【python实现卷积神经网络】定义训练和测试过程

代码来源：https://github.com/eriklindernoren/ML-From-Scratch

卷积神经网络中卷积层Conv2D（带stride、padding）的具体实现：https://www.cnblogs.com/xiximayou/p/12706576.html

激活函数的实现（sigmoid、softmax、tanh、relu、leakyrelu、elu、selu、softplus）：https://www.cnblogs.com/xiximayou/p/12713081.html

损失函数定义（均方误差、交叉熵损失）：https://www.cnblogs.com/xiximayou/p/12713198.html

优化器的实现（SGD、Nesterov、Adagrad、Adadelta、RMSprop、Adam）：https://www.cnblogs.com/xiximayou/p/12713594.html

卷积层反向传播过程：https://www.cnblogs.com/xiximayou/p/12713930.html

全连接层实现：https://www.cnblogs.com/xiximayou/p/12720017.html

批量归一化层实现：https://www.cnblogs.com/xiximayou/p/12720211.html

池化层实现：https://www.cnblogs.com/xiximayou/p/12720324.html

padding2D实现：https://www.cnblogs.com/xiximayou/p/12720454.html

Flatten层实现：https://www.cnblogs.com/xiximayou/p/12720518.html

上采样层UpSampling2D实现：https://www.cnblogs.com/xiximayou/p/12720558.html

Dropout层实现：https://www.cnblogs.com/xiximayou/p/12720589.html

激活层实现：https://www.cnblogs.com/xiximayou/p/12720622.html

首先是所有的代码：

from __future__ import print_function, division
from terminaltables import AsciiTable
import numpy as np
import progressbar
from mlfromscratch.utils import batch_iterator
from mlfromscratch.utils.misc import bar_widgets


class NeuralNetwork():
    """Neural Network. Deep Learning base model.
    Parameters:
    -----------
    optimizer: class
        The weight optimizer that will be used to tune the weights in order of minimizing
        the loss.
    loss: class
        Loss function used to measure the model's performance. SquareLoss or CrossEntropy.
    validation: tuple
        A tuple containing validation data and labels (X, y)
    """
    def __init__(self, optimizer, loss, validation_data=None):
        self.optimizer = optimizer
        self.layers = []
        self.errors = {"training": [], "validation": []}
        self.loss_function = loss()
        self.progressbar = progressbar.ProgressBar(widgets=bar_widgets)

        self.val_set = None
        if validation_data:
            X, y = validation_data
            self.val_set = {"X": X, "y": y}

    def set_trainable(self, trainable):
        """ Method which enables freezing of the weights of the network's layers. """
        for layer in self.layers:
            layer.trainable = trainable

    def add(self, layer):
        """ Method which adds a layer to the neural network """
        # If this is not the first layer added then set the input shape
        # to the output shape of the last added layer
        if self.layers:
            layer.set_input_shape(shape=self.layers[-1].output_shape())

        # If the layer has weights that needs to be initialized 
        if hasattr(layer, 'initialize'):
            layer.initialize(optimizer=self.optimizer)

        # Add layer to the network
        self.layers.append(layer)

    def test_on_batch(self, X, y):
        """ Evaluates the model over a single batch of samples """
        y_pred = self._forward_pass(X, training=False)
        loss = np.mean(self.loss_function.loss(y, y_pred))
        acc = self.loss_function.acc(y, y_pred)

        return loss, acc

    def train_on_batch(self, X, y):
        """ Single gradient update over one batch of samples """
        y_pred = self._forward_pass(X)
        loss = np.mean(self.loss_function.loss(y, y_pred))
        acc = self.loss_function.acc(y, y_pred)
        # Calculate the gradient of the loss function wrt y_pred
        loss_grad = self.loss_function.gradient(y, y_pred)
        # Backpropagate. Update weights
        self._backward_pass(loss_grad=loss_grad)

        return loss, acc

    def fit(self, X, y, n_epochs, batch_size):
        """ Trains the model for a fixed number of epochs """
        for _ in self.progressbar(range(n_epochs)):
            
            batch_error = []
            for X_batch, y_batch in batch_iterator(X, y, batch_size=batch_size):
                loss, _ = self.train_on_batch(X_batch, y_batch)
                batch_error.append(loss)

            self.errors["training"].append(np.mean(batch_error))

            if self.val_set is not None:
                val_loss, _ = self.test_on_batch(self.val_set["X"], self.val_set["y"])
                self.errors["validation"].append(val_loss)

        return self.errors["training"], self.errors["validation"]

    def _forward_pass(self, X, training=True):
        """ Calculate the output of the NN """
        layer_output = X
        for layer in self.layers:
            layer_output = layer.forward_pass(layer_output, training)

        return layer_output

    def _backward_pass(self, loss_grad):
        """ Propagate the gradient 'backwards' and update the weights in each layer """
        for layer in reversed(self.layers):
            loss_grad = layer.backward_pass(loss_grad)

    def summary(self, name="Model Summary"):
        # Print model name
        print (AsciiTable([[name]]).table)
        # Network input shape (first layer's input shape)
        print ("Input Shape: %s" % str(self.layers[0].input_shape))
        # Iterate through network and get each layer's configuration
        table_data = [["Layer Type", "Parameters", "Output Shape"]]
        tot_params = 0
        for layer in self.layers:
            layer_name = layer.layer_name()
            params = layer.parameters()
            out_shape = layer.output_shape()
            table_data.append([layer_name, str(params), str(out_shape)])
            tot_params += params
        # Print network configuration table
        print (AsciiTable(table_data).table)
        print ("Total Parameters: %d
" % tot_params)

    def predict(self, X):
        """ Use the trained model to predict labels of X """
        return self._forward_pass(X, training=False)

接着我们来一个一个函数进行分析：

1、初始化__init__：这里面定义好优化器optimizer、模型层layers、错误errors、损失函数loss_function、用于显示进度条progressbar，这里从mlfromscratch.utils.misc中导入了bar_widgets，我们看看这是什么：

bar_widgets = [
    'Training: ', progressbar.Percentage(), ' ', progressbar.Bar(marker="-", left="[", right="]"),
    ' ', progressbar.ETA()
]

2、set_trainable()：用于设置哪些模型层需要进行参数的更新

3、add()：将一个模块放入到卷积神经网络中，例如卷积层、池化层、激活层等等。

4、test_on_batch()：使用batch进行测试，这里不需要进行反向传播。

5、train_on_batch()：使用batch进行训练，包括前向传播计算损失以及反向传播更新参数。

6、fit()：喂入数据进行训练或验证，这里需要定义好epochs和batch_size的大小，同时有一个读取数据的函数batch_iterator()，位于mlfromscratch.utils下的data_manipulation.py中：

def batch_iterator(X, y=None, batch_size=64):
    """ Simple batch generator """
    n_samples = X.shape[0]
    for i in np.arange(0, n_samples, batch_size):
        begin, end = i, min(i+batch_size, n_samples)
        if y is not None:
            yield X[begin:end], y[begin:end]
        else:
            yield X[begin:end]

7、_forward_pass()：模型层的前向传播。

8、_backward_pass()：模型层的反向传播。

9、summary()：用于输出模型的每层的类型、参数数量以及输出大小。

10、predict()：用于输出预测值。

不难发现，该代码是借鉴了tensorflow中的一些模块的设计思想。