Try–except–finally behavior in Python - Mini Project: Build & Apply

Try-except-finally behavior

📖 Scenario: Imagine you are writing a small program that divides numbers. Sometimes, the user might enter zero as the divisor, which causes an error. You want to handle this error gracefully and always print a message at the end.

🎯 Goal: You will create a program that divides two numbers, handles division by zero using try and except, and always prints a final message using finally.

📋 What You'll Learn

Create two variables with exact values for dividend and divisor

Create a variable to store the result initialized to None

Use a try-except-finally block to perform division and handle ZeroDivisionError

Print the result or an error message

Always print a final message regardless of error

💡 Why This Matters

🌍 Real World

Handling errors like division by zero is common in programs that do calculations, such as calculators or financial apps.

💼 Career

Understanding try-except-finally is important for writing robust code that doesn't crash unexpectedly, a key skill for any programmer.

1

DATA SETUP: Create variables for dividend and divisor

Create two variables called dividend and divisor with values 10 and 0 respectively.

Python

# Your code here

Hint

Use simple assignment like dividend = 10 and divisor = 0.

2

CONFIGURATION: Create a variable to store the result

Create a variable called result and set it to None.

Python

dividend = 10
divisor = 0
# Your code here

Hint

Initialize result to None before the try block.

3

CORE LOGIC: Use try-except-finally to divide and handle errors

Write a try block that divides dividend by divisor and assigns it to result. Add an except ZeroDivisionError block that sets result to the string 'Cannot divide by zero'. Add a finally block that prints 'Execution completed'.

Python

dividend = 10
divisor = 0
result = None
# Your code here

Hint

Use try, except ZeroDivisionError, and finally blocks exactly as described.

4

OUTPUT: Print the result

Write a print(result) statement to display the value of result.

Python

dividend = 10
divisor = 0
result = None

try:
    result = dividend / divisor
except ZeroDivisionError:
    result = 'Cannot divide by zero'
finally:
    print('Execution completed')
# Your code here

Hint

Print the result after the try-except-finally block.

Practice

1. What does the finally block do in a try-except-finally structure?

easy

A. It always runs, whether an error occurs or not.

B. It runs only if an error occurs.

C. It runs only if no error occurs.

D. It runs before the try block.

Solution

1. Step 1: Understand the role of try and except

   The try block runs code that might cause an error, and except runs only if an error happens.

2. Step 2: Understand the finally block behavior

   The finally block always runs after try and except, no matter if an error occurred or not.

3. Final Answer:

   It always runs, whether an error occurs or not. -> Option A

4. Quick Check:

   finally always runs = A [OK]

Hint: Remember: finally always runs last, no matter what. [OK]

Common Mistakes:

• Thinking finally runs only on errors
• Confusing except and finally blocks
• Believing finally runs before try

2. Which of the following is the correct syntax for a try-except-finally block in Python?

easy

A. except:
pass
try:
pass
finally:
pass

B. try:
pass
finally:
pass
except:
pass

C. try:
pass
except:
pass
finally:
pass

D. try:
pass
except:
pass
else:
pass

Solution

1. Step 1: Recall the order of blocks

   The correct order is try, then except, then finally.

2. Step 2: Check each option's order

   try: pass except: pass finally: pass follows the correct order. try: pass finally: pass except: pass places finally before except, which is invalid. except: pass try: pass finally: pass starts with except, which is wrong. try: pass except: pass else: pass uses else but no finally.

3. Final Answer:

   try, except, finally in correct order -> Option C

4. Quick Check:

   try-except-finally order = C [OK]

Hint: Remember order: try, except, then finally. [OK]

Common Mistakes:

• Placing finally before except
• Starting with except block
• Confusing else with finally

3. What will be the output of this code?

try:
    print('Start')
    x = 1 / 0
except ZeroDivisionError:
    print('Error caught')
finally:
    print('Always runs')

medium

A. Start\nAlways runs

B. Start\nError caught

C. Error caught\nAlways runs

D. Start\nError caught\nAlways runs

Solution

1. Step 1: Trace the try block

   The code prints 'Start' then tries to divide by zero, causing a ZeroDivisionError.

2. Step 2: Handle the exception and finally block

   The except block catches the error and prints 'Error caught'. Then the finally block runs and prints 'Always runs'.

3. Final Answer:

   Start\nError caught\nAlways runs -> Option D

4. Quick Check:

   try prints + except prints + finally prints = A [OK]

Hint: finally always prints last, even after except. [OK]

Common Mistakes:

• Forgetting finally runs
• Assuming code stops after error
• Missing the initial print before error

4. Find the error in this code snippet:

try:
    print('Hello')
except:
    print('Error')
finally
    print('Done')

medium

A. Missing colon after except

B. Missing colon after finally

C. Indentation error in try block

D. No error, code is correct

Solution

1. Step 1: Check syntax of try-except-finally

   Each block header must end with a colon (:). The finally line is missing a colon.

2. Step 2: Verify other parts

   The except line has a colon, and indentation is correct.

3. Final Answer:

   Missing colon after finally -> Option B

4. Quick Check:

   Colon needed after finally = B [OK]

Hint: Check colons after try, except, finally lines. [OK]

Common Mistakes:

• Ignoring missing colon errors
• Confusing except and finally syntax
• Assuming indentation fixes missing colon

5. Consider this code:

def test():
    try:
        return 'try'
    except:
        return 'except'
    finally:
        return 'finally'

result = test()
print(result)

What will be printed?

hard

A. finally

B. except

C. None

D. try

Solution

1. Step 1: Understand return in try and finally

   The try block returns 'try', but the finally block also has a return statement.

2. Step 2: Know that finally return overrides others

   In Python, if finally has a return, it overrides any previous return from try or except.

3. Step 3: Determine final output

   The function returns 'finally', so print(result) outputs 'finally'.

4. Final Answer:

   finally -> Option A

5. Quick Check:

   finally return overrides try/except returns = D [OK]

Hint: Return in finally overrides try/except returns. [OK]

Common Mistakes:

• Thinking try return is final
• Ignoring finally's return effect
• Assuming except runs without error