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

Try-except-else 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 nicely and also do something special when there is no error.

🎯 Goal: You will create a program that tries to divide two numbers, catches division errors, and uses else to print a success message only when no error happens.

📋 What You'll Learn

Create two variables numerator and denominator with exact values

Create a variable result and set it to None

Use a try block to divide numerator by denominator and store in result

Use an except ZeroDivisionError block to set result to the string 'Cannot divide by zero'

Use an else block to print 'Division successful'

Finally, print the value of result

💡 Why This Matters

🌍 Real World

Handling errors like division by zero is common in calculators, data processing, and user input validation.

💼 Career

Understanding try-except-else helps you write robust code that doesn't crash and gives clear feedback, a key skill for any programmer.

1

Create numerator and denominator variables

Create a variable called numerator and set it to 10. Create another variable called denominator and set it to 2.

Python

# Your code here

Hint

Use simple assignment to create the two variables with the exact numbers.

2

Create a result variable

Create a variable called result and set it to None.

Python

numerator = 10
denominator = 2
# Your code here

Hint

Initialize result to None before the try block.

3

Write try-except-else to divide numbers

Write a try block where you divide numerator by denominator and assign it to result. Add an except ZeroDivisionError block that sets result to the string 'Cannot divide by zero'. Add an else block that prints 'Division successful'.

Python

numerator = 10
denominator = 2
result = None
# Your code here

Hint

Use the exact keywords and variable names as shown. The else block runs only if no error happens.

4

Print the result

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

Python

numerator = 10
denominator = 2
result = None
try:
    result = numerator / denominator
except ZeroDivisionError:
    result = 'Cannot divide by zero'
else:
    print('Division successful')
# Your code here

Hint

The program should print Division successful first, then the number 5.0.

Practice

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

easy

A. Runs only if no error occurs in the try block

B. Runs only if an error occurs in the try block

C. Always runs regardless of errors

D. Runs before the try block

Solution

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

   The try block runs code that might cause an error. If an error happens, the except block runs.

2. Step 2: Role of else block

   The else block runs only if no error occurs in the try block, meaning the code succeeded without exceptions.

3. Final Answer:

   Runs only if no error occurs in the try block -> Option A

4. Quick Check:

   else runs if no error = A [OK]

Hint: Else runs only when try succeeds without errors [OK]

Common Mistakes:

• Thinking else runs after except
• Assuming else runs always
• Confusing else with finally

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

easy

A. try:
pass
finally:
pass
else:
pass

B. try:
pass
else:
pass
except:
pass

C. try:
pass
except:
pass
else:
pass

D. except:
pass
try:
pass
else:
pass

Solution

1. Step 1: Recall correct order of blocks

   The correct order is try, then except, then else. The else block must come after except.

2. Step 2: Check each option

   try: pass except: pass else: pass follows the correct order and syntax. Options A, B, and D have wrong order or misplaced blocks.

3. Final Answer:

   try: pass except: pass else: pass -> Option C

4. Quick Check:

   try-except-else order = C [OK]

Hint: Remember order: try, except, else [OK]

Common Mistakes:

• Placing else before except
• Using else after finally
• Starting with except block

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

try:
    print("Start")
    x = 1 / 1
except ZeroDivisionError:
    print("Error")
else:
    print("No Error")
print("End")

medium

A. Start\nNo Error\nEnd

B. Start\nEnd

C. Error\nNo Error\nEnd

D. Start\nError\nEnd

Solution

1. Step 1: Analyze try block execution

   The code prints "Start" and calculates 1/1 which is 1, no error occurs.

2. Step 2: Determine which blocks run

   Since no error, except block is skipped, else block runs printing "No Error", then "End" prints after.

3. Final Answer:

   Start No Error End -> Option A

4. Quick Check:

   No error means else runs = D [OK]

Hint: If no error, else runs after try [OK]

Common Mistakes:

• Thinking except runs without error
• Ignoring else block output
• Missing that print("End") always runs

4. Identify the error in this code snippet:

try:
    print(10 / 0)
else:
    print("No error")
except ZeroDivisionError:
    print("Error occurred")

medium

A. Syntax is correct

B. except block is missing

C. try block is empty

D. else block is before except block

Solution

1. Step 1: Check block order in try-except-else

   The correct order is try, except, then else. Here, else comes before except which is invalid syntax.

2. Step 2: Confirm syntax error

   Python raises a syntax error because else must follow except, not precede it.

3. Final Answer:

   else block is before except block -> Option D

4. Quick Check:

   else must come after except = A [OK]

Hint: Else must follow except, not before [OK]

Common Mistakes:

• Placing else before except
• Forgetting except block
• Misordering try-except-else blocks

5. Consider this code:

def check_value(val):
    try:
        result = 10 / val
    except ZeroDivisionError:
        return "Cannot divide by zero"
    else:
        return f"Result is {result}"

print(check_value(0))
print(check_value(5))

What is the output?

hard

A. Result is 0.0\nCannot divide by zero

B. Cannot divide by zero\nResult is 2.0

C. Cannot divide by zero\nResult is 0

D. Error at runtime

Solution

1. Step 1: Analyze call with 0

   When val=0, division causes ZeroDivisionError, so except block returns "Cannot divide by zero".

2. Step 2: Analyze call with 5

   When val=5, division succeeds (10/5=2.0), so else block returns "Result is 2.0".

3. Final Answer:

   Cannot divide by zero Result is 2.0 -> Option B

4. Quick Check:

   ZeroDivision triggers except, else runs if no error = B [OK]

Hint: Except returns on error; else returns on success [OK]

Common Mistakes:

• Assuming else runs even if error occurs
• Confusing output order
• Expecting runtime error instead of handled exception