Running Code

How code execution works in the browser.

How It Works

Your code runs directly in your browser using Pyodide, a Python interpreter compiled to WebAssembly. This means:

• No server needed — Code executes locally
• Instant feedback — No waiting for a server response
• Privacy — Your code stays in your browser until you submit

Execution Flow

Click "Run Code"
    ↓
Pyodide parses your code
    ↓
If syntax error → Show error, stop
    ↓
Execute code line by line
    ↓
If runtime error → Show error with traceback
    ↓
Display output in Output tab

Input Handling

The input() Function

Python's input() function works inline:

age = input("How old are you? ")
print(f"You are {age} years old")

When executed:

1. "How old are you?" appears in the output
2. An input field appears below
3. You type your answer and press Enter
4. Execution continues with your input

Multiple Inputs

Each input() call waits for you to respond:

first = input("First name: ")
last = input("Last name: ")
print(f"Hello, {first} {last}!")

Output Display

Normal Output

print() statements appear in green text:

print("Hello, World!")
# Output: Hello, World!

Error Output

Errors appear in red with details:

print(undefined_variable)
# Output: NameError: name 'undefined_variable' is not defined

Supported Python Features

What Works

• All basic Python syntax
• Standard library modules (most)
• print(), input(), range(), len(), etc.
• Lists, dictionaries, sets, tuples
• Functions and classes
• File operations (in-memory)
• math, random, datetime, json, etc.

What Doesn't Work

• Network requests (requests, urllib)
• System calls (os.system(), subprocess)
• GUI libraries (tkinter, pygame)
• Some C-extension modules

Loading Additional Packages

Some packages can be loaded on demand:

import micropip
await micropip.install('numpy')
import numpy as np

INFO

For most assignments, you won't need additional packages.

Performance

Speed

Pyodide runs at near-native speed for most code. You might notice slowness with:

• Very large loops (millions of iterations)
• Heavy computation
• Large data structures

Memory

Browser memory is limited. Avoid:

• Creating huge lists
• Infinite loops
• Loading large files

Clearing Output

Click the Clear button to:

• Erase all code from the editor
• Start fresh

WARNING

Clear removes all your code! Make sure you've saved it elsewhere if needed.

Troubleshooting

"Python interpreter failed to load"

• Check your internet connection
• Refresh the page
• Wait for "Loading Python..." to complete

Code runs forever

If your code has an infinite loop:

1. Refresh the page
2. Fix the loop condition
3. Try again

Output looks wrong

• Check for missing print() statements
• Verify your logic
• Add debug prints to trace execution