sigmoid prime python

1class Sigmoid:
2    def __call__(self, z):
3        """
4        Compute the sigmoid of z
5        Arguments:
6        z -- scalar or numpy array of any size.
7        Return:
8        sigmoid(z)
9        """
10        sigmoid = 1/np.exp(-z)
11        return sigmoid
12    
13    def prime(self, z):  # Questions need some help --- Thanks!
14        """
15        Compute the derivative of sigmoid of z
16        Arguments:
17        z -- scalar or numpy array of any size.
18        Return:
19        Sigmoid prime
20        """
21        ###return sigmoid * (1 - sigmoid)