Using boolean operators in an if statement

I'm a beginner in coding, and I'm trying to create a quadratic equation calculator using python.

while True:
    print("""
        Welcome to DJIGURDA Supreme Quadratic Calculator
        Please enter three values in the form of /'ax^2 + bx +c/'. """)
    a = input("a (first constant)")
    b = input("b (second constant)")
    c = input("c (third constant)")

    if not a.isalpha and not b.isalpha and not c.isalpha:
        d = (float(b)**2) - (4 * float(a) * float(c))
        print(d)
        if d >= 0:
            x_1 = (float(-b) + (d**0.5)) / (2*float(a))
            x_2 = (float(-b) - (d**0.5)) / (2*float(a))
            print("The first variable is equal to %s./n The second variable is equal to %s")[str(x_1), str(x_2)]
        else: 
            print("No real roots.")
    else:
        print("Please enter numerical values.")

This code keeps returning "Please enter numerical values." Why is the code not making it past the first "if" statement?