Custom error messages in Python - Step-by-Step Execution

Concept Flow - Custom error messages

Start

↓

Run code

↓

Error occurs?

No→Continue normal flow

Yes↓

Raise error with custom message

↓

Catch error (optional)

↓

Show custom error message

↓

End

The program runs code and if an error happens, it raises a custom message to explain what went wrong.

Execution Sample

Python

def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero!")
    return a / b

result = divide(10, 0)

This code tries to divide 10 by 0 and raises a custom error message instead of crashing silently.

Execution Table

Step	Action	Condition	Result	Output/Error
1	Call divide(10, 0)	b == 0?	True	Raise ValueError
2	Raise error	N/A	Error raised with message	"Cannot divide by zero!"
3	Program stops or error caught	N/A	Error message shown	ValueError: Cannot divide by zero!

💡 Error raised because divisor b is zero, stopping normal execution

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
a	N/A	10	10	10
b	N/A	0	0	0
result	N/A	N/A	N/A	No value due to error

Key Moments - 3 Insights

Why does the program stop after raising the error?

Because raising an error interrupts normal flow immediately, as shown in execution_table step 2, so no further code runs unless the error is caught.

What happens if the condition b == 0 is False?

The function returns the division result normally without raising an error, continuing the program flow.

Can we change the error message to be more helpful?

Yes, the message inside raise ValueError(...) can be any string to explain the problem clearly, as seen in the custom message "Cannot divide by zero!".

Visual Quiz - 3 Questions

Look at the execution_table, what is the condition checked at step 1?

A b == 0

B a == 0

C result == 0

D a / b == 0

💡 Hint

Check the 'Condition' column in execution_table row 1

At which step does the program raise the custom error message?

A Step 1

B Step 2

C Step 3

D No error is raised

💡 Hint

Look at the 'Action' and 'Result' columns in execution_table row 2

If we change b to 2, what would happen in the execution_table?

A Error raised at step 2

B Program stops immediately

C Condition b == 0 is False, function returns result

D Error message changes automatically

💡 Hint

Refer to key_moments about condition being False and normal return

Concept Snapshot

Custom error messages in Python:
Use raise with an error type and a string message.
Example: raise ValueError("message")
This stops code and shows your message.
Helps explain problems clearly.
Can be caught with try-except to handle errors.

Full Transcript

This visual trace shows how custom error messages work in Python. The code calls a function divide(10, 0). It checks if the divisor b is zero. Since it is, the program raises a ValueError with the message "Cannot divide by zero!". This stops the program unless the error is caught. Variables a and b hold 10 and 0 respectively. The result variable never gets a value because the error stops execution. Key moments explain why the program stops and how the message helps. The quiz asks about the condition checked, when the error is raised, and what happens if b changes. The snapshot summarizes how to use raise with a message to make errors clear and helpful.

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