Python Online Debugger

How to debug Python online

1. Open the Python editor and write or paste your program, or open one of the examples below.
2. Click beside a line number to set a red breakpoint. Set as many as you need.
3. Select Debug. The program runs and pauses on the first breakpoint it reaches. If you switched the editor to Browser Python, switch back to Execution Server first: the debugger runs there.
4. Read the Variables panel for the current values and the Call Stack panel for the functions that led to this line.
5. Use Step Over to run the next line, Step Into to enter a function call, Step Out to finish the current function, or Continue to run to the next breakpoint.
6. Select Stop when you have found the problem, fix the code and run it again.

What the debugger shows

• BreakpointsPause exactly where a value goes wrong instead of adding print() calls everywhere.
• Step by stepStep over, into and out of functions, and see which line runs next.
• VariablesThe variables of the function that is running, or the top-level variables outside a function, with their current values every time the program pauses.
• Call stackThe chain of function calls that reached the current line, which makes recursion easy to follow.
• Real Python, with input()Your program runs with real Python 3 on our execution server, the editor's default mode, so it can ask for keyboard input and import the installed libraries while you debug.
• AI helpAsk the AI to explain an error or suggest a fix, right in the editor.

Try it: why is the average wrong?

The average of 80, 72, 95 and 73 is 80, but the function returns 60.0. Set a breakpoint on the return line and select Debug: total is 240, not 320, because range(1, len(scores)) starts at the second item.

def average(scores):
    total = 0
    for i in range(1, len(scores)):
        total += scores[i]
    # Expected 80.0, but this prints 60.0.
    # Set a breakpoint on the next line and look at total and i.
    return total / len(scores)


print(average([80, 72, 95, 73]))

Follow recursion in the call stack

Set a breakpoint on return n * factorial(n - 1) and use Step Into. Each call adds a frame to the call stack with its own n. The second call, factorial(0), skips the base case and keeps going until Python raises RecursionError; the stack shows why.

def factorial(n):
    if n == 1:
        return 1
    return n * factorial(n - 1)


# Step Into each call and watch the Call Stack grow, then shrink.
print(factorial(4))
# factorial(0) never reaches n == 1: debug it to see why it fails.
print(factorial(0))

Check a condition inside a loop

Pears are out of stock but still appear in the list. Put a breakpoint on the if line and press Continue to stop once per item: when count is 0, the condition is still true.

inventory = {"apples": 3, "pears": 0, "plums": 5}

in_stock = []
for name, count in inventory.items():
    if count >= 0:  # Bug: should be > 0
        in_stock.append(name)

print("In stock:", in_stock)

Debugger, visualizer or print()?

Frequently asked questions