Install new Python modules from within Addon (without sudo/admin privileges)

Question

I am working under Blender 2.90 installed by snap.

If I try to run

subprocess.check_call([bpy.app.binary_path_python, "-m", "ensurepip"])

or

import ensurepip
ensurepip.bootstrap()

from within my Addon, I get told, that the site-packages of my Blender's Python are read-only. Is there any way to circumvent this?

Answer 1

Thanks to all the help, I was able to piece together a fully automatic solution that works at least on my system (tests on other Linux and Windows machines planned). It looks something like this:

import bpy
import sys
import importlib
import subprocess
Blender's Python executable
pybin = sys.executable
def add_user_site():
    # Locate users site-packages (writable)
    user_site = subprocess.check_output([pybin, "-m", "site", "--user-site"])
    user_site = user_site.decode("utf8").rstrip("\n")   # Convert to string and remove line-break
    # Add user packages to sys.path (if it exits)
    user_site_exists = user_site is not None
    if user_site not in sys.path and user_site_exists:
        sys.path.append(user_site)
    return user_site_exists
def enable_pip():
    if importlib.util.find_spec("pip") is None:
        subprocess.check_call([pybin, "-m", "ensurepip", "--user"])
        subprocess.check_call([pybin, "-m", "pip", "install", "--upgrade", "pip", "--user"])
def install_module(module : str):
    if importlib.util.find_spec(module) is None:
        subprocess.check_call([pybin, "-m", "pip", "install", module, "--user"])
user_site_added = add_user_site()
enable_pip()
All the modules you need, that don't come shipped with Blender
modules = ["module1", "module2", ...] 
for module in modules:
    install_module(module)
If there was no user-site before...
if not user_site_added:
    add_user_site()

Answer 2

The paths available in Blender 3.x include /home/my-user/.config/blender/3.3/scripts/addons/modules which is writable. It is possible to install your Python packages in this directory directly. For example:

pip install --target=$HOME/.config/blender/3.3/scripts/addons/modules/ my_package

Or on Windows:

pip install --target=%appdata%\Blender Foundation\Blender\3.5\scripts\addons\modules my_package