PLL Reference

In this class, we write Python with Python Language Levels (PLL), an extension for the VSCode editor. PLL runs your programs right in the editor, and shows the results in an interactions panel — output, errors, images, tables, and a prompt where you can try extra Python after a run. It works in vscode.dev, in the browser, so you do not need to install Python on your computer.

These pages describe everything PLL adds to Python:

• Language levels — the #level line at the top of your file, and type annotations.
• Tests — test_ functions, assert, pytest.approx, and checking your tests against the course's code.
• Images — shapes, combining images, and loading pictures.
• Tables and charts — tables, loading CSV files, charts, pandas, and matplotlib.
• Reactors — animations and interactive programs.
• Input and files — input(), files next to your program, and reading from the web.

For the Python language itself, see the Python documentation.

Installing PLL

1. Open vscode.dev (or desktop VSCode).
2. Open the Extensions view (the four squares in the left sidebar).
3. Search for Python Language Levels.
4. Click Install. If you are asked to trust the publisher, do that.

You do not need Microsoft's Python extension. If VSCode offers to install it, don't: PLL is all you need.

Running a program

1. Create a file whose name ends in .py, for example hello.py.

2. Type a small program:

   #level beginner
   
   print("hello")

3. Click PLL: Run Python File, at the top right of the editor, on the same bar as the file name.

   If you do not see it, open the Command Palette (Ctrl+Shift+P on Windows, Cmd+Shift+P on a Mac) and run PLL: Run Python File.

The first run can take a little while: PLL is starting Python in the browser. Later runs are faster.

Results appear in the PLL panel at the bottom of the window (near Problems and Terminal). If that panel is hidden, run PLL: Show Interactions from the Command Palette.

When PLL runs a file, it runs the file, and then any tests in it. If a line in your file is just an expression (like 1 + 1, or circle(20, "solid", "red")), PLL shows its value, in order with anything you print (except at #level raw, which shows nothing it isn't asked to, as plain Python does).

Before you have a file

With no Python file open, the interactions panel has a session of its own, called No file: type Python at its prompt and press Enter. It is for the very first things you try, before you have a repository to make a file in, and it always uses #level beginner. Once you open a Python file, the panel shows that file's session instead; PLL: Start REPL switches back to the session with no file.

The interactions panel

The interactions panel has a prompt at the bottom. After a file has run, you can type extra Python there and press Enter. Names you defined in the file are still available. The panel's header shows the file and the level the prompt uses (for example hello.py [beginner]). Closing a file ends its session: its output in the panel, and the names its runs defined, go with it.

• Enter runs what you typed.
• Shift+Enter adds another line (for a longer snippet).
• If Python is waiting for more input (for example after if True:), PLL will keep prompting until what you typed is complete.
• Up and Down move through things you typed earlier.
• Ctrl+L (Windows) or Cmd+L (Mac) clears the panel. You can also run PLL: Clear Interactions.

Stopping a program

If a program runs longer than you expect — a loop that never ends, say — click Stop in the interactions panel. Ctrl+C (with nothing selected) and PLL: Stop Program in the Command Palette do the same thing. The program stops with a KeyboardInterrupt, and anything it printed first is kept. Stop works the same way while your tests run: the test that was running is marked as stopped, and the rest of the tests are not run.

This works for ordinary Python code. If your program is stuck inside a library (a very long pandas operation, for example), or it catches KeyboardInterrupt itself, PLL tells you it could not stop it; reload the window in that case.

Errors

When your program has a mistake, PLL explains what went wrong, and how you might fix it, in the interactions panel, and marks the line in the editor. Click the location in the message to jump to that line. Many mistakes are found before the file runs at all (see Language levels). The marks in the editor go as soon as you edit the file, since its lines may have moved; the panel keeps the message, and the next run marks whatever is still wrong.

Commands

All of these are available from the Command Palette:

Command	What it does
PLL: Run Python File	Run the file, then its tests (if any).
PLL: Show Interactions	Open the interactions panel.
PLL: Start REPL	Show the session with no file, at #level beginner, whatever file is open.
PLL: Change Level	Change the file's #level line (or give a file with no line one).
PLL: Stop Program	Stop the program that is running.
PLL: Clear Interactions	Clear the panel.

Copy and paste

Ctrl/Cmd+C, X, and V work as usual, both in the editor and in the interactions panel. If they do nothing in the browser and you use a keyboard layout other than QWERTY (Dvorak, Colemak, …), add this to your user settings (Command Palette → Preferences: Open User Settings (JSON)):

{
  "keyboard.dispatch": "keyCode"
}