I wrote a recursive function to compute the square root of a number using the Newton-Raphson method. I am using an inner function, is that a bad thing? How can I use just one function definition?

Square_root_recursive.py

def r_sqrt(a):
    
    a = float(a)
    if a == 0.0:
        return 0.0
    x = a

    def sqrt(x):
        epsilon = 0.000000000000001
        y = (x + a/x) / 2
        
        if abs(y - x) < epsilon:
            return x
        else:
            return sqrt(y)
    return sqrt(a)

I wrote a recursive function to compute the square root of a number using the Newton-Raphson method. I am using an inner function, is that a bad thing? How can I use just one function definition and still be recursive?