Try–except execution flow in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of a try block in Python?

The try block lets you test a block of code for errors. If an error occurs, Python stops running the try block and looks for an except block to handle the error.

beginner

What happens when an exception occurs inside a try block?

Python immediately stops executing the try block and jumps to the matching except block to handle the error. If no matching except block is found, the program crashes.

beginner

Can code after the error inside a try block run if an exception occurs?

No. Once an exception happens, the rest of the try block is skipped. The program moves to the except block.

intermediate

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

The else block runs only if no exceptions were raised in the try block. It is useful for code that should run only when everything in try succeeds.

intermediate

How does the finally block behave in a try-except structure?

The finally block always runs after the try and except blocks, no matter what. It is used for cleanup actions like closing files or releasing resources.

What happens if no exception occurs in the try block and there is an except block?

A The <code>try</code> block runs again.

B The <code>except</code> block runs anyway.

C The program crashes.

D The <code>except</code> block is skipped.

If no error happens, Python skips the except block and continues normally.

Where does Python jump when an exception occurs inside a try block?

A To the <code>except</code> block.

B To the next line in the <code>try</code> block.

C To the <code>else</code> block.

D To the <code>finally</code> block only.

Python stops the try block and jumps to the matching except block to handle the error.

When does the else block run in a try-except structure?

A Always, before the <code>try</code> block.

B Only if an exception occurs.

C Only if no exception occurs.

D Only if the <code>finally</code> block is missing.

The else block runs only if the try block finishes without errors.

What is guaranteed about the finally block?

A It runs only if an exception occurs.

B It always runs, no matter what.

C It runs only if no exception occurs.

D It runs only if there is an <code>else</code> block.

The finally block always runs after try and except, even if the program crashes.

If an exception is not caught by any except block, what happens?

A The program crashes and shows an error.

B The <code>else</code> block runs.

C The program continues normally.

D The <code>finally</code> block is skipped.

If no except block matches the error, the program stops and shows the error message.

Explain the flow of execution in a try-except-else-finally structure when an exception occurs.

Think about which blocks run and which are skipped when an error happens.

Describe what happens when no exception occurs in a try-except-else-finally structure.

Focus on the normal flow without errors.

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