19-debugging

19 · Debugging

Part 4 — Real-World Python · Estimated time: 25–35 min · Prerequisite: 17 · Exceptions & Error Handling

Every programmer writes bugs — debugging is the skill of finding and fixing them, and it's just as important as writing code. The good news: Python's error messages are genuinely helpful once you know how to read them. This lesson is about staying calm and systematic when something breaks.

What you'll learn

• The most common Python errors and what each one means
• How to read a traceback (hint: from the bottom up)
• Print debugging — the simplest, most useful tool
• The built-in interactive debugger (breakpoint())
• A calm, repeatable approach to fixing bugs

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1. Common errors and what they mean

Error	Usually means	Typical fix
SyntaxError	Python can't parse the code	Check for a missing :, ), or quote.
NameError	a name isn't defined (often a typo)	Check spelling / that it's defined first.
TypeError	wrong type for an operation	e.g. don't add str + int — convert first.
IndexError	list index out of range	Valid indexes are 0..len-1.
KeyError	dictionary has no such key	Use .get() or check in first.
ValueError	right type, wrong value	e.g. int("abc").
AttributeError	object has no such attribute/method	Check the object's type and method name.

try:
    print([1, 2, 3][10])
except IndexError as e:
    print("IndexError:", e)

▶️ Run it: python examples/01_common_errors.py

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2. Reading a traceback

When code crashes, Python prints a traceback. Read it bottom-up: the last line is the actual error and message; the lines above show the chain of calls that led there, with file names and line numbers.

import traceback

def step_three(): return 1 / 0
def step_two():   return step_three()
def step_one():   return step_two()

try:
    step_one()
except ZeroDivisionError:
    traceback.print_exc()   # shows the same traceback you'd see on a crash

The bottom line says ZeroDivisionError: division by zero, and just above it you'll see it happened in step_three. That's your starting point.

▶️ Run it: python examples/02_reading_traceback.py

💡 Try it yourself: what's the very first thing you should look at in a traceback? (The bottom line — the error type and message.)

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3. Print debugging

The fastest way to understand what your code is actually doing is to print values as it runs. Add temporary print() calls, see what's happening, then remove them.

def double_all(numbers):
    result = []
    for n in numbers:
        print(f"DEBUG: processing {n}")   # temporary — watch the loop
        result.append(n * 2)
    return result

print(double_all([1, 2, 3]))

▶️ Run it: python examples/03_print_debugging.py

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4. The interactive debugger

For trickier bugs, Python's built-in debugger lets you pause and inspect. Add breakpoint() where you want to stop, then run the file normally:

def buggy(x):
    breakpoint()    # execution pauses here and gives you a (Pdb) prompt
    return x * 2

At the (Pdb) prompt:

Command	Does
p name	print a variable's value
n	run the next line
c	continue until the next breakpoint or the end
l	list the code around the current line
q	quit the debugger

(The debugger is interactive, so it isn't runnable in the in-browser version — try it in a terminal.)

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A calm debugging method

1. Read the error — the bottom of the traceback tells you what and where.
2. Reproduce it — make the bug happen reliably.
3. Inspect — print the values right before the failing line. Are they what you expected?
4. Form one hypothesis, test it — change one thing at a time.
5. Fix, then re-run — confirm it's gone, and that you didn't break anything else.

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Common mistakes

Mistake	What happens	Fix
Panicking at a red wall of text	you miss the useful line	Read the bottom line first.
Changing many things at once	you can't tell what fixed it	Change one thing, re-run, repeat.
Ignoring the line number	you look in the wrong place	The traceback names the exact file and line.
Leaving debug prints in	noisy output	Remove them once the bug is fixed.

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Recap / cheat-sheet

# Read tracebacks bottom-up: last line = the error.
# Common errors: SyntaxError, NameError, TypeError, IndexError, KeyError, ValueError

print(f"DEBUG: x = {x}")   # print-debugging

breakpoint()               # drop into the interactive debugger
# (Pdb) p x   n   c   l   q

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Exercises

Each file below has a bug. Read the comment, fix the code so it produces the expected output, then check against solutions/.

1. exercises/01_fix_the_nameerror.py — squash a NameError.
2. exercises/02_fix_the_indexerror.py — fix an IndexError.
3. exercises/03_fix_the_typeerror.py — fix a TypeError.

Try each yourself before opening the solution.

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Next → 20 · Classes & Objects (coming soon)