Paste it, run it, step through it

Python code visualizer

Paste a Python function and its arguments, or a short script. It runs in your browser, each line it runs is a step, and the stage shows what that line read, what it changed and which way it went.

def two_sum(numbers, target):
    left, right = 0, len(numbers) - 1
    while left < right:
        total = numbers[left] + numbers[right]
        if total == target:
            return [left, right]
        if total < target:
            left += 1
        else:
            right -= 1
    return None

two_sum

or Ctrl+Enter

How it works

Real Python, in your browser: Your code runs on Pyodide, Python 3.13 built for WebAssembly. The standard library is there, heapq and collections included. Packages like numpy aren't.
Every line is a step: Each line of your functions that runs is a step you can play, scrub or link to. A run stops at 100,000 lines or 2 seconds, so a loop that never ends can't hang the page.
Every value by its shape: A list of dicts is a table, a dict of numbers is a ledger, a list of lists is a grid, an object is a card. Each step marks what its line read and changed, works out its test (`2 < 1` is False) and says which way the run went.
The view comes from the run: Rules read what your variables do. Two names indexing a list are pointers, a list pushed and popped is a stack, heapq is a heap, a table filled from its own cells is DP. Those runs get the pattern's own stage too.
Change one thing: Every number and value in your code is a control. Change one, the code runs again, and the stage says what came out differently.
Links keep your code private: A link holds the code after its #, which your browser never sends, so it stays out of server logs. Code from a link runs in a sandbox with no access to your account or this page.