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Section 1. Errors in Python

When Coding Meets Real Life

Example 1: The Missing Classroom

open("non_existent_file.txt") 
# Like searching for classroom 404 that doesn't exist

When Coding Meets Real Life

Example 2: Zero Division Drama

total_people = 0
cake_slices = 8 / total_people
# Like preparing cake slices when no one comes to party

Meet Python’s Error Messages

The 5 Most Common Errors

  1. SyntaxError
while True
  1. NameError
print(new_variable)
  1. TypeError
1+'a'
  1. FileNotFoundError
fi=open('okokokokok.txt','r')
  1. IndentationError
while True:
print('yes')

Let’s Read Error Messages Together!

Sample Error Stack

python3 demo.py

Traceback (most recent call last):
  File "demo.py", line 3, in <module>
    print(undefined_var)
NameError: name 'undefined_var' is not defined

Error Message Breakdown

What Each Part Means:

  1. Traceback: Error roadmap
  2. File demo.py: Where error happened
  3. Line 3: Exact line number
  4. NameError: Error type
  5. undefined_var: variable leading to error

Question: What’s the error here?

def greet():
print("Hello!")  # Missing indentation

greet()

Question: What’s the error here?

age = "20"
years_to_drive = age - 16

Section 2. Exception Handling in Python

Basic Try-Except Structure

try:
    # Code that might fail
    result = 10 / 0
except ZeroDivisionError:
    print("Oops! Can't divide by zero!")
  • try: Contains risky code
  • except: Handles specific errors
  • Like seatbelts for your code!

Case: Let’s Break a Calculator!

What if someone tries to calculate BMI with zero height?

try:
    weight = 70
    height = 0
    bmi = weight / height
except ZeroDivisionError:
    print("Alert! Did you enter height as zero?")
  • Catches specific error type
  • Prevents program crash

Case: File Detective Game

What if student list file is missing?

try:
    with open("ghost_file.txt") as f:
        print(f.read())
except FileNotFoundError:
    print("Pssst... this file doesn't exist!")
  • FileNotFoundError specific handler
  • Gives clear feedback

The Safety Net (Catch-All)

When you’re not sure what might go wrong:

try:
    magic_number = "100" + 5
except Exception as e:
    print(e)
  • Exception catches all errors
  • as e stores error details

Friendly Error Messages when building software

Make technical errors human-friendly:

try:
    user_age = int(input("Enter age: "))
except ValueError:
    print("Hey! Numbers only please :)")
  • Converts confusing errors
  • Helps users understand mistakes

Guess: What will happen ?

try:
    guests = int(input("Number of guests: "))
    1+'2'
    sugar_per_person = 50
    total_sugar = sugar_per_person * guests
    print("Needed sugar: "+str(total_sugar)+"g")
except ValueError:
    print("Numbers only for guests please!")
except Exception as e:
    print(e)

Section 3. Handling Multiple Exceptions

Basic Exception Handling

try:
    num = int(input("Enter number: "))
except ValueError:
    print("Numbers only please!")

Handling Different Error Types

Multiple except Blocks

try:
    age = int(input("Age: "))
    print(str(100/age))
except ValueError:
    print("Numbers only!")  
except ZeroDivisionError:
    print("Age can't be zero!")

Question: What will happen if we input 0 ? string ? 4 ?

How It Works

  • First checks for ValueError
  • Then checks for ZeroDivisionError
  • Only one except block executes

Handling Cases Together

Tuple-style Catching

try:
    with open("grades.txt") as f:
        print(f.read())
except (FileNotFoundError, PermissionError):
    print("File problem! Check if:")
    print("- File exists")
    print("- You have access")

Grade Calculator: Base Code

try:
    total = float(input("Total points: "))
    earned = float(input("Earned points: "))
    percentage = (earned/total)*100
except ValueError:
    print("Numbers only!")
except ZeroDivisionError:
    print("Total can't be zero!")

Question: Change it into Tuple-style ?

