RuntimeWarning: overflow encountered in expdef sigmoid(z):

def sigmoid(z):
		return 1.0 / (1 + np.exp(-z)) 

改为

def sigmoid(z):
    #防止溢出
    if z >= 0:
	    return 1.0 / (1 + np.exp(-z))
    else:
        return np.exp(z) / (1 + np.exp(z))