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

Start with the cppyy-cling package (cppyy-backend repo, subdirectory “cling”), which requires source to be pulled in from upstream, and thus takes a few extra steps:

$ git clone
$ cd cppyy-backend/cling
$ python egg_info
$ python
$ python -m pip install . --upgrade

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 using pip.

The cppyy-cling package, because it contains Cling/Clang/LLVM, is rather large to build, so by default the setup script will use all cores (x2 if hyperthreading is enabled). You can change this behavior with the MAKE_NPROCS envar. The wheel of cppyy-cling is reused by pip for all versions of CPython and PyPy, thus the long compilation is needed only once for all different versions of Python on the same machine.

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

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.