The original Python plugin is incompatible with usrmerged systems because the virtual environment directory structure that it needs is very hardcoded. This new plugin does not use virtual environments; instead, during the build the plugin uses the part's install dir as the base for "pip install --user" calls. A sitecustomize file is provided so that at runtime the interpreter can pick up the packages installed by pip. Since this sitecustomize has to be discoverable, this means that the plugin assumes that the interpreter will be included in the primed payload (which is true for Rockcraft, and will be true for Snapcraft starting with core26). This new plugin is not registered by default; applications wanting to use it, typically in conjunction with usrmerged install dirs, need to explicitly register it *over* the default Python plugin.