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.