COMPATIBILITY.md

Web VPython WASM compatibility notes

The WASM runner executes programs with CPython through Pyodide. This gives Web VPython real Python semantics in the browser, but a few older GlowScript/RapydScript conveniences are intentionally different.

Define functions before passing or calling them

In classic GlowScript/RapydScript, this pattern often worked even though the function was defined later:

scene.bind('mousedown', clicked)

def clicked(event):
    print(event.pos)

In the WASM runner, names are resolved using standard Python rules. Define the function before it is referenced:

def clicked(event):
    print(event.pos)

scene.bind('mousedown', clicked)

The same rule applies to other callbacks, button bindings, and ordinary function calls.

range() requires integer arguments

The WASM runner uses CPython's built-in range(), which only accepts integer start, stop, and step values:

for x in range(1, 10, 0.9):   # TypeError in CPython / WASM
    print(x)

For non-integer steps, use an explicit floating-point loop or a library helper such as NumPy:

import numpy as np

for x in np.arange(1, 10, 0.9):
    print(x)

As with normal Python floating-point arithmetic, decimal steps such as 0.1 may produce values with small binary floating-point roundoff. Use round(...) when displaying values if exact decimal formatting is important.

Raw JavaScript syntax is not valid in WASM programs

Classic GlowScript/RapydScript programs sometimes include JavaScript syntax directly, for example:

audioCtx = new window.AudioContext();

The WASM runner parses programs as CPython, so JavaScript-only syntax such as new ... is a Python SyntaxError. Use Pyodide's JavaScript proxy syntax instead. For example, Web Audio constructors can be called with .new():

audioCtx = window.AudioContext.new()

def playFreq(freq, time):
    toneGen = audioCtx.createOscillator()
    toneGen.frequency.value = freq
    toneGen.type = "sine"
    toneGen.connect(audioCtx.destination)
    toneGen.start()
    toneGen.stop(audioCtx.currentTime + time)

As noted above, callback functions still need to be defined before they are passed to scene.bind(...), button(bind=...), and similar APIs.