The cppyy module is a frontend that requires an intermediate (Python interpreter dependent) layer, and a backend (see Package Structure). Because of this layering and because it leverages several existing packages through reuse, the relevant codes are contained across a number of repositories.

The backend repo contains both the cppyy-cling (under “cling”) and cppyy-backend (under “clingwrapper”) packages.

Building from source

Except for cppyy-cling, the structure in the repositories follows a normal PyPA package and they are thus ready to build with setuptools: simply clone the package and either run python, or use pip.

It is highly recommended to follow the dependency chain when manually upgrading packages individually (i.e. cppyy-cling, cppyy-backend, CPyCppyy if on CPython, and then finally cppyy), because upstream packages expose headers that are used by the ones downstream. Of course, if only building for a patch/point release, there is no need to re-install the full chain (or follow the order). Always run the local updates from the package directories (i.e. where the file is located), as some tools rely on the package structure.

The STDCXX envar can be used to control the C++ standard version; use MAKE to change the make command; and MAKE_NPROCS to control the maximum number of parallel jobs. Compilation of the backend, which contains a customized version of Clang/LLVM, can take a long time, so by default the setup script will use all cores (x2 if hyperthreading is enabled).

On MS Windows, some temporary path names may be too long, causing the build to fail. To resolve this issue, point the TMP and TEMP envars to an existing directory with a short name before the build: For example:

> set TMP=C:\TMP
> set TEMP=C:\TMP

The first package to build is cppyy-cling. It may take a long time, especially on a laptop (Mac ARM being a notable exception), since Cling comes with a builtin version of LLVM/Clang. Consider therefore for a moment your reasons for building from source: there being no pre-built wheel for the platform that you’re interested in or simply needing the latest version from the repository; or perhaps you are planning to develop/modify the sources.

If the former, clone the repository, check out a specific tagged release as needed, then run the following steps to add Cling and build a wheel. Once built, install the wheel as appropriate:

$ git clone
$ cd cppyy-backend/cling
$ python egg_info
$ python
$ python bdist_wheel
$ python -m pip install dist/cppyy_cling-* --upgrade


cppyy-cling wheels do not depend on the Python interpreter and can thus be re-used for any version of Python or PyPy.

The egg_info setup command is needed for to find the right version. That script in turn downloads the proper release from upstream, trims and patches it, and installs the result in the “src” directory. When done, the structure of cppyy-cling looks again like a PyPA package and can be used/installed as expected, here done with pip.

By default, the setup script will use all cores (x2 if hyperthreading is enabled). You can change this behavior by setting the MAKE_NPROCS envar to the desired number of allowable sub jobs.

If on the other hand you are building from source to develop/modify cppyy-cling, consider using the cmake interface. The first installation will still be just as slow, but subsequent builds can be incremental and thus much faster. For this use, first install the latest version from a pre-built wheel, which will setup the proper directory structure, then use cmake to build and install the latest or modified version of cppyy-cling into that:

$ python -m pip install cppyy-cling
$ git clone
$ cd cppyy-backend/cling
$ python egg_info
$ python
$ mkdir dev
$ cd dev
$ cmake ../src -Wno-dev -DCMAKE_CXX_STANDARD=17 -DLLVM_ENABLE_EH=0 -DLLVM_ENABLE_RTTI=0 -DLLVM_ENABLE_TERMINFO=0 -DLLVM_ENABLE_ASSERTIONS=0 -Dminimal=ON -Druntime_cxxmodules=OFF -Dbuiltin_zlib=ON -Dbuiltin_cling=ON -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_INSTALL_PREFIX=<path to environment python site-packages>
$ make -j <N> install

where the cmake command needs to be given the full path to site-packages/cppyy_backend in the virtual environment or other installation location. Adjust other options (esp. CMAKE_CXX_STANDARD) as needed. For the build command, adjust the cmake command as appropriate for your favorite, or platform-specific, build system and/or use cmake --build instead of make directly. See the cmake documentation for details.

Next up is cppyy-backend (cppyy-backend, subdirectory “clingwrapper”; omit the first step if you already cloned the repo for cppyy-cling):

$ git clone
$ cd cppyy-backend/clingwrapper
$ python -m pip install . --upgrade --no-use-pep517 --no-deps

Note the use of --no-use-pep517, which prevents pip from needlessly going out to and creating a local “clean” build environment from the cached or remote wheels. Instead, by skipping PEP 517, the local installation will be used. This is imperative if there was a change in public headers or if the version of cppyy-cling was locally updated and is thus not available on PyPI.

Upgrading CPyCppyy (if on CPython; it’s not needed for PyPy) and cppyy is very similar:

$ git clone
$ cd CPyCppyy
$ python -m pip install . --upgrade --no-use-pep517 --no-deps

Just like cppyy-cling, CPyCppyy has cmake scripts which are the recommended way for development, as incremental builds are faster:

$ mkdir build
$ cmake ../CPyCppyy
$ make -j <N>

then simply point the PYTHONPATH envar to the build directory above to pick up the local module.

Finally, the top-level package cppyy:

$ git clone
$ cd cppyy
$ python -m pip install . --upgrade --no-deps

Please see the pip documentation for more options, such as developer mode.