Guard clauses

We know this routine will always fail with negative numbers so we can detect this precondition early on and raise an exception at that point, a technique called a guard clause:

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.

    Raises:
        ValueError: If x is negative.
    """

    if x < 0:
        raise ValueError("Cannot compute square root of negative   
                         number{}".format(x))

    guess = x
    i = 0
    while guess * guess != x and i < 20:
        guess = (guess + x / guess) / 2.0
        i += 1
    return guess

The test is a simple if-statement and a call to raise passing a newly minted exception object. The ValueError() constructor accepts an error message...