Bisection algorithm in Python (to compare with the Haskell version).

bisection.py

# bisection(lambda x: x**2 - 2.0, 0, 100, 1e-15)
def bisection(f, a, b, err):
    if err < 1e-15 or abs(b - a) / 2 < err:
        return None
    if a > b:
        a, b = b, a
    m = (b + a) / 2.0
    while (b - a) / 2.0 > err and f(m) != 0.0:
        if f(a) * f(m) < 0.0:
            b = m
        elif f(m) * f(b) < 0.0:
            a = m
        else:
            return None
        m = (b + a) / 2.0
    return m