Practice Example: Pet Age

try:
    human_years = int(input("Dog's age: "))
    dog_years = human_years * 7
    print(f"Dog age: {dog_years}")
except ValueError:
    print("Enter whole number!")
except Exception:
    print("Something unexpected happened!")

Question: Change it into Tuple-style ? Do we really need to do that?

Section 4: Exception Handling - finally and else

finally

Always Executes!

  • Runs whether exceptions occur or not
  • Perfect for cleanup operations
  • Example situations:
    • Closing files
    • Releasing database connections
    • Freeing network resources

File Handling with finally

Guess: What will happen?

try:
    file = open("diary.txt", "r")
    print(file.read())
except FileNotFoundError:
    print("Diary not found!")
finally:
    file.close()  # This always runs
    print("File handler cleaned up")

The Special else Clause

Runs Only When…

  • No exceptions occurred in try block
  • Useful for separating success logic 
    try:
  result = 10 / num
except ZeroDivisionError:
  print("Can't divide by zero!")
else:
  print(f"Result is {result}")  # Only if successful

Why Use else?

  • Makes code clearer than putting code at end of try block
  • Avoids catching unexpected exceptions
  • Example before/after comparison:
# Without else
try:
    config = open("settings.cfg")
    # More code here...
    # Could accidentally catch exceptions from this code

# With else
try:
    config = open("settings.cfg")
except:
    #...
else:
    # Safe code here

Question: who can explain the following program?

Real-World Example: Safe Config Loader

config_file = False
try:
    config_file = open("game_config.cfg")
except FileNotFoundError:
    print("⚠ Config file missing!")
else:
    settings = config_file.read()
    print(" Game configuration loaded!")
finally:
    if config_file:
        config_file.close()
    print("Resource cleanup completed")

Common Patterns

Resource Management Trio

  1. try: Attempt risky operation
  2. except: Handle known errors
  3. else: Process success case
  4. finally: Cleanup resources

Question: who can explain the following function?

def safe_divide(a, b):
    try:
        result = a / b
    except ZeroDivisionError:
        print("Division by zero!")
    else:
        print(f"Result is {result}")
    finally:
        print("Operation complete")

safe_divide(10, 2)
safe_divide(5, 0)

Section 5. Exception Handling in Action

1 Network Requests: Connection Timeout

import requests
try:
    response = requests.get("https://www.google.com", timeout=3)
except requests.exceptions.ConnectTimeout:
    print("Oops! Connection took too long!")

What happens:

  • Waits 3 seconds for connection

  • Shows message if server doesn’t respond

2 Retry

import time,requests
wait_time=3

for attempt in range(3):
    try:
        requests.get("https://www.google.com",timeout=3)
        break
    except Exception:
        print(f"Attempt {attempt+1} failed")
        time.sleep(wait_time)

Question: How can we dynamically change the wait-time?

First time: 3s Second time: 2s Third time: 1s

3 Checking Duplicate Usernames

existing_users = ["alice", "bob", "charlie"]

try:
    new_user = input("Choose username: ")
    if new_user in existing_users:
        raise ValueError("Username taken!")
except ValueError as e:
    print(e)

Question: How can we use while-loop to add multiple users?

4 Password Strength Check (Use “raise” to define error)

def check_password(pwd):
    if len(pwd) < 8:
        raise Exception("Password too short!")
    if not any(c.isdigit() for c in pwd):
        raise Exception("Add numbers!")

   
try:
    check_password("weak")
except Exception as e:
    print("Weak password: "+str(e))

5 Fahrenheit Input Validation

try:
    fahr = float(input("Enter Fahrenheit: "))
except ValueError:
    print("Numbers only please!")
else:
    celsius = (fahr - 32) * 5/9
    print(f"{fahr}F = {celsius:.1f}C")

Question: celsius to fahrenheit?

6 Temperature Range Check (use “raise”)

try:
    temp = float(input("Enter temp (-100 to 212F): "))
    if not (-100 <= temp <= 212):
        raise ValueError("Unrealistic temperature!")
except ValueError as e:
    print(f"Invalid input: {e}")

Question: put “Range Check” into the “Fahrenheit-to-Celsius”?