Exceptions are an important aspect of the API of a function. Callers of a function need to know which exceptions to expect under various conditions so that they can ensure appropriate exception handlers are in place. We'll use square-root finding as an example, using a home-grown square-root function, courtesy of Heron of Alexandria (although he probably didn't use Python).

Callers of a function need to know which exceptions to expect:

Figure 6.1: Callers need to know now

Place the following code in a file roots.py:

def sqrt(x):
    """Compute square roots using the method of Heron of Alexandria.

    Args:
        x: The number for which the square root is to be computed.

    Returns:
        The square root of x.
    """
    guess = x
    i = 0
    while guess * guess != x and i < 20:
        guess = (guess + x / guess) / 2.0
        i += 1
    return guess

def main():
    print(sqrt(9))
    print(sqrt(2))

if __name__ == '__main__':
    main...