python initiated with systemd cannot start subprocess

Question

I have a python script inside a virtualenv which is started using systemd.

[Unit]
Description=app
After=network.target

[Service]
Type=simple
User=user
Group=user
Environment=VIRTUAL_ENV=/home/user/Projects/app/venv
Environment=PATH=$VIRTUAL_ENV/bin:$PATH
WorkingDirectory=/home/user/Projects/app
ExecStart=/home/user/Projects/app/venv/bin/python app.py

[Install]
WantedBy=multi-user.target

The thing is that the script uses subprocess.Popen(['python', 'whatever.py']) to open another python script. I got a not found error, and discovered that python should be invoked with an absolute path, so I changed it and it worked well.

However, now I use a third party library, pygatt, which inside uses subprocess to open gatttool or hcitool which are in $PATH (system wide binaries, usually in /usr/bin).
So now I cannot change that library (I could by forking it, but I hope I don't have to).

How comes that systemd cannot spawn python subprocesses without using an absolute path? Without systemd (running from console), everything works.

Answer 1

I'm not sure but it is very possible that setting environment in one configuration line isn't taken into account in the following ones.

Environment=VIRTUAL_ENV=/home/user/Projects/app/venv
Environment=PATH=$VIRTUAL_ENV/bin:$PATH

here you're expecting VIRTUAL_ENV to be set to $VIRTUAL_ENV is evaluated the next line, but that may not work. I would try hardcoding the second line:

Environment=VIRTUAL_ENV=/home/user/Projects/app/venv
Environment=PATH=/home/user/Projects/app/venv/bin:$PATH