Try–except–else behavior in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the try block in Python?

The try block contains code that might cause an error. Python runs this code and watches for exceptions (errors).

beginner

What happens when an exception occurs inside a try block?

Python stops running the try block and looks for a matching except block to handle the error.

intermediate

What is the role of the else block in a try–except–else structure?

The else block runs only if the try block did NOT raise any exceptions. It is for code that should run when everything goes well.

intermediate

When does the else block NOT run in a try–except–else structure?

The else block does NOT run if an exception was raised in the try block, even if the exception was caught by except.

advanced

Why use else instead of putting code after the try-except blocks?

Using else clearly shows which code runs only when no errors happen. It keeps the code organized and avoids accidentally running code after an error.

What part of try–except–else runs only if no error occurs in the try block?

A else

B except

C finally

D try

The else block runs only if the try block completes without any exceptions.

If an exception occurs in the try block and is caught, does the else block run?

A Yes, always

B Only if the exception is not caught

C Only if the exception is ValueError

D No, never

The else block does not run if any exception occurs in the try block, even if it is caught.

Where should you put code that must run regardless of errors?

A try block

B finally block

C else block

D except block

The finally block runs no matter what, whether an exception occurs or not.

What happens if no except block matches the exception raised in try?

A The except block runs anyway

B The else block runs

C The program crashes with the error

D The finally block is skipped

If no except block matches, the error is not handled and the program crashes.

Why might you use an else block after try-except?

A To run code only if no error happened

B To catch exceptions

C To run code only if an error happened

D To always run code

The else block runs only if no exceptions occur in the try block.

Explain the flow of execution in a try–except–else structure when no exceptions occur.

Think about what happens when everything goes smoothly.

Describe what happens when an exception is raised and caught in the try–except–else structure.

Focus on how the program handles errors.

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