Raising Exceptions with `raise` in Python

The keyword raise is used to throw (or propagate) an exception in Python. By using raise, we can generate custom errors or re-throw captured exceptions to be handled at a higher level of the program.

The raise statement in Python is used to intentionally throw an exception. This can be useful in various situations, such as:

Basic Syntax

The basic syntax for throwing an exception with raise is as follows:

raise ExceptionType("Error message")

Basic usage example

Suppose we want to throw an exception when a value is invalid:

def check_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative.")
    print("Valid age.")

try:
    check_age(-5)
except ValueError as e:
    print(f"An error occurred: {e}")

Re-throwing exceptions

Sometimes, after catching an exception, we may want to re-throw it so that it can be handled in a higher context.

This is useful if you have performed some kind of processing in the except block but wish for the exception to continue propagating.

def process_data(data):
    try:
        if not isinstance(data, int):
            raise TypeError("An integer was expected.")
        print(f"Processing {data}")
    except TypeError as e:
        print(f"Error detected: {e}")
        raise  # Re-throws the exception to be handled higher up

try:
    process_data("text")
except TypeError as e:
    print(f"Exception handled at the upper level: {e}")