Improve Python install path detection

[roehling]

[ This is the companion PR to colcon/colcon-core/pull/601 ]

This PR makes the customized Python path scheme logic in ament_cmake_python contingent on an install prefix other than /usr.

This makes ament more distribution-friendly and prevents the accidental overwriting of system packages in case someone decides to install their Python packages to /usr. There is no other change in behavior.

[sloretz]

This makes ament more distribution-friendly and prevents the accidental overwriting of system packages in case someone decides to install their Python packages to /usr

Would you mind giving an example of when this is a problem? I'm not sure when a user would choose to install packages to /usr.

Colcon installs into its own install space which by default is a folder called install at the root of the workspace, and when building debian packages on the ROS buildfarm it's set to a path in /opt/ros per ROS distro (example). If they were to use cmake install the default CMAKE_INSTALL_PREFIX seems to be /usr/local.

I'm hesitant to merge this without a careful review because the logic here is working around issues we've had with patches to setuptools that differ for debian, fedora, and homebrew.

• Pip version of colcon unable to build ROS2 on Jammy colcon/colcon-core#503
• Use posix_prefix scheme to avoid 'local' in path #386
• Use deb_system scheme when available to avoid 'local' in path colcon/colcon-core#504
• Error when using --symlink-install - error: option --install-layout not recognized colcon/colcon-core#506
• Avoid deb_system and --install-layout deb colcon/colcon-core#507
• Add workaround for Fedora's rpm_prefix distribution patch colcon/colcon-core#510
• Use sitecustomize to emulate venv during python install colcon/colcon-core#512
• Add sysconfig scheme workaround for osx_framework_library colcon/colcon-core#514

[roehling]

The main motivation for the change is to make ament more distribution-friendly by not overriding the Python schema for distribution-owned locations. As this affects the /usr prefix only, everything else remains exactly as before, including /opt/ros and $HOME workspaces.

[cottsay]

It doesn't feel very good to have a hard-coded check for /usr specifically, especially if your goal is to be more friendly to diverse distribution configurations.

What if we skip the scheme override when we're installing to the same prefix that Python suggests system packages install to? In other words, rather than checking if CMAKE_INSTALL_PREFIX is /usr, we check if CMAKE_INSTALL_PREFIX is the same as sysconfig.get_config_var('base')?

[roehling]

What if we skip the scheme override when we're installing to the same prefix that Python suggests system packages install to? In other words, rather than checking if CMAKE_INSTALL_PREFIX is /usr, we check if CMAKE_INSTALL_PREFIX is the same as sysconfig.get_config_var('base')?

I like that idea, as it will also do the right thing for virtual environments. I'll update the PR(s) as soon as I find time.