Raising Exceptions with `raise` in Python

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

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

Basic Syntax

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

raise ExceptionType("Error message")

Basic Usage Example

Suppose we want to raise an exception when a value is not valid:

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-raising Exceptions

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

This is useful if you have performed some kind of processing in the except block but want 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}")