Writing extension modules for pypy

This document tries to explain how to interface the PyPy python interpreter with any external library.

Right now, there are the following possibilities of providing third-party modules for the PyPy python interpreter (in order of usefulness):


CFFI is the recommended way. It is a way to write pure Python code that accesses C libraries. The idea is to support either ABI- or API-level access to C — so that you can sanely access C libraries without depending on details like the exact field order in the C structures or the numerical value of all the constants. It works on both CPython (as a separate pip install cffi) and on PyPy, where it is included by default.

PyPy’s JIT does a quite reasonable job on the Python code that call C functions or manipulate C pointers with CFFI. (As of PyPy 2.2.1, it could still be improved, but is already good.)

See the documentation here.


The goal of the ctypes module of PyPy is to be as compatible as possible with the CPython ctypes version. It works for large examples, such as pyglet. PyPy’s implementation is not strictly 100% compatible with CPython, but close enough for most cases.

We also used to provide ctypes-configure for some API-level access. This is now viewed as a precursor of CFFI, which you should use instead. More (but older) information is available here. Also, ctypes’ performance is not as good as CFFI’s.

PyPy implements ctypes as pure Python code around two built-in modules called _ffi and _rawffi, which give a very low-level binding to the C library libffi. Nowadays it is not recommended to use directly these two modules.


This method is still experimental. It adds the cppyy module. The method works by using the Reflex package to provide reflection information of the C++ code, which is then used to automatically generate bindings at runtime. From a python standpoint, there is no difference between generating bindings at runtime, or having them “statically” generated and available in scripts or compiled into extension modules: python classes and functions are always runtime structures, created when a script or module loads. However, if the backend itself is capable of dynamic behavior, it is a much better functional match to python, allowing tighter integration and more natural language mappings. Full details are available here.


The cppyy module is written in RPython, which makes it possible to keep the code execution visible to the JIT all the way to the actual point of call into C++, thus allowing for a very fast interface. Reflex is currently in use in large software environments in High Energy Physics (HEP), across many different projects and packages, and its use can be virtually completely automated in a production environment. One of its uses in HEP is in providing language bindings for CPython. Thus, it is possible to use Reflex to have bound code work on both CPython and on PyPy. In the medium-term, Reflex will be replaced by cling, which is based on llvm. This will affect the backend only; the python-side interface is expected to remain the same, except that cling adds a lot of dynamic behavior to C++, enabling further language integration.


C++ is a large language, and cppyy is not yet feature-complete. Still, the experience gained in developing the equivalent bindings for CPython means that adding missing features is a simple matter of engineering, not a question of research. The module is written so that currently missing features should do no harm if you don’t use them, if you do need a particular feature, it may be necessary to work around it in python or with a C++ helper function. Although Reflex works on various platforms, the bindings with PyPy have only been tested on Linux.

RPython Mixed Modules

This is the internal way to write built-in extension modules in PyPy. It cannot be used by any 3rd-party module: the extension modules are built-in, not independently loadable DLLs.

This is reserved for special cases: it gives direct access to e.g. the details of the JIT, allowing us to tweak its interaction with user code. This is how the numpy module is being developed.