用python编写一个三层的BP神经网络的代码

好的，以下是三层BP神经网络的Python代码示例：

import numpy as np

class NeuralNetwork:
    def __init__(self, input_size, hidden_size, output_size):
        self.W1 = np.random.randn(input_size, hidden_size)
        self.W2 = np.random.randn(hidden_size, output_size)

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

    def forward(self, X):
        self.z2 = np.dot(X, self.W1)
        self.a2 = self.sigmoid(self.z2)
        self.z3 = np.dot(self.a2, self.W2)
        y_hat = self.sigmoid(self.z3)
        return y_hat

    def sigmoid_prime(self, z):
        return np.exp(-z) / ((1 + np.exp(-z)) ** 2)

    def cost(self, X, y):
        y_hat = self.forward(X)
        J = 0.5 * sum((y - y_hat) ** 2)
        return J

    def cost_prime(self, X, y):
        y_hat = self.forward(X)

        delta3 = np.multiply(-(y - y_hat), self.sigmoid_prime(self.z3))
        dJdW2 = np.dot(self.a2.T, delta3)

        delta2 = np.dot(delta3, self.W2.T) * self.sigmoid_prime(self.z2)
        dJdW1 = np.dot(X.T, delta2)

        return dJdW1, dJdW2

希望这个代码对你有帮助！