Migrating from Cython 0.29 to 3.0¶
Cython 3.0 is a major revision of the compiler and the language that comes with some backwards incompatible changes. This document lists the important ones and explains how to deal with them in existing code.
Python 3 syntax/semantics¶
Cython 3.0 now uses Python 3 syntax and semantics by default, which previously
required setting the
language_level directive <compiler-directives> to
The new default setting is now
language_level=3str, which means Python 3
semantics, but unprefixed strings are
str objects, i.e. unicode text strings
under Python 3 and byte strings under Python 2.7.
You can revert your code to the previous (Python 2.x) semantics by setting
Further semantic changes due to the language level include:
/-division uses the true (float) division operator, unless
Python classes that are defined without bases (
class C: ...) are “new-style” classes also in Py2.x (if you never heard about “old-style classes”, you’re probably happy without them).
Annotations (type hints) are now stored as strings. (PEP 563)
StopIterationhandling in generators has been changed according to PEP 479.
Some Python compatibility bugs were fixed, e.g.
The binding directive is now enabled by default.
This makes Cython compiled Python (
def) functions mostly compatible
with normal (non-compiled) Python functions, regarding signature introspection,
It also makes them bind as methods in Python classes on attribute assignments,
thus the name.
If this is not intended, i.e. if a function is really meant to be a function
and never a method, you can disable the binding (and all other Python function
features) by setting
binding=False or selectively adding a decorator
In pure Python mode, the decorator was not available in Cython 0.29.16 yet,
but compiled code does not suffer from this.
We recommend, however, to keep the new function features and instead deal
with the binding issue using the standard Python
def func(self, b): ... class MyClass(object): binding_method = func no_method = staticmethod(func)
Cython now has support for loading pxd files also from namespace packages according to PEP-420. This might have an impact on the import path.
Cython used to generate code that depended on the deprecated pre-NumPy-1.7 C-API. This is no longer the case with Cython 3.0.
You can now define the macro
to get rid of the long-standing build warnings that the compiled C module
uses a deprecated API. Either per file:
# distutils: define_macros=NPY_NO_DEPRECATED_API=NPY_1_7_API_VERSION
or by setting it in your Extensions in
Extension(... define_macros=[("NPY_NO_DEPRECATED_API", "NPY_1_7_API_VERSION")] )
One side-effect of the different C-API usage is that your code may now require a call to the NumPy C-API initialisation function where it previously got away without doing so.
In order to reduce the user impact here, Cython 3.0 will now call it
automatically when it sees
numpy being cimported, but the function
not being used.
In the (hopefully rare) cases where this gets in the way, the internal
C-API initialisation can be disabled by faking the use of the function
without actually calling it, e.g.
# Explicitly disable the automatic initialisation of NumPy's C-API. <void>import_array
Class-private name mangling¶
Cython has been updated to follow the Python rules for class-private names
more closely. Essentially any name that starts with and doesn’t end with
__ within a class is mangled with the class name. Most user code
should be unaffected – unlike in Python unmangled global names will
still be matched to ensure it is possible to access C names
cdef extern void __foo() class C: # or "cdef class" def call_foo(self): return __foo() # still calls the global name
What will no-longer work is overriding methods starting with
cdef class Base: cdef __bar(self): return 1 def call_bar(self): return self.__bar() cdef class Derived(Base): cdef __bar(self): return 2
Base.__bar is mangled to
call_bar will always call
_Base__bar. This matches established Python behaviour and applies
cpdef methods and attributes.
Arithmetic special methods¶
The behaviour of arithmetic special methods (for example
__pow__) of cdef classes has changed in Cython 3.0. They now
support separate “reversed” versions of these methods (e.g.
__rpow__) that behave like in pure Python.
The main incompatible change is that the type of the first operand
__self__) is now assumed to be that of the defining class,
rather than relying on the user to test and cast the type of each operand.