Raising exceptions in Python - Step-by-Step Execution

Concept Flow - Raising exceptions

Start

↓

Check condition

Yes↓

Raise exception

↓

Exception propagates

↓

Handle exception or crash

No↓

Continue normal flow

↓

End

The program checks a condition, raises an exception if true, then the exception moves up until handled or program stops.

Execution Sample

Python

def check_age(age):
    if age < 18:
        raise ValueError("Too young")
    return "Allowed"

This function raises a ValueError if age is less than 18, otherwise returns 'Allowed'.

Execution Table

Step	Action	Condition	Result	Output/Exception
1	Call check_age(16)	age < 18?	16 < 18 is True	Raise ValueError('Too young')
2	Exception propagates	No further code runs	Exception not caught here	ValueError('Too young')
3	Call check_age(20)	age < 18?	20 < 18 is False	Return 'Allowed'

💡 Execution stops at exception if not caught; otherwise continues normally.

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
age	N/A	16	20	20

Key Moments - 3 Insights

Why does the function stop running after raising the exception?

When the exception is raised (see Step 1 in execution_table), the function immediately stops and does not run the return statement.

What happens if the exception is not caught?

The exception moves up the call stack and if no code catches it, the program stops with an error (Step 2).

Why does check_age(20) return 'Allowed'?

Because the condition age < 18 is false (Step 3), so no exception is raised and the function returns normally.

Visual Quiz - 3 Questions

Look at the execution_table, what happens at Step 1 when age is 16?

A The function returns 'Allowed'

B The function raises a ValueError

C The function continues without any action

D The function prints 'Too young'

💡 Hint

Check the 'Output/Exception' column at Step 1 in execution_table.

At which step does the function return a normal value instead of raising an exception?

A Step 3

B Step 1

C Step 2

D None

💡 Hint

Look for the row where 'Output/Exception' shows a return value in execution_table.

If we change the age to 18, what would happen according to the execution_table logic?

A Raise ValueError

B Raise a different exception

C Return 'Allowed'

D Function does nothing

💡 Hint

Check the condition 'age < 18' and what happens when it is false in execution_table.

Concept Snapshot

Raising exceptions:
Use 'raise ExceptionType("message")' to stop normal flow.
Exception moves up until caught or program stops.
If condition triggers, raise exception.
Otherwise, continue normal execution.

Full Transcript

This visual execution shows how raising exceptions works in Python. The function check_age checks if age is less than 18. If yes, it raises a ValueError and stops running. The exception moves up until caught or program ends with error. If age is 20, no exception is raised and function returns 'Allowed'. This helps stop wrong inputs and handle errors clearly.