Bisection method to find roots of a non-linear equation

bisection.py

#Bisection Method to find roots of a non linear equation
#Praveen Sridhar 
#psbots.blogspot.com
def bisection(function,lower_bound,upper_bound,error_tolerance=1.0e-4):
    fu=function(upper_bound)
    if fu==0.0 :  return upper_bound
    fl=function(lower_bound)
    if fl==0.0 :  return lower_bound
    if fu*fl > 0.0:  #if the product of the function values at the two bounds is positive, then they do not bracket a root
        return None
    else:
        mid=(lower_bound+upper_bound)/2
        error=(upper_bound-lower_bound)/2
        while(error>error_tolerance): #the loop executes till the desired level of accuracy is attainedfm=function(mid)
            fu=function(upper_bound)
            fl=function(lower_bound)

            #if the middle value is indeed the root, it is returned as the root
            if fm==0.0 : return mid

            #if the actual root lies between upper bound and the middle value, the lower bound is set to be the middle value
            elif fu*fm < 0.0 : lower_bound=mid

            ##if the actual root lies between lower bound and the middle value, the upper bound is set to be the middle value
            elif fl*fm < 0.0 : upper_bound=mid
            
            mid=(lower_bound+upper_bound)/2
            error=(upper_bound-lower_bound)/2
        return mid

def func(x):           #defining the non-linear equation to be solved as a function
    return x**3-3*x+1

x=bisection(func,1.0,2.0) #running the bisection function for the function 'func'

print x