Debugging Basics
📚 What is Debugging? Debugging is the process of finding and fixing mistakes (bugs) in your code. Every programmer debugs every day. The word 'bug' comes from 1947 when an actual moth was found inside a computer causing it to fail! Today bugs are logic errors, typos, or wrong assumptions in your c…
Appy Says…
Every professional developer spends 30–50% of their time debugging. It's not a sign you're bad at coding — it means you're coding. The best developers aren't the ones who write perfect code first time; they're the ones who find bugs fast.
What is Debugging?
A bug is an error or unexpected behaviour in your code. Debugging is the process of finding it and fixing it. There are three types of errors:
- •Syntax errors — code that Python can't even read (missing colon, wrong indent). Python tells you before running.
- •Runtime errors — code that runs but crashes partway through (dividing by zero, index out of range). Python shows a traceback.
- •Logic errors — code that runs without crashing but gives the wrong answer. These are the hardest to find.
Think of it like QA testing a game
Before a game ships, testers play it repeatedly looking for anything that breaks — falling through the floor, enemies stuck in walls, scores not adding up. Debugging code is the same job: run it, observe what it does, find where it diverges from what it should do, fix it, test again.
Debugging Strategy — Step by Step
- •Read the error message — it tells you the file, line number, and type of error
- •Add print() statements to see the value of variables at different points
- •Isolate the problem — comment out sections until you find which line causes the issue
- •Check your assumptions — is the variable the type you think? Print
type(x) - •Search the error — copy the error message into Google/Stack Overflow, others have had it
- •Fix one thing at a time — don't change everything at once or you won't know what worked
Common Python Errors
- •NameError: using a variable before creating it —
print(score)beforescore = 0 - •TypeError: wrong type in an operation —
"age: " + 15(string + int) - •IndexError: list index out of range — accessing
list[5]when list has 3 items - •IndentationError: wrong indentation — Python's indentation is syntax, not style
- •ZeroDivisionError: dividing by zero — check
if divisor != 0first
Key Facts
- •The word 'bug' comes from a literal moth found in a 1940s computer that caused a fault
- •Python's traceback shows the exact chain of calls that led to the error — read it bottom to top
- •
print()debugging is used by professionals daily — simple but powerful - •Python has a built-in debugger called
pdb— advanced but useful for complex bugs - •Good variable names prevent many logic errors —
total_scoreis harder to misuse thants
Watch Out: Don't Guess — Investigate
The worst debugging habit is randomly changing code hoping something fixes it. Every change should be a hypothesis: 'I think the problem is X, so I'll change Y and see if the result changes.' This is how professionals debug efficiently.
Remember
When you hit an error: (1) Read the message. (2) Find the line. (3) Print nearby variables. (4) Fix one thing. (5) Test. Repeat. Debugging is detective work — and once you're good at it, it actually becomes quite satisfying.
What You Learned
- •Three types: syntax errors (can't read), runtime errors (crashes), logic errors (wrong result)
- •Read the traceback — it shows the exact line and type of error
- •Use
print()to inspect variable values at different points - •Check types with
type(x)when a variable behaves unexpectedly - •Fix one thing at a time and test after each change
- •Debugging skill matters as much as writing code — it gets faster with practice
Key Facts
- →The word 'bug' comes from a literal moth found in a 1940s computer that caused a fault
- →Python's traceback shows the exact chain of calls that led to the error — read it bottom to top
- →
print()debugging is used by professionals daily — simple but powerful - →Python has a built-in debugger called
pdb— advanced but useful for complex bugs - →Good variable names prevent many logic errors —
total_scoreis harder to misuse thants
Real-World Examples
Remember
When you hit an error: (1) Read the message. (2) Find the line. (3) Print nearby variables. (4) Fix one thing. (5) Test. Repeat. Debugging is detective work — and once you're good at it, it actually becomes quite satisfying.
Quick Quiz
What is a bug?