Support generating conda binaries for ROBOTOLOGY_USES_PYTHON option

[traversaro]

In the conda-forge based CI and in https://github.com/robotology/robotology-superbuild/blob/master/doc/conda-forge.md, the option ROBOTOLOGY_USES_PYTHON has not been tested until now, and this may be quickly bites us as while conda is now a language-agnostic package manager, it has its roots and it is mainly used as of 2021 by the Python community.

Support for the building from source workflow

This is tracked in #615, so this issue is now just about generating binaries for ROBOTOLOGY_USES_PYTHON .

[traversaro]

Related issues:

• Install the python package in python3 folder ami-iit/bipedal-locomotion-framework#217
• Installation path for Python bindings is not searched by the interpreter if CMAKE_INSTALL_PREFIX is /usr/local yarp#2511

[traversaro]

Support for generating conda binaries

For the generation of conda binary packages that is currently tracked in #620, some additional tweaks are necessary. In particular, while on macOS or Linux the usual installation process will work fine, on Windows the layout of the installed packages are a bit different. In particular on Windows given a <conda_env_prefix>, the MSVC-compatible C++ libraries are all installed in <conda_env_prefix>\Library\include, <conda_env_prefix>\Library\lib and <conda_env_prefix>\Library\bin, and this is typically obtained by setting CMAKE_INSTALL_PREFIX to <conda_env_prefix>\Library on Windows. However, the Python packages instead would need to be installed in <conda_env_prefix>\Lib\site-packages, so they should be installed outside of CMAKE_INSTALL_PREFIX . To do so, we need to instruct each superbuild package that installs Python extension of code to install it <conda_env_prefix>\Lib\site-packages.

The CMake options that can be used in this context are:

• Options that permits to specify an absolute path for the bindings installation directory (as CMAKE_INSTALL_PYTHON3DIR for YARP, see Installation path for Python bindings is not searched by the interpreter if CMAKE_INSTALL_PREFIX is /usr/local yarp#2511 (comment))
• Options that permits to install directly in Python3_SITELIB as returned by find_package(Python3), (as FRAMEWORK_DETECT_ACTIVE_PYTHON_SITEPACKAGES for bfl, see https://github.com/dic-iit/bipedal-locomotion-framework/blob/381ad4623ce709ad7966e75412b45814152cf009/bindings/CMakeLists.txt#L7 )

A quick check shows that the appropriate CMake options on Windows are:

• 🔄 YARP: -DCMAKE_INSTALL_PYTHON3DIR:PATH=%CONDA_PREFIX%/Lib/site-packages , once Installation path for Python bindings is not searched by the interpreter if CMAKE_INSTALL_PREFIX is /usr/local yarp#2511 (comment) is solved
• ❌ iDynTree: no option is present, it should be added, see Add a CMake option to explicitly control the installation path of Python bindings idyntree#834
• ✅ bipedal-locomotion-framework: the FRAMEWORK_DETECT_ACTIVE_PYTHON_SITEPACKAGES option can be used https://github.com/dic-iit/bipedal-locomotion-framework/blob/381ad4623ce709ad7966e75412b45814152cf009/bindings/CMakeLists.txt#L7

To insert them, we need also a mechanism to inject this CMake options only when a conda recipe is generated, and not when the the projects are compiled via the robotology-superbuild .

[traversaro]

As @singhbal-baljinder is experimenting with Python-based learning library and for several of them the installation via conda is the easiest option, it is a good idea to fix this.

[traversaro]

❌ iDynTree: no option is present, it should be added, see robotology/idyntree#834

Will be fixed by robotology/idyntree#852 .

[traversaro]

Another aspect to handle is that, differently from most other C++ dependencies, Python is supported in several versions in conda-forge (at this time, if I am not wrong 3.6-3.9) so we would need to add some kind of variant (https://docs.conda.io/projects/conda-build/en/latest/resources/variants.html) support in our generate recipes. Probably a possible strategy to try is just to rely on the existing conda-forge pinning file (see https://github.com/conda-forge/conda-forge-pinning-feedstock) that can be just installed as conda install -c conda-forge conda-forge-pinning and then passed as additional conda_build_config file in https://github.com/robotology/robotology-superbuild/blob/master/.github/workflows/generate-conda-packages.yaml#L107 .