0release notes
Unicode 16.0.0 is now used to parse identifiers.
3.2.0release notes

Cython tools and frontend scripts now use python3 instead of python in their shebang line.
3.2.0release notes

Fixed cases where indexing bytes failed to optimise.
3.2.0release notes
cython.critical_section now works with PyMutex, as it already did with Python objects.
3.2.0release notes
Extension arguments defined for the shared code module are no longer ignored.
3.2.0release notes
Fixed embedsignatures failing when lambda was used in function default arguments.
3.2.0release notes
Several improvements were made in freethreaded Python code.
3.2.0release notes
Several improvements were made for the Limited API.
3.2.0release notes
Fixed a crash when iterating over temporary memoryviews.
3.2.0release notes
Fixed an incorrect call to PyList_GetItemRef in PyPy and GraalPython before Python 3.13.
3.2.0release notes
Value conversion errors no longer fail tracing C return statements; tracing falls back to reporting None returns.
3.1.5release notes
Conversion now validates the length of C++ strings longer than PY_SSIZE_T_MAX.
3.1.5release notes
Generated header files no longer use some non-Limited API code.
3.1.5release notes
Empty return statements are now reported consistently when tracing.
3.1.5release notes