Debugging Techniques

Introduction

Debugging is the process of finding and fixing the cause of unexpected behaviour. The goal is not just to fix the symptom but to understand why it happened.

1. Read the Traceback

The traceback is your first source of information. Read it bottom to top — the last line names the exception, the lines above show the call chain that led to it.

Traceback (most recent call last):
  File "main.py", line 12, in <module>
    process(data)
  File "main.py", line 7, in process
    total = sum(values)
TypeError: unsupported operand type(s) for +: 'int' and 'str'

Here the problem is a string in a list that should contain only integers, on line 7 of process().

2. Print Debugging

The simplest technique — add print() calls to inspect values at different points.

def calculate_bonus(salary, multiplier):
    print(f"DEBUG salary={salary}, multiplier={multiplier}")
    bonus = salary * multiplier
    print(f"DEBUG bonus={bonus}")
    return bonus

Remove or comment out debug prints before sharing your code.

3. The Python Debugger (pdb)

pdb lets you pause execution and step through code line by line.

import pdb

def calculate_bonus(salary, multiplier):
    pdb.set_trace()   # execution pauses here
    return salary * multiplier

Useful pdb commands at the prompt:

Command	Action
n	Next line
s	Step into a function
c	Continue to next breakpoint
p variable	Print a variable's value
q	Quit the debugger

In Python 3.7+ you can use breakpoint() instead of import pdb; pdb.set_trace().

def calculate_bonus(salary, multiplier):
    breakpoint()
    return salary * multiplier

4. VS Code Debugger

VS Code has a built-in debugger with a visual interface:

1. Click the line number in the editor to set a breakpoint (a red dot appears).
2. Press F5 or click Run → Start Debugging.
3. Execution pauses at the breakpoint.
4. Use the debug toolbar to step through, inspect variables, and continue.

This is the most productive approach for anything beyond a quick print check.

5. Common Bugs and How to Find Them

Symptom	Likely cause	Where to look
TypeError on arithmetic	A string where a number is expected	Check type() of inputs
KeyError on dict access	Key does not exist	Use .get() or check with in
Wrong output from a loop	Off-by-one error	Print the loop variable on each iteration
Function returns None	Missing return	Check every code path has a return
Variable has unexpected value	Modified elsewhere	Search for all assignments to that name

Practice Exercises

• Take a function with a deliberate bug (wrong operator, missing return) and use print debugging to find and fix it.
• Set a breakpoint() in a function and step through it, printing the value of a variable at each step.
• Write a function that receives a list of mixed types (integers and strings) and raises a TypeError when it hits a string. Use the traceback to identify exactly which value caused the problem.