写一个bp神经网络代码用python语言

好的，下面是一个简单的 BP 神经网络的 Python 代码：

import numpy as np

class BPNeuralNetwork:
    def __init__(self, input_size, hidden_size, output_size):
        self.input_size = input_size
        self.hidden_size = hidden_size
        self.output_size = output_size
        
        # 初始化权重矩阵
        self.W1 = np.random.randn(self.input_size, self.hidden_size)
        self.W2 = np.random.randn(self.hidden_size, self.output_size)
        
    def sigmoid(self, x):
        return 1 / (1 + np.exp(-x))
        
    def forward(self, X):
        # 前向传播
        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_derivative(self, x):
        return x * (1 - x)
        
    def backward(self, X, y, y_hat, learning_rate):
        # 反向传播
        delta3 = np.multiply(-(y - y_hat), self.sigmoid_derivative(y_hat))
        dJdW2 = np.dot(self.a2.T, delta3)
        delta2 = np.dot(delta3, self.W2.T) * self.sigmoid_derivative(self.a2)
        dJdW1 = np.dot(X.T, delta2)
        
        # 更新权重矩阵
        self.W1 -= learning_rate * dJdW1
        self.W2 -= learning_rate * dJdW2
        
    def train(self, X, y, epochs, learning_rate):
        for i in range(epochs):
            y_hat = self.forward(X)
            self.backward(X, y, y_hat, learning_rate)

希望这个代码对你有所帮助。如果你有其他问题，请继续提问。如果你想听一个笑话，这里有一个：

为什么猪不能上太空？因为它们的火箭只能飞一会儿，然后就变成了烤猪！