python 3.4 spirograph error
Question: 
    from turtle import*                                                                      
    from math import*
    
    def Xcord(R,r,p,t):  
    
        x= (R-r) * cos(t) - (r+p) * cos((R-r)//r*t)
    
    def Ycord (R,r,p,t):
    
        y= (R-r) * sin(t) - (r+p) * sin((R-r)//r*t)
    
    def t_iter(R,r,p):
    
          t=0
          down()
          goto(Xcord(R,r,p,t),Ycord(R,r,p,t))
          while (t < 2 * pi):
              t = t+.01
              Xcord(R,r,p,t)
              Ycord(R,r,p,t)
              up()
          return
    
    def main():
    
          R=100
          r=4
          p=int(input("Please enter a number between 10 and 100: "))
          if p < 10 or p > 100:
                input(" Incorrect value of p!")
          t_iter(R,r,p)
          input("Hit enter to close porgram")
          bye()
    
    main()
    

I get this error:

    
    
    Traceback (most recent call last):
    
      File "C:/Users/Coscio/Desktop/spirals.py", line 31, in <module>
        main()
    
      File "C:/Users/Coscio/Desktop/spirals.py", line 27, in main
        t_iter(R,r,p)
    
      File "C:/Users/Coscio/Desktop/spirals.py", line 13, in t_iter
        goto(Xcord(R,r,p,t),Ycord(R,r,p,t))
    
      File "<string>", line 1, in goto
    
      File "D:\Python\lib\turtle.py", line 1774, in goto
        self._goto(Vec2D(*x))
    
    TypeError: type object argument after * must be a sequence, not NoneType
    


Answer: 
    from turtle import*
    
    from math import*
    
    def Xcord(R,r,p,t):
    
            return (R-r) * cos(t) - (r+p) * cos((R-r)//r*t)
    
    def Ycord (R,r,p,t):
        return (R-r) * sin(t) - (r+p) * sin((R-r)//r*t)
    
    def t_iter(R,r,p):
        t=0
        up()
        Xcord(R,r,p,t)
        Ycord(R,r,p,t)
        while (t < 2 * pi):
            t = t+.01
            goto(Xcord(R,r,p,t),Ycord(R,r,p,t))
            down()
        return float(Xcord(R,r,p,t))
        return float(Ycord(R,r,p,t))
    
    def validate():
        while True:
            p=int(input("Please enter a number between 10 and 100: "))
            if p >= 10 or p <= 100:
                break
        return p
    
    
    def main():
        speed(0)
        R=100
        r=4
        p = validate()
        t_iter(R,r,p)
        Xcord(R,r,p,t_iter(R,r,p))
        Ycord(R,r,p,t_iter(R,r,p))
        input("Hit enter to close porgram")
        bye()
    
    main()
    

