[Leejohnston]

Chapter 17 — Errors & Exception Handling
Programs crash when something unexpected happens. Exception handling prevents that — keeping your software safe, professional, and user-friendly.

Every programmer makes mistakes. 
Every user enters bad input. 
Every program encounters unexpected situations.

Python gives us tools to handle all of this safely.

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17.1 Common Types of Errors

SyntaxError 
Code is written incorrectly.

Code:

print("Hello"  # missing parenthesis

TypeError 
Wrong type used.

Code:

"age" + 5

ValueError 
Bad value inside correct type.

Code:

int("hello")

ZeroDivisionError 
Division by zero.

Code:

10 / 0

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17.2 Try & Except

Basic structure:

Code:

try:
    # code that may fail
except:
    # what to do if it fails

Example:

Code:

try:
    num = int(input("Enter a number: "))
except:
    print("That was not a valid number.")

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17.3 Catching Specific Errors

Better practice:

Code:

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

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17.4 Multiple Except Blocks

Code:

try:
    value = int(input("Enter age: "))
except ValueError:
    print("Please enter numbers only.")
except TypeError:
    print("Type error occurred.")

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17.5 Using else

Runs ONLY if no error happens.

Code:

try:
    age = int(input("Age: "))
except ValueError:
    print("Invalid age.")
else:
    print(f"You are {age} years old.")

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17.6 Using finally

Always runs — useful for closing files.

Code:

try:
    f = open("data.txt")
except FileNotFoundError:
    print("File missing.")
finally:
    print("Program finished.")

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17.7 Raising Your Own Errors

For validation:

Code:

def set_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative.")
    return age

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17.8 Validating User Input Safely

Code:

while True:
    try:
        num = int(input("Enter a number: "))
        break
    except ValueError:
        print("Numbers only, try again.")

This pattern is used in many real programs.

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17.9 Handling File Errors

Code:

try:
    with open("notes.txt") as f:
        print(f.read())
except FileNotFoundError:
    print("File not found.")

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17.10 Real-World Example — Safe Calculator

Code:

try:
    a = float(input("A: "))
    b = float(input("B: "))
    print(a / b)
except ZeroDivisionError:
    print("Cannot divide by zero.")
except ValueError:
    print("Numbers only.")

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17.11 Mini Project — Safe Login

Requirements:

• ask for username 
• ask for password 
• catch empty input 
• catch spaces 
• deny login if invalid 

Example errors to catch:

• ValueError (empty input) 
• anything unexpected 

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17.12 Challenge — Fault-Tolerant Data Loader

Your program must:

1. Ask for a filename 
2. Try to open it 
3. Catch:
  • FileNotFoundError 
  • PermissionError 
  • UnicodeDecodeError 
4. If opened successfully:
  • read file 
  • show the number of lines 

BONUS: 
Ask the user to “try again” without crashing.

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17.13 Chapter Summary

• try/except prevents crashes 
• catch specific errors for clarity 
• else runs when no error occurs 
• finally always runs 
• raising errors helps validate values 
• safe loops ensure correct user input 

Next:
Chapter 18 — Object-Oriented Programming (Beginner Overview)

This is where we introduce *classes*, *objects*, and the structure behind real-world software.

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Written and Compiled by Lee Johnston — Founder of The Lumin Archive