The python example is missing the most important part, which is how to make your script work with external dependencies. Here is a working example.py with external dependencies:

    #! /usr/bin/env nix-shell
    #! nix-shell -i python3 -p python3 -p python3Packages.requests -p python3Packages.fire

    import requests
    import fire

    class CLI:
        
        def get(self, url):
            return requests.get(url).status_code

    fire.Fire(CLI)

Run with chmod +x example.py ./example.py get https://example.com