raise Exception in Python

Learn how to manually raise exceptions in Python using the raise statement.

What is raise?

The raise statement is used to manually generate an exception in Python.

We can use raise when a program detects an invalid situation and wants to stop the current operation.

raise Flow

Basic Syntax

raise Exception("Error message")

The raise keyword is followed by an exception object.

Simple Example

age = 15

if age < 18:

    raise Exception("Age must be 18 or above")

Output:

Exception: Age must be 18 or above

The program manually raises an exception because the age is less than 18.

Raising ValueError

We can raise a specific exception such as ValueError.

marks = -10

if marks < 0:

    raise ValueError("Marks cannot be negative")

Output:

ValueError: Marks cannot be negative

raise with try and except

A manually raised exception can be handled using try and except.

try:

    age = 15

    if age < 18:

        raise ValueError("Age must be 18 or above")

except ValueError as error:

    print(error)

Output:

Age must be 18 or above

raise with User Input

We can use raise to validate user input.

try:

    age = int(
        input("Enter your age: ")
    )

    if age < 18:

        raise ValueError("You must be 18 or above")

    print("You are eligible")

except ValueError as error:

    print("Error:", error)

If the user enters an age below 18, the program manually raises ValueError.

Example: Validate Student Marks

try:

    marks = int(
        input("Enter marks: ")
    )

    if marks < 0 or marks > 100:

        raise ValueError("Marks must be between 0 and 100")

    print("Valid marks:", marks)

except ValueError as error:

    print("Error:", error)

Example Output:

Enter marks: 120
Error: Marks must be between 0 and 100

Raising Different Exception Types

Python allows us to raise different types of exceptions depending on the situation.

number = -5

if number < 0:

    raise ValueError("Number cannot be negative")

Here, ValueError is raised because the supplied value is invalid.

raise Without a Message

An exception can also be raised without providing a custom message.

age = 15

if age < 18:

    raise ValueError

This raises a ValueError without a custom error message.

Raising an Existing Exception

An exception can be caught and raised again using the raise statement.

try:

    number = int("abc")

except ValueError:

    print("Handling error")

    raise

The raise statement raises the same exception again.

Practical Uses of raise

Best Practices

Common Mistakes

• Raising an exception without a meaningful reason.
• Using the wrong exception type.
• Raising exceptions for normal program flow.
• Providing unclear or misleading error messages.

Practice Programs

1. Raise a ValueError if age is below 18.
2. Raise an exception when marks are outside the range 0 to 100.
3. Create a calculator that raises an exception when division by zero is attempted.
4. Take user input and manually raise an exception for invalid input.
5. Catch an exception and raise it again using the raise statement.