Editor

It is a code editor with a file tree on one side and a working shell underneath it, all running in the browser tab. You write a script and press Run, or Ctrl+Enter, and it is saved and run at the prompt below as "sh deploy.sh" or "python report.py", which you are equally free to type yourself. Whatever the script writes appears in the tree, because the editor, the prompt and Python are all looking at one filesystem rather than at three copies of one. It shares that filesystem with the Command Line tool, so a file made at either place is available at the other.

Three, and all of them are real. The shell is a shell: pipes, redirects, variables, if, for, while, case, functions, heredocs and getopts, with around sixty commands behind them. Python is CPython compiled to WebAssembly, so it is the actual interpreter with the actual standard library, and a traceback is the traceback you would get anywhere else. JavaScript is your browser's own engine, reached with "js build.js", with top-level await and a small fs object for the files. All three read and write the same tree, so a script that produces out.csv leaves it somewhere you can open and look at. What none of them has is a package manager: no pip, no npm, no require and no import, because fetching a package means asking somebody else's server for it.

No. Everything runs inside the tab: there is no server behind this page, no account, and no request to any other host. Your files are kept, but they are kept in this browser and nowhere else, so a reload or a visit tomorrow finds them where you left them and nobody else ever sees them. The plain version: a file you write here is on the disk of the computer you wrote it on until you remove it, so on a shared machine use the Forget everything button under the file tree, which deletes every file and the record of them together. "rm" removes one at a time. Only files are kept; variables, functions and Python names go when the tab does. To take work off the machine, "save" downloads a file, "save" on its own downloads the whole tree as a zip, and "load" brings files back in.

Safer than running it anywhere else, and still not a reason to stop reading it. A script runs in a worker built for that one run and killed afterwards, so it never touches the page, the terminal or the next script. Inside it there is no network and no storage: fetch, XMLHttpRequest, WebSocket, indexedDB and caches are all absent, and so is Worker, which is what stops a script starting a fresh one and getting them back. So it can read the files it was handed and print, and that is the list. Two honest caveats. That list is a list, written down in the source, and a browser that grows a new way out grows it there too. And "the files it was handed" is your whole home directory, so a script can still quietly rewrite or delete your work, exactly as a shell script could. Ctrl+C stops one that will not end.

No, and the same answer covers require and import. This site makes no requests to any other host, which is why the Python runtime is served from here rather than from a CDN, and a package manager is a request to somebody else's server by definition. So what you have is each language and what it brings with it: Python's standard library, which is json, re, csv, datetime, itertools, collections, math, decimal, hashlib and the rest of it, and JavaScript's built-ins, which is everything from JSON and Intl to crypto.subtle. That covers a surprising amount of the scripting people actually do. What it does not cover is a project with a dependency list, and this is the wrong tool for one.

If you have not typed into it, the editor takes the change in and shows you the new text, so a sed or a Python script that rewrote the file is visible immediately rather than being the older of two answers on your screen. If you have typed into it, your typing stays and the change is left on disk, because throwing away unsaved work to make room for a command is the one thing no editor is forgiven for. A file that gets deleted keeps its tab, and saving it puts the file back. The same three answers cover a command run in another tab of this site, since both windows are looking at one filesystem.

Yes, and both are looking at the same files rather than at two copies. A file written in one window appears in the other tree straight away, and one deleted disappears from it, with the rules above deciding what happens to anything you have open: a clean tab takes the new text, a tab you have typed into keeps your typing. Each window keeps its own working directory, variables and Python names, because those belong to a shell rather than to a disk. Saving in both at once does not cost you either save: each window keeps the files it changed and the two are put together, which covers two tabs working on different files. A file changed in both windows in the same moment is the one case that has to be decided, and this window wins it, with a line under the tree naming the file so you are not left to find out later. Forget everything empties every window at once.

Right-click it in the tree for Rename and Delete, or use "mv old.py new.py" and "rm old.py" at the prompt, which is the same filesystem either way and so the same result. A rename brings the open tab with it rather than closing it; a delete asks a second time first, and closes whatever tabs were open under it. Folders work the same, and a folder with anything in it says so instead of taking it with it. "mkdir" makes one.

Press Run at the top of the editor, or Ctrl+Enter. It saves the file first and then types the line at the prompt below, so what runs is what is on your screen rather than the version before the change you just made. Run only appears for a file this session can run, which means Python and shell, worked out from the suffix or from a "#!" line when the name says nothing. It goes grey while the prompt is busy, in the middle of a block, or at the Python prompt, because a line arriving in any of those lands somewhere other than where it was meant to. Nothing stops you typing "python report.py" yourself, and the button is only ever typing it for you.