Raise an exception in Python

Your own functions can raise exceptions too. When a function gets an argument it cannot use, raise stops it at once and tells the caller what is wrong. This is better than returning a wrong answer without any warning.

The program

def bmi(weight_kg, height_m):
    if weight_kg <= 0:
        raise ValueError(f"weight_kg must be more than 0, got {weight_kg}")
    if not 0.5 <= height_m <= 2.5:
        raise ValueError(f"height_m must be in metres (0.5 to 2.5), got {height_m}")
    return weight_kg / height_m ** 2


print(f"BMI: {bmi(68, 1.75):.1f}")

for weight, height in [(68, 175), (-5, 1.75)]:
    try:
        print(f"BMI: {bmi(weight, height):.1f}")
    except ValueError as error:
        print("Error:", error)

BMI: 22.2
Error: height_m must be in metres (0.5 to 2.5), got 175
Error: weight_kg must be more than 0, got -5

How it works

• raise ValueError("...") creates an exception object with a message and raises it. The function stops on that line, and the return never runs.
• The second call passes the height in centimetres (175) by mistake. Without the check, the function would return about 0.002, a wrong result with no error.
• A good message says which argument is wrong, what was expected, and what value was received. got {height_m} saves the caller from guessing.
• The function only raises. The caller decides what to do: here it catches the error with except ValueError as error and prints the message.

If nobody catches the exception, the program stops with a traceback that ends with your message:

ValueError: height_m must be in metres (0.5 to 2.5), got 175

Which exception to raise

Use one of Python's built-in types when it fits:

• ValueError: the type is right but the value is not allowed (a negative weight, an empty name).
• TypeError: the argument is the wrong type (a string where a number is needed).
• KeyError or IndexError: something you looked up does not exist.

def repeat(text, times):
    if not isinstance(times, int):
        raise TypeError(f"times must be an int, not {type(times).__name__}")
    return text * times


try:
    repeat("ab", "3")
except TypeError as error:
    print("Error:", error)

Error: times must be an int, not str

When none of these fits, make your own type. The custom exception program shows how.