I don't know how to power off my PC while running virtualenv and Python.

Run the command in the background by combining nohup with &.

$nohup python sample.py&

In such cases, it is convenient to use the screen command to operate on a virtual terminal.

If you know the following commands, you can move them.

screen
Create Virtual Terminal

Ctrl+ad
Detach the screen from the current terminal.

screen-r
Resume the detached screen session.

Also, if done, stop with Ctrl+z and use disown as stated in the comment.