Custom error messages in Python - Time & Space Complexity

Time Complexity: Custom error messages

O(n)

Understanding Time Complexity

When we add custom error messages in code, it helps us understand problems better.

We want to see how adding these messages affects how long the program takes to run.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

    def divide_numbers(a, b):
        if b == 0:
            raise ValueError("Cannot divide by zero!")
        return a / b
    
    for i in range(1, 1000):
        try:
            result = divide_numbers(10, i - 500)
        except ValueError as e:
            print(e)
    

This code tries to divide 10 by numbers from -499 to 499, raising a custom error if dividing by zero.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for-loop runs 999 times, calling the divide function each time.
• How many times: Exactly 999 times, once per loop iteration.

How Execution Grows With Input

Explain the growth pattern intuitively.

Input Size (n)	Approx. Operations
10	About 10 calls to divide_numbers
100	About 100 calls to divide_numbers
1000	About 999 calls to divide_numbers

Pattern observation: The number of operations grows directly with the input size; doubling input doubles work.

Final Time Complexity

Time Complexity: O(n)

This means the time it takes grows in a straight line as the input size grows.

Common Mistake

[X] Wrong: "Adding custom error messages makes the program much slower because of the extra text."

[OK] Correct: The error message text is just a small part and only runs when an error happens, so it does not change the overall growth with input size.

Interview Connect

Understanding how error handling affects performance shows you can write clear and efficient code, a skill valued in real projects.

Self-Check

"What if we changed the loop to run inside another loop of the same size? How would the time complexity change?"

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