Is there already a tool like this?
An in-browser tool for scripted text/data processing:
On the left, an input box (plaintext). In the middle, a script (using JS to being with followed quickly by python). On the right, an output box (plaintext).
That's it—for starters. I have not been able to find something like this out there. There ARE, of course, a million codesandbox/jsfiddle/etc lookalikes, but they don't use this exact pattern of input -> script -> output. You can obviously accomplish this in many other different ways, but I have frequently found myself wanting such a domain-specific tool instead of opening text editor, touching file, remembering how to parse csvs, and so on and so forth.
I have MANY ideas for more features, maybe some "pro" ones: different data sources (csv, live data via url, etc), POST output to url, etc.
I didn't understand exactly what this does from your description. Could you say what it would be used for, what problem it would solve, who would use it? A few examples would really help.