I use nix-shell, and mostly I love it. But it’s important to be aware that the above means “Go get the latest(*) versions of python, pillow and ansicolor and run this code in an environment where they’re available.” It doesn’t do any version-pinning of your dependencies. That might be what you want, but maybe not: it’s frustrating when a script that worked yesterday won’t work today, or will only work after some big download.

My own rule of thumb is that nix-shell is great for quick one-offs and for sharing environments. For local tools and anything else I’m sharing with my future self, it’s usually better to write a nix expression and install it, which gives me access to Nix’s (excellent) rollback system, and lets me upgrade on my schedule, not upstream’s.

* - ‘Latest’ according to whatever Nix channel checkout currently applies. Which you can change, of course, but the point is it’s external to the script.