Find a root of f(x) = 0 with Ridder’s method in Python

''' root = ridder(f,a,b,tol=1.0e-9).
    Finds a root of f(x) = 0 with Ridder's method.
    The root must be bracketed in (a,b).
'''
import error
from math import sqrt
 
def ridder(f,a,b,tol=1.0e-9):   
    fa = f(a)
    if fa == 0.0: return a
    fb = f(b)
    if fb == 0.0: return b
    if fa*fb > 0.0: error.err('Root is not bracketed')
    for i in range(30):
      # Compute the improved root x from Ridder's formula
        c = 0.5*(a + b); fc = f(c)
        s = sqrt(fc**2 - fa*fb)
        if s == 0.0: return None
        dx = (c - a)*fc/s
        if (fa - fb) < 0.0: dx = -dx
        x = c + dx; fx = f(x)
      # Test for convergence
        if i > 0:
            if abs(x - xOld) < tol*max(abs(x),1.0): return x
        xOld = x
      # Re-bracket the root as tightly as possible
        if fc*fx > 0.0:
            if fa*fx < 0.0: b = x; fb = fx
            else:           a = x; fa = fx
        else:
            a = c; b = x; fa = fc; fb = fx
    return None
    print 'Too many iterations'

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