使用二分法（bisection method）求平方根。
def sqrtbi(x, epsilon):
assert x>0, 'x must be non-nagtive, not ' + str(x)
assert epsilon > 0, 'epsilon must be postive, not ' + str(epsilon)
low = 0
high = x
guess = (low + high)/2.0
counter = 1
while (abs(guess ** 2 - x) > epsilon) and (counter <= 100):
if guess ** 2 >> sqrtbi(2,0.000001)
>>> 1.41421365738
上面的方法，如果 x> sqrtbi(0.25,0.000001)
>>> 0.25
那如何求0.25的平方根呢？
只要略微改动上面的代码即可。注意6行和7行的代码。
def sqrtbi(x, epsilon):
assert x>0, 'x must be non-nagtive, not ' + str(x)
assert epsilon > 0, 'epsilon must be postive, not ' + str(epsilon)
low = 0
high = max(x, 1.0)
## high = x
guess = (low + high)/2.0
counter = 1
while (abs(guess ** 2 - x) > epsilon) and (counter <= 100):
if guess ** 2 >> sqrtbi(0.25,0.000001)
>>> 0.5