Lesson: Debugging in Python

[introduction]

Welcome back! Today we’re exploring debugging, one of the most essential skills for every programmer. Debugging is simply the process of finding and fixing errors in your code. By the end of this lesson, you’ll understand common error types, debugging techniques, and tools that will help you solve problems confidently.

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[concept_explanation]

Debugging involves identifying where and why something is going wrong in your program.

Python provides several types of errors:

1. Syntax Errors

Mistakes in the structure of your code.
Example: missing parentheses, colon, or using wrong indentation.

2. Runtime Errors

Errors that happen while the program runs.
Example: dividing by zero, using a variable that doesn’t exist.

3. Logic Errors

The code runs, but the output is incorrect.
Example: Using + instead of *.

To debug effectively, you must learn to:

• Read error messages

• Trace what the code is doing

• Test small parts of your code

• Print variable values to see what's happening

• Use debugging tools

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[python_examples]

1. Example of a bug

age = "20"
print(age + 5)   # TypeError!

Fix

age = "20"
print(int(age) + 5)

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2. Using print statements for debugging

def add(a, b):
    print("a =", a, "b =", b)  # debug helper
    return a + b

print(add(3, 7))

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3. Using try/except for debugging

try:
    x = 10 / 0
except ZeroDivisionError:
    print("You cannot divide by zero.")

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4. Using Python’s built-in debugger (pdb)

import pdb

pdb.set_trace()

x = 5
y = x + 10
print(y)

This allows you to step through your code line by line.

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[analogy]

Debugging is like being a detective:

• You gather clues (error messages)

• You trace the sequence of events

• You test your theories

• You slowly narrow down the problem until you find the exact line causing trouble

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[practice_task]

Try debugging this code (there are two errors):

numbers = [1, 2, 3, 4]
for n in numbers
    print(n + "!")

Fix the errors so it prints:

1!
2!
3!
4!

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[quiz]

1. What type of error occurs when Python can’t understand your code?

2. What type of error occurs when your code runs but gives the wrong output?

3. What will this code do?

try:
    print(10 / 0)
except:
    print("An error happened")

4. What tool lets you pause and step through your code?

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[common_mistakes]

• Ignoring error messages instead of reading them

• Trying to debug too much code at once

• Assuming the issue is in one place without testing

• Forgetting to convert values before using them

• Using incorrect indentation (one of the most common Python errors!)

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[extra_notes]

• IDEs like VS Code and PyCharm have built-in visual debuggers.

• You can use assert statements for quick checks:

  assert x > 0, "x must be positive"
  

• Logging (import logging) is better than print debugging for bigger projects.

• Debugging skill grows with experience—every programmer relies on it daily.

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[encouragement]

Congratulations! Debugging is the final major topic in this module, and you're now equipped with the skills that real Python developers use. You’ve learned variables, loops, functions, lists, dictionaries, strings, OOP, file handling, and debugging — an amazing achievement!