Creating exception classes in Python - Visual Walkthrough

Concept Flow - Creating exception classes

Define new Exception class

↓

Raise the new Exception

↓

Catch the Exception with try-except

↓

Handle or print error message

↓

End

You first create a new exception class, then raise it in code, catch it using try-except, and finally handle or display the error.

Execution Sample

Python

class MyError(Exception):
    pass

try:
    raise MyError("Oops!")
except MyError as e:
    print(e)

This code defines a new exception class, raises it, catches it, and prints the error message.

Execution Table

Step	Action	Evaluation	Result
1	Define class MyError(Exception)	Class created	MyError is ready to use
2	Enter try block	No error yet	Proceed
3	Execute raise MyError("Oops!")	Raise exception	Exception MyError("Oops!") raised
4	Exception caught by except MyError as e	Catch exception	Variable e holds MyError("Oops!")
5	Execute print(e)	Print error message	Output: Oops!
6	End of try-except block	No more code	Program ends normally

💡 Exception raised and caught, program ends after handling

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
e	undefined	undefined	MyError('Oops!')	MyError('Oops!')

Key Moments - 3 Insights

Why do we inherit from Exception when creating MyError?

Inheriting from Exception makes MyError a proper error type that can be raised and caught, as shown in step 1 and step 3 of the execution_table.

What happens if we raise MyError but do not catch it?

If not caught, the program stops with an error message. Here, step 4 shows catching the error to prevent a crash.

Why do we use 'as e' in except MyError as e?

'as e' stores the caught exception in variable e, so we can access its message, as seen in step 4 and step 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of variable 'e' after step 4?

A None

B undefined

C MyError('Oops!')

D Exception

💡 Hint

Check the variable_tracker row for 'e' after step 4.

At which step is the exception actually raised?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Action' column in execution_table where 'raise' is mentioned.

If we remove the except block, what happens after step 3?

A Program crashes with error

B Program continues normally

C Exception is ignored

D Exception is caught automatically

💡 Hint

Refer to key_moments about what happens if exception is not caught.

Concept Snapshot

Create a new exception by defining a class inheriting Exception.
Raise it with 'raise MyError("message")'.
Catch it using try-except: 'except MyError as e:'.
Use 'e' to access the error message.
This helps handle errors clearly and safely.

Full Transcript

We start by defining a new exception class called MyError that inherits from Exception. This makes it a proper error type. Then, inside a try block, we raise this error with a message 'Oops!'. The program jumps to the except block that catches MyError and stores it in variable e. We print e, which shows the message. Finally, the program ends normally after handling the error. This process helps us create and manage custom errors in Python.