1. Custom Exception Classes

Python allows you to create your own exception classes by inheriting from the built-in Exception class. Custom exceptions make error messages more meaningful and help handle application-specific errors.

Syntax

class CustomException(Exception):
    pass

Example

class InvalidAgeError(Exception):
    pass

age = 15

if age < 18:
    raise InvalidAgeError("Age must be 18 or above.")

print("Eligible")

Output

Traceback (most recent call last):
...
InvalidAgeError: Age must be 18 or above.

Example with Exception Handling

class InvalidAgeError(Exception):
    pass

try:
    age = 15

    if age < 18:
        raise InvalidAgeError("Age must be 18 or above.")

except InvalidAgeError as e:
    print(e)

Output

Age must be 18 or above.

2. Raising vs Catching Exceptions

Raising Exceptions

Raising an exception means deliberately generating an error when an invalid condition occurs. The raise keyword is used for this purpose.

Example

age = 16

if age < 18:
    raise ValueError("Not eligible to vote.")

Output

Traceback (most recent call last):
...
ValueError: Not eligible to vote.

Catching Exceptions

Catching an exception means handling the error using a try and except block so that the program continues running.

Example

try:
    age = 16

    if age < 18:
        raise ValueError("Not eligible to vote.")

except ValueError as e:
    print("Error:", e)

Output

Error: Not eligible to vote.

Difference Between Raising and Catching

Raising Exceptions	Catching Exceptions
Generates an exception	Handles an exception
Uses the `raise` keyword	Uses `try` and `except`
Stops normal program flow unless handled	Prevents program termination by handling the error
Used to report invalid conditions	Used to recover from errors gracefully

3. Basic Logging with the `logging` Module

The logging module is used to record events that happen while a program runs. It is more flexible than using print() statements and is commonly used for debugging and monitoring applications.

Basic Logging Example

import logging

logging.basicConfig(level=logging.INFO)

logging.info("Program started")
logging.warning("Low disk space")
logging.error("File not found")

Output

INFO:root:Program started
WARNING:root:Low disk space
ERROR:root:File not found

Logging Levels

Level	Purpose
`DEBUG`	Detailed information for debugging
`INFO`	General information about program execution
`WARNING`	Indicates a possible problem
`ERROR`	Reports an error that affected an operation
`CRITICAL`	Reports a serious error that may stop the program

Example

import logging

logging.basicConfig(level=logging.DEBUG)

logging.debug("Debug message")
logging.info("Information message")
logging.warning("Warning message")
logging.error("Error message")
logging.critical("Critical error")

Output

DEBUG:root:Debug message
INFO:root:Information message
WARNING:root:Warning message
ERROR:root:Error message
CRITICAL:root:Critical error

Logging to a File

You can store log messages in a file instead of displaying them on the console.

Example

import logging

logging.basicConfig(
    filename="app.log",
    level=logging.INFO
)

logging.info("Application started")
logging.error("An error occurred")

Contents of app.log

INFO:root:Application started
ERROR:root:An error occurred

Best Practices

Create custom exceptions for application-specific errors.
Use raise to report invalid conditions.
Use try and except to handle exceptions gracefully.
Prefer the logging module over print() for debugging and application monitoring.
Use appropriate logging levels (DEBUG, INFO, WARNING, ERROR, CRITICAL) based on the importance of the message.

Quick Revision Table

Topic	Summary
Custom Exception Classes	User-defined exceptions created by inheriting from `Exception`
`raise`	Generates an exception when an invalid condition occurs
`try` / `except`	Catches and handles exceptions
Raising Exceptions	Creates an error using `raise`
Catching Exceptions	Handles errors using `try` and `except`
`logging` Module	Records application events and errors
Logging Levels	`DEBUG`, `INFO`, `WARNING`, `ERROR`, `CRITICAL`
File Logging	Stores log messages in a file using `filename` in `basicConfig()`

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Python Exception Handling & Logging

1. Custom Exception Classes

Syntax

Example

Example with Exception Handling

2. Raising vs Catching Exceptions

Raising Exceptions

Example

Catching Exceptions

Example

Difference Between Raising and Catching

3. Basic Logging with the `logging` Module

Basic Logging Example

Logging Levels

Example

Logging to a File

Example

Best Practices

Quick Revision Table

​1. Custom Exception Classes

​Syntax

​Example

​Example with Exception Handling

​2. Raising vs Catching Exceptions

​Raising Exceptions

​Example

​Catching Exceptions

​Example

​Difference Between Raising and Catching

​3. Basic Logging with the logging Module

​Basic Logging Example

​Logging Levels

​Example

​Logging to a File

​Example

​Best Practices

​Quick Revision Table

1. Custom Exception Classes

Syntax

Example

Example with Exception Handling

2. Raising vs Catching Exceptions

Raising Exceptions

Example

Catching Exceptions

Example

Difference Between Raising and Catching

3. Basic Logging with the `logging` Module

Basic Logging Example

Logging Levels

Example

Logging to a File

Example

Best Practices

Quick Revision Table