My code is right but it's not executing

[Bhavani Kasiviswanathan]

def tribonacci(signature,n)

   for x in range (n-len(a)):
      b= a[0]+a[1]+a[2]
      a[0] = a[1]
      a[1] = a[2]
      a[2] = b
      signature.append(b)
   print (signature)

could you please tell me why it's not executing in codewars

[RD916]

not calling function