SICP 1.1.7 in Python3

>>> def goodEnough(guess, x):
    return abs(guess * guess - x) < 0.001

>>> def average(x, y):
    return (x + y) / 2

>>> def improve(guess, x):
    return average(guess, x / guess)

>>> def sqrtIter(guess, x):
    if goodEnough(guess, x):
        return guess
    else:
        return sqrtIter(improve(guess, x), x)


>>> def sqrt(x):
    return sqrtIter(1.0, x)

>>> sqrt(9)
3.00009155413138