Can be used to set up per dir environments, e.g using direnv to automatically activate them.
## Python
https://nixos.org/manual/nixpkgs/stable/#python

```nix
{ pkgs ? import <nixpkgs> {}}:

pkgs.mkShell {
    packages = [ pkgs.virtualenv ];
}
```

```nix
with import <nixpkgs> {};
let
	my_toolz = python311.pkgs.buildPythonPackage rec { 
		pname = "toolz";
		version = "0.10.0";
		pyproject = true;
		src = fetchPypi {
			inherit pname version;
			hash = "sha256-CP3V73yWSArRHBLUct4hrNMjWZlvaaUlkpm1QP66RWA=";
		};
		nativeBuildInputs = [
			python311.pkgs.setuptools
			python311.pkgs.wheel
		]; # has no tests
		doCheck = false;
		meta = {
			homepage = "https://github.com/pytoolz/toolz/";
			description = "List processing tools and functional utilities";
			# [...]
		};
	};
in python311.withPackages (ps: with ps; [ numpy my_toolz ]) ).env
```

The [[keyword-import]] is required here because imports a nix expression from another source, in this case nixpkgs.
It is not required at line two, because python310Packages itself is already in the local scope.