Try–except execution flow in Python - Mini Project: Build & Apply

Try-except execution flow

📖 Scenario: You are writing a simple calculator program that divides two numbers. Sometimes users might enter zero as the divisor, which causes an error. You want to handle this error gracefully.

🎯 Goal: Build a program that safely divides two numbers using try and except to catch division errors and print a friendly message.

📋 What You'll Learn

Create two variables with exact values for numerator and denominator

Create a variable to store the error message

Use a try-except block to perform division and catch ZeroDivisionError

Print the result or the error message

💡 Why This Matters

🌍 Real World

Handling errors like division by zero is common in calculators, data processing, and user input validation to keep programs running smoothly.

💼 Career

Knowing how to use try-except blocks is essential for writing robust software that can handle unexpected problems without crashing.

1

DATA SETUP: Create numerator and denominator variables

Create two variables called numerator and denominator with values 10 and 0 respectively.

Python

# Create numerator and denominator variables
# Your code here

Hint

Use = to assign values to variables.

2

CONFIGURATION: Create an error message variable

Create a variable called error_message and set it to the string 'Cannot divide by zero.'.

Python

numerator = 10
denominator = 0
# Create error_message variable
# Your code here

Hint

Remember to use quotes for strings.

3

CORE LOGIC: Use try-except to divide safely

Write a try block that divides numerator by denominator and stores the result in result. Add an except ZeroDivisionError block that sets result to error_message.

Python

numerator = 10
denominator = 0
error_message = 'Cannot divide by zero.'
# Use try-except to divide numerator by denominator
# Your code here

Hint

Use try: and except ZeroDivisionError: blocks.

4

OUTPUT: Print the result

Write a print statement to display the value of result.

Python

numerator = 10
denominator = 0
error_message = 'Cannot divide by zero.'
try:
    result = numerator / denominator
except ZeroDivisionError:
    result = error_message
# Print the result
# Your code here

Hint

Use print(result) to show the output.

Practice

1. What is the main purpose of using a try-except block in Python?

easy

A. To speed up the program execution

B. To handle errors and prevent the program from crashing

C. To repeat a block of code multiple times

D. To define a new function

Solution

1. Step 1: Understand the role of try block

   The try block contains code that might cause an error during execution.

2. Step 2: Understand the role of except block

   The except block catches and handles the error so the program does not stop abruptly.

3. Final Answer:

   To handle errors and prevent the program from crashing -> Option B

4. Quick Check:

   Try-except handles errors = B [OK]

Hint: Try-except blocks catch errors to keep programs running [OK]

Common Mistakes:

• Thinking try-except speeds up code
• Confusing try-except with loops
• Using try-except to define functions

2. Which of the following is the correct syntax to catch a ZeroDivisionError in Python?

easy

A. try:
x = 1/0
except ZeroDivisionError:
print('Cannot divide by zero')

B. try:
x = 1/0
catch ZeroDivisionError:
print('Cannot divide by zero')

C. try:
x = 1/0
except:
print('Error')
finally ZeroDivisionError:

D. try:
x = 1/0
except ZeroDivisionError then:
print('Cannot divide by zero')

Solution

1. Step 1: Identify correct try-except syntax

   Python uses try: followed by except ExceptionType: to catch errors.

2. Step 2: Check each option for syntax errors

   try: x = 1/0 except ZeroDivisionError: print('Cannot divide by zero') uses correct except ZeroDivisionError: syntax; others use invalid keywords like catch or incorrect formatting.

3. Final Answer:

   try: x = 1/0 except ZeroDivisionError: print('Cannot divide by zero') -> Option A

4. Quick Check:

   Correct except syntax = A [OK]

Hint: Use 'except ExceptionType:' to catch specific errors [OK]

Common Mistakes:

• Using 'catch' instead of 'except'
• Adding 'then' after except
• Misplacing 'finally' keyword

3. What will be the output of the following code?

try:
    print('Start')
    x = 5 / 0
    print('End')
except ZeroDivisionError:
    print('Error caught')
print('Done')

medium

A. Start
End
Error caught
Done

B. Error caught
Done

C. Start
Done

D. Start
Error caught
Done

Solution

1. Step 1: Trace code inside try block

   It prints 'Start', then tries to divide 5 by 0, which raises ZeroDivisionError before printing 'End'.

2. Step 2: Handle exception and continue

   The except block catches the error and prints 'Error caught'. After that, the program continues and prints 'Done'.

3. Final Answer:

   Start Error caught Done -> Option D

4. Quick Check:

   Exception stops try block, except runs = C [OK]

Hint: Error stops try; except runs; code after try-except runs [OK]

Common Mistakes:

• Assuming 'End' prints after error
• Missing that except block runs
• Thinking program stops after error

4. Identify the error in this code snippet:

try:
    print('Hello')
except ValueError
    print('Value error occurred')

medium

A. try block cannot have print statements

B. Missing parentheses after except

C. Missing colon ':' after except ValueError

D. except block must come before try block

Solution

1. Step 1: Check syntax of except statement

   The except line must end with a colon ':' to define the block.

2. Step 2: Identify missing colon

   In the code, except ValueError is missing the colon, causing a syntax error.

3. Final Answer:

   Missing colon ':' after except ValueError -> Option C

4. Quick Check:

   except line needs ':' = A [OK]

Hint: Always put ':' after except ExceptionType [OK]

Common Mistakes:

• Forgetting colon after except
• Adding parentheses after except
• Misordering try and except blocks

5. You want to safely convert user input to an integer, using try-except-else and catching ValueError specifically, printing 'Invalid input' for invalid input and 'Input is', num for valid input, without stopping the program. Which code does this?

hard

A. try:
num = int(input('Enter number: '))
except ValueError:
print('Invalid input')
else:
print('Input is', num)

B. try:
num = int(input('Enter number: '))
except:
print('Invalid input')
else:
print('Input is', num)

C. try:
num = int(input('Enter number: '))
except ValueError:
print('Invalid input')
finally:
print('Done')

D. try:
num = int(input('Enter number: '))
except ValueError:
print('Invalid input')

Solution

1. Step 1: Understand try-except-else structure

   The try block attempts conversion; except handles errors; else runs if no error occurs.

2. Step 2: Check which option prints 'Invalid input' on error and shows input if valid

   try: num = int(input('Enter number: ')) except ValueError: print('Invalid input') else: print('Input is', num) correctly prints 'Invalid input' on ValueError and prints the number if conversion succeeds.

3. Final Answer:

   try: num = int(input('Enter number: ')) except ValueError: print('Invalid input') else: print('Input is', num) -> Option A

4. Quick Check:

   Use except for errors, else for success = D [OK]

Hint: Use except for errors and else for success actions [OK]

Common Mistakes:

• Not using else to handle successful input
• Catching all exceptions without specifying
• Missing error handling causing crash