Activating a venv using the configuration.nix

Hey Guys,
So I have a python virtual environment where I have my pypi packages.
Before nixos, I used to put a “source bla/bla/bin/activate” command in my .bashrc to activate the venv whenever I open a new shell. Now as a new nixos user, I wanna learn how to do that via the configuration.nix file. So what are your thoughts?