The Python Apprentice

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. ...