Why exceptions occur in Python - See It in Action

Why Exceptions Occur

📖 Scenario: Imagine you are writing a simple program that asks a user to enter two numbers and divides the first number by the second. Sometimes, things can go wrong, like if the user types a word instead of a number or tries to divide by zero. These problems cause exceptions in the program.

🎯 Goal: You will create a small program that shows why exceptions happen by trying to divide two numbers. You will see what causes errors and how Python tells you about them.

📋 What You'll Learn

Create two variables with exact names and values

Add a variable to hold the divisor

Write code that divides the numbers and can cause an exception

Print the result or the error message

💡 Why This Matters

🌍 Real World

Handling exceptions is important in real programs to avoid crashes and give users helpful messages.

💼 Career

Knowing why exceptions occur and how to handle them is a key skill for all programmers to write reliable software.

1

Create two variables with numbers

Create two variables called num1 and num2 with the exact values 10 and 0 respectively.

Python

# Create variables num1 and num2 with values 10 and 0
# Your code here

Hint

Use the equals sign = to assign values to variables.

2

Add a variable for the divisor

Create a variable called divisor and set it to the value of num2.

Python

num1 = 10
num2 = 0
# Create a variable divisor and set it to num2
# Your code here

Hint

Assign the value of num2 to divisor using =.

3

Divide the numbers to cause an exception

Write a line that divides num1 by divisor and stores the result in a variable called result.

Python

num1 = 10
num2 = 0
divisor = num2
# Divide num1 by divisor and store in result
# Your code here

Hint

Use the division operator / to divide numbers.

4

Print the result or the error message

Use a try and except block to print result if division works, or print "Error: Division by zero is not allowed." if an exception occurs.

Python

num1 = 10
num2 = 0
divisor = num2
result = num1 / divisor
# Use try-except to print result or error message
# Your code here

Hint

Put the division inside try, and catch ZeroDivisionError in except.

Practice

1. Why do exceptions occur in a Python program?

easy

A. Because the program encounters an unexpected error during execution

B. Because the program runs perfectly without any errors

C. Because the program finishes all tasks successfully

D. Because the program has no code to execute

Solution

1. Step 1: Understand what exceptions mean

   Exceptions happen when the program faces an unexpected problem it cannot handle normally.

2. Step 2: Identify the cause of exceptions

   Unexpected errors like dividing by zero or accessing missing files cause exceptions.

3. Final Answer:

   Because the program encounters an unexpected error during execution -> Option A

4. Quick Check:

   Unexpected error = Exception occurs [OK]

Hint: Exceptions happen only when errors occur during running [OK]

Common Mistakes:

• Thinking exceptions occur when program runs fine
• Confusing exceptions with normal program flow
• Believing exceptions happen without any error

2. Which of the following is the correct way to catch an exception in Python?

easy

A. try:
# code
except Exception:
# handle error

B. catch:
# code
try Exception:
# handle error

C. handle:
# code
catch Exception:
# handle error

D. try:
# code
catch Exception:
# handle error

Solution

1. Step 1: Recall Python syntax for exception handling

   Python uses try to run code and except to catch errors.

2. Step 2: Match the correct syntax

   try: # code except Exception: # handle error uses try and except Exception, which is correct.

3. Final Answer:

   try:\n # code\nexcept Exception:\n # handle error -> Option A

4. Quick Check:

   Use try and except keywords [OK]

Hint: Remember: try and except catch errors in Python [OK]

Common Mistakes:

• Using catch instead of except
• Swapping try and except keywords
• Incorrect keyword order or spelling

3. What will be the output of this code?

try:
    x = 5 / 0
except ZeroDivisionError:
    print('Cannot divide by zero')

medium

A. No output

B. 5

C. ZeroDivisionError

D. Cannot divide by zero

Solution

1. Step 1: Analyze the code inside try block

   The code tries to divide 5 by 0, which causes a ZeroDivisionError.

2. Step 2: Check the except block

   The except block catches ZeroDivisionError and prints 'Cannot divide by zero'.

3. Final Answer:

   Cannot divide by zero -> Option D

4. Quick Check:

   ZeroDivisionError caught prints message [OK]

Hint: Division by zero triggers ZeroDivisionError [OK]

Common Mistakes:

• Expecting program to crash without output
• Confusing error name with printed message
• Ignoring except block handling

4. Find the error in this code snippet:

try:
    print(10 / 2)
except ZeroDivisionError
    print('Error')

medium

A. No error in the code

B. Wrong exception type used

C. Missing colon after except statement

D. Missing try keyword

Solution

1. Step 1: Check syntax of except statement

   The except line lacks a colon at the end, which is required in Python.

2. Step 2: Confirm other parts are correct

   try keyword and exception type are correct; only colon is missing.

3. Final Answer:

   Missing colon after except statement -> Option C

4. Quick Check:

   except line must end with colon [OK]

Hint: Always put colon after except statement [OK]

Common Mistakes:

• Forgetting colon after except
• Assuming wrong exception type causes syntax error
• Thinking try keyword is missing

5. You want to open a file and read its content, but the file might not exist. Which code correctly handles this exception?

hard

A. try:
with open('data.txt') as f:
print(f.read())
except:
pass

B. try:
with open('data.txt') as f:
print(f.read())
except FileNotFoundError:
print('File not found')

C. try:
with open('data.txt') as f:
print(f.read())
except ZeroDivisionError:
print('File not found')

D. with open('data.txt') as f:
print(f.read())
except FileNotFoundError:
print('File not found')

Solution

1. Step 1: Understand the problem

   Opening a file that may not exist can cause FileNotFoundError.

2. Step 2: Check which option correctly catches FileNotFoundError

   try: with open('data.txt') as f: print(f.read()) except FileNotFoundError: print('File not found') uses try-except with FileNotFoundError and prints a message, which is correct.

3. Final Answer:

   try:\n with open('data.txt') as f:\n print(f.read())\nexcept FileNotFoundError:\n print('File not found') -> Option B

4. Quick Check:

   Catch FileNotFoundError to handle missing files [OK]

Hint: Catch FileNotFoundError to handle missing files [OK]

Common Mistakes:

• Placing except outside try block
• Catching wrong exception type
• Ignoring exception handling completely