User-installed Python modules do not have precedence over those pre-installed in the snap #167

The snap comes with a series of Python modules preinstalled, which on the host system are installed at:

/snap/freecad/current/usr/lib/python3.10/dist-packages/$MODULE

The user can install new Python modules using the provided freecad.pip command. Those modules are then installed at:

$HOME/snap/freecad/common/.local/lib/python3.10/site-packages/$MODULE

Sometimes it is useful for one of the preinstalled Python modules (e.g. ifcopenshell) to be upgraded. Be it to fix a bug, or to use a new feature. This works in principle. E.g.

freecad.pip install ifcopenshell

As expected, we then end up with:

/snap/freecad/current/usr/lib/python3.10/dist-packages/ifcopenshell         # The version we just installed: 0.8.1
$HOME/snap/freecad/common/.local/lib/python3.10/site-packages/ifcopenshell  # The preinstalled version: 0.8.0

If we then import ifcopenshell from the Python console within FreeCAD:

>>> import ifcopenshell
>>> ifcopenshell.__version__
'0.8.0'

Notice how FreeCAD's Python only saw the preinstalled version. When the snap is launched, a message seems to indicate that the user path is searched for before the system path:

Adding snap-specific PYTHONPATH to sys.path: $HOME/snap/freecad/common/.local/lib/python3.10/site-packages:/snap/freecad/1296/lib/python3.10/site-packages:/snap/freecad/1296/usr/lib/python3/dist-packages

However, that does not seem to be the case.

Running:

>>> import sys
>>> print(sys.path)

Returns PYTHONPATH, of which here's an edited extract:

/snap/freecad/1296/usr/lib/python3/dist-packages
/snap/freecad/1296/lib/python3.10/site-packages
$HOME/freecad/common/.local/lib/python3.10/site-packages
$HOME/freecad/common/.local/lib/python3.10/site-packages  # <= Injected by $SNAP_PYTHONPATH?
/snap/freecad/1296/usr/local/lib/python3.10/dist-packages # <= Injected by $SNAP_PYTHONPATH?
/snap/freecad/1296/usr/lib/python3/dist-packages          # <= Injected by $SNAP_PYTHONPATH?
$HOME/freecad/common/AdditionalPythonPackages/py310
$HOME/freecad/common/AdditionalPythonPackages
$HOME/freecad/common/
$HOME/freecad/common
$/snap/freecad/1296/lib/python3.10/site-packages/ifcopenshell/lib/linux/64bit/python3.10

Notice how /snap/freecad/1296/usr/lib/python3/dist-packages is listed first, instead of $HOME/freecad/common/.local/lib/python3.10/site-packages

Running this snippet from FreeCAD's Python console:

>>> import os
>>> pythonpath = os.environ.get("SNAP_PYTHONPATH")
>>> pythonpath
'$HOME/snap/freecad/common/.local/lib/python3.10/site-packages:/snap/freecad/1296/lib/python3.10/site-packages:/snap/freecad/1296/usr/lib/python3/dist-packages'

That list of paths seems to be injected somewhere in the middle of the PYTHONPATH. Since it's inserted after /snap/freecad/1296/usr/lib/python3/dist-packages, it has no effect. Also note that SNAP_PYTHONPATH is a custom environment variable that we define in snapcraft.yaml:

In summary, Python packages are forever pinned to the version preinstalled with the snap.