Debugging Your Code

Solving the Mystery

Bugs are a natural part of programming. A “bug” is just a mistake in your logic that causes the program to behave unexpectedly. Today, we learn the mindset and the tools needed to fix them.

Type of Bugs

1. Syntax Errors: Mistakes in your typing (e.g., prnit("Hello")). Python finds these for you.
2. Runtime Errors: Mistakes that happen while the app is running (e.g., 1 / 0).
3. Logic Errors: The app runs fine, but the result is wrong (e.g., calculating price + discount instead of price - discount).

The Power of print()

The simplest way to debug is to “trace” your code by printing variable values at different steps.

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def calculate_total(price, tax):
    print(f"DEBUG: Price is {price}") # Trace point 1
    total = price * tax 
    print(f"DEBUG: Total is {total}") # Trace point 2
    return total

# If tax is 0.10 (10%), why is the total wrong?
calculate_total(100, 0.10) 

Using the Debugger (pdb)

Python comes with a built-in debugger called pdb. It allows you to pause your code and look around.

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import pdb

x = 10
y = 20

# Pause the program here!
pdb.set_trace() 

z = x + y
print(z)

When the program hits set_trace(), it stops. You can type variable names (like x) to see their values before the next line runs.

Rubber Duck Debugging

Sometimes, the best way to find a bug is to explain your code line-by-line to an inanimate object (like a rubber duck). As you explain what you think the code is doing, you often realize what it is actually doing.

Interactive Practice

Imagine a loop that should print numbers 1 to 5 but only prints 1 to 4. Where would you put a print() statement to check the loop condition?