How to stop python from automatically converting a float number to a complex number?

Question

I'm creating a function to compute Newton's method. However, the functions f and df keep returning a complex number. I am getting a type error: "can't convert complex to float" at the line where I am defining f. How do I stop python from returning a complex number and not a float?

from cmath import cos, sin
import numpy as np
import matplotlib.pyplot as plt
import numpy as np

def Newtons_method(f, df, x0, i_max, eps_x, eps_f):

    errors = []
    conv = 0
    xk = x0
    for k in range(i_max):

        res = f(xk)
        approx_err = -1.0 * res / df(xk)
        errors.append([k, abs(approx_err), res])

        if (abs(approx_err) < eps_x) and (res < eps_f):

            array_errors = np.array(errors)
            conv = 1
            return xk, array_errors, conv

        der = df(xk)

        if der == 0:

            print("Derivative is zero")
            return None

        xk = xk + approx_err

    print("Exceeded maximum iterations")
    return None

def f(x):
    return float(x * x - 2 * x * cos(x) + cos(x) * cos(x))

def df(x):
    return float(2 * x - 2 * (cos(x) - x * sin(x)) - sin(2 * x))

x0 = 1.0
i_max = 50
eps_x = 1.0e-12
eps_f = 1.0

solution_Newton, record_Newton, flag_Newton = Newtons_method(f, df, x0, i_max, eps_x, eps_f)

x = record_Newton[:,0]
y = record_Newton[:,1]

plt.plot(x, y)
plt.yscale("log")
plt.title("Newton's method")
plt.xlabel("Number of iterations")
plt.ylabel("Approximate errors")
plt.show()