snap adds hidden --user flag when running pip from within FC

[luzpaz]

Gist

So there's something unusual about the copy of Python that's executing inside the FreeCAD Snap -- it doesn't operate the same way as the normal system Python. The results of the earlier testing make it seem like it has an addition hidden "--user" parameter, or something along those lines.

Background

Trying to install the new BlenderBIM workbench I ran in to some problems that @chennes has been helping me troubleshoot. For some reason the snap doesn't like installing the ifcopenshell 3rd party dependency. We opened a ticket for it at FreeCAD/FreeCAD#12967 but most of the troubleshooting happened in our DMs

At first FreeCAD would just hang: https://forum.freecadweb.org/viewtopic.php?p=635481#p635481
Chennes provided a fix to the addon manager: FreeCAD/FreeCAD#7634
Results discussion thread

Then we tried on the FC console: discussion thread
We then moved to DMs eventually narrowing it down to the code block below.
Important note: I was able to install via pip in my OS's CLI

$ python -m pip install --target /home/beast/snap/freecad/common/AdditionalPythonPackages ifcopenshell
Collecting ifcopenshell
  Downloading ifcopenshell-0.7.0-221014-py310-none-manylinux1_x86_64.whl (40.9 MB)
     ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 40.9/40.9 MB 2.2 MB/s eta 0:00:00
Installing collected packages: ifcopenshell
Successfully installed ifcopenshell-0.7.0

About info

OS: Ubuntu Core 20 (KDE/plasma)
Word size of FreeCAD: 64-bit
Version: 0.21.30769 (Git) Snap 396
Build type: Release
Branch: master
Hash: 1282fdb7e8a721e9b411358d68daf8921b0bf50a
Python 3.8.10, Qt 5.15.5, Coin 4.0.0, Vtk 7.1.1, OCC 7.6.3
Locale: English/United States (en_US)
Installed mods: 
  * TabBar
  * ExtremeProDark 2.3.3
  * FreeCAD-BlenderBIM 2022.10.20

Reproduce

• Start FreeCAD snap from the CLI
• In the FreeCAD UI code console paste the following:

import subprocess
import addonmanager_utilities as utils
python_exe = utils.get_python_exe()
args = [python_exe, '-m', 'pip', 'install', '--disable-pip-version-check', '--target', '/home/beast/snap/freecad/common/AdditionalPythonPackages', 'ifcopenshell']
try:
    proc = subprocess.run(args, check=True)
except Exception as e:
    print(e)

Result: This will error out but...

• Look at the CLI output
  Result: ERROR: Can not combine '--user' and '--target'

Notice how --user wasn't invoked at all in the args array