Double a point on a elliptic curve

ECdouble.py

# An example of predfined parameters
a = 1; b = 1
P = 23
x1 = 0; y1 = 1
G = (x1,y1)

# Function
def ECdouble(G,a,b,P):  # G = (x1,y1) where x1,y1 are integers.
    lambda_mod = (3*G[0]** 2 + a)% P * modular_inverse(2*G[1],P) 
    x3 = (lambda_mod * lambda_mod - G[0] - G[0]) % P
    y3 = (lambda_mod * (G[0]-x3)-G[1]) % P
    return (x3,y3)