Custom error messages in Python - Mini Project: Build & Apply

Custom error messages

📖 Scenario: You are creating a simple program that asks a user to enter their age. You want to make sure the age is a positive number. If the user enters a wrong value, you will show a clear custom error message to help them understand the mistake.

🎯 Goal: Build a program that checks the user's age input and shows a custom error message if the input is not a positive number.

📋 What You'll Learn

Create a variable called age_input with the value '-5' (a string).

Create a variable called error_message with the value 'Age must be a positive integer.'.

Write a try block that converts age_input to an integer called age.

If age is less than or equal to zero, raise a ValueError with the error_message.

Write an except block that catches ValueError and prints the error_message.

Print age if no error occurs.

💡 Why This Matters

🌍 Real World

Custom error messages help users understand what went wrong and how to fix it, improving user experience in apps and websites.

💼 Career

Many programming jobs require writing clear error handling to make software reliable and user-friendly.

1

Create the input variable

Create a variable called age_input and set it to the string '-5'.

Python

# Create a variable called age_input with the value '-5'
age_input = ''  # Your code here

Hint

Use age_input = '-5' to create the variable.

2

Create the error message variable

Create a variable called error_message and set it to the string 'Age must be a positive integer.'.

Python

age_input = '-5'
# Create a variable called error_message with the value 'Age must be a positive integer.'
error_message = ''  # Your code here

Hint

Use error_message = 'Age must be a positive integer.' to create the variable.

3

Add the try block and check age

Write a try block that converts age_input to an integer called age. Then check if age is less than or equal to zero. If yes, raise a ValueError with the error_message.

Python

age_input = '-5'
error_message = 'Age must be a positive integer.'

# Write a try block to convert age_input to int and check if age <= 0
try:
    pass  # Your code here

Hint

Use age = int(age_input) inside the try block. Then check if age <= 0 and raise ValueError(error_message).

4

Add except block and print age

Add an except ValueError block that prints the error_message. After the try-except, print the variable age if no error occurs.

Python

age_input = '-5'
error_message = 'Age must be a positive integer.'

try:
    age = int(age_input)
    if age <= 0:
        raise ValueError(error_message)
except ValueError:
    pass  # Your code here

# Print age if no error

Hint

Use except ValueError: and inside it print(error_message). Use else: to print age if no error.

Practice

1. What is the main purpose of using raise ValueError('Custom message') in Python?

easy

A. To print a warning message without stopping the program.

B. To stop the program and show a specific error message when a condition is not met.

C. To automatically fix errors in the code.

D. To ignore errors and continue running the program.

Solution

1. Step 1: Understand the raise statement

   The raise keyword is used to stop the program and throw an error.

2. Step 2: Purpose of custom messages

   Adding a message like 'Custom message' helps explain why the error happened.

3. Final Answer:

   To stop the program and show a specific error message when a condition is not met. -> Option B

4. Quick Check:

   raise with message = stop and explain error [OK]

Hint: Raise errors to stop and explain problems clearly [OK]

Common Mistakes:

• Thinking raise only prints messages without stopping
• Confusing raise with print or logging
• Believing raise fixes errors automatically

2. Which of the following is the correct syntax to raise a custom error with message "Invalid input"?

easy

A. raise 'Invalid input' ValueError

B. throw ValueError('Invalid input')

C. error ValueError('Invalid input')

D. raise ValueError('Invalid input')

Solution

1. Step 1: Identify the correct keyword

   In Python, raise is used to throw errors, not throw or error.

2. Step 2: Correct order of error and message

   The syntax is raise ErrorType('message'), so the error type comes first, then the message in parentheses.

3. Final Answer:

   raise ValueError('Invalid input') -> Option D

4. Quick Check:

   raise + ErrorType('message') = correct syntax [OK]

Hint: Use raise ErrorType('message') to create custom errors [OK]

Common Mistakes:

• Using throw instead of raise
• Placing message before error type
• Missing parentheses around the message

3. What will be the output of this code?

def check_age(age):
    if age < 18:
        raise ValueError('Age must be 18 or older')
    return 'Access granted'

print(check_age(16))

medium

A. Access granted

B. None

C. ValueError: Age must be 18 or older

D. SyntaxError

Solution

1. Step 1: Check the condition in the function

   The function raises a ValueError if age is less than 18. Here, age is 16, so the error triggers.

2. Step 2: Understand what happens on raise

   When the error is raised, the program stops and shows the error message instead of returning 'Access granted'.

3. Final Answer:

   ValueError: Age must be 18 or older -> Option C

4. Quick Check:

   raise triggers error output = ValueError message [OK]

Hint: If condition fails, raise stops and shows error message [OK]

Common Mistakes:

• Expecting function to return 'Access granted' anyway
• Confusing error message with print output
• Thinking raise prints message but continues

4. Find the error in this code snippet:

def check_number(num):
    if num < 0:
        raise 'Negative number error'
    return 'Number is positive'

print(check_number(-5))

medium

A. You cannot raise a string directly; it must be an Exception type.

B. The raise statement is missing parentheses.

C. The function should return None instead of a string.

D. The if condition should be num > 0.

Solution

1. Step 1: Check the raise statement

   The code tries to raise a string directly, which is not allowed in Python. Only Exception types can be raised.

2. Step 2: Correct way to raise errors

   Use raise ValueError('message') or another Exception class, not a plain string.

3. Final Answer:

   You cannot raise a string directly; it must be an Exception type. -> Option A

4. Quick Check:

   raise must use Exception type, not string [OK]

Hint: Always raise Exception objects, not strings [OK]

Common Mistakes:

• Raising strings instead of Exception classes
• Forgetting to include parentheses with message
• Changing condition incorrectly

5. You want to create a function validate_score(score) that raises a ValueError with the message "Score must be between 0 and 100" if the score is outside this range. Which code correctly implements this?

hard

A. def validate_score(score):
if score < 0 or score > 100:
raise ValueError('Score must be between 0 and 100')
return 'Valid score'

B. def validate_score(score):
if 0 <= score <= 100:
raise ValueError('Score must be between 0 and 100')
return 'Valid score'

C. def validate_score(score):
if score < 0 and score > 100:
raise ValueError('Score must be between 0 and 100')
return 'Valid score'

D. def validate_score(score):
if score == 0 or score == 100:
raise ValueError('Score must be between 0 and 100')
return 'Valid score'

Solution

1. Step 1: Understand the valid range condition

   The score is valid if it is between 0 and 100 inclusive. So invalid means less than 0 or greater than 100.

2. Step 2: Check the if condition logic

   def validate_score(score): if score < 0 or score > 100: raise ValueError('Score must be between 0 and 100') return 'Valid score' correctly uses if score < 0 or score > 100 to detect invalid scores and raise the error.

3. Final Answer:

   def validate_score(score): if score < 0 or score > 100: raise ValueError('Score must be between 0 and 100') return 'Valid score' -> Option A

4. Quick Check:

   Use or for invalid range, raise error if outside [OK]

Hint: Raise error if score is less than 0 or greater than 100 [OK]

Common Mistakes:

• Using and instead of or in condition
• Raising error for valid scores instead of invalid
• Checking only equality instead of range