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Raising Exceptions

You can raise an exception in your Python code using the raise keyword.

There are many predefined exceptions in Python. You can also create your own exceptions, typically by creating a subclass of the builtin Exception class.

Here’s a program that calculates an area. If you pass negative values to the calculate_area function, it raises a ValueError exception. This type of exception is typically used when a function receives arguments of the right type but the wrong value.

def calculate_area(length, width):

    if length < 0:
        raise ValueError("Length parameter cannot be negative.")
    if width < 0:
        raise ValueError("Width parameter cannot be negative.")

    return length * width

def main():
    area = calculate_area(5, -1)
    print(f"The area is {area} square metres")

Traceback (most recent call last):
  File "/Users/john/Documents/Python WordPress/./strings.py", line 15, in <module>
  File "/Users/john/Documents/Python WordPress/./strings.py", line 12, in main
    area = calculate_area(5, -1)
  File "/Users/john/Documents/Python WordPress/./strings.py", line 7, in calculate_area
    raise ValueError("Width parameter cannot be negative.")
ValueError: Width parameter cannot be negative.

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