题图来自: github
本文主要介绍了PrettyTensor，用来快速构建神经网络。当然，原文写于16年，现在有更方便的API，后续会介绍。本文有大段前篇教程的文字及代码，如果看过上一篇的朋友可以快速翻到下文PrettyTensor实现的那一部分去。
by Magnus Erik Hvass Pedersen / GitHub / Videos on YouTube中文翻译 thrillerist / Github
介绍
之前的教程演示了如何在TensorFlow中实现一个卷积神经网络，这需要了解一些TensorFlow工作的底层原理。它有点复杂，实现起来还容易犯错。
这篇教程为我们说明了如何使用TensorFlow的一个附加包PrettyTensor，它也是Google开发的。PrettyTensor提供了在TensorFlow中创建神经网络的更简单的方法，让我们可以关注自己想要实现的想法，而不用过多担心底层的实现细节。这也让代码更短、更容易阅读和修改。
除了用PrettyTensor构造图之外，这篇教程的大部分代码和教程 #02 中的一样，当然还有一些细微的变化。
这篇教程是基于教程 #02 之上的，如果你是TensorFlow新手的话，推荐先学完上一份教程。你需要熟悉基本的线性代数、Python和Jupyter Notebook编辑器。
流程图
下面的图表直接展示了之后实现的卷积神经网络中数据的传递。关于卷积的详细描述请看上一篇教程
from IPython.display import Image
Image('images/02_network_flowchart.png')
输入图像在第一层卷基层中用权重过滤器处理。结果在16张新图里，每个代表了卷积层里一个过滤器（的处理结果）。图像也经过降采样，因此图像分辨率从28x28减少到14x14。
这16张小图在第二个卷积层中处理。这16个通道都需要一个权重过滤，这层的输出的每个通道也各需要一个权重过滤。总共有36个输出，所以在第二个卷积层有16 x 36 = 576个滤波器。输出图再一次降采样到7x7个像素。
第二个卷积层的输出是36张7x7像素的图像。它们被压到一个长为7 x 7 x 36 = 1764的向量中去，它作为一个有128个神经元（或元素）的全连接网络的输入。这些又输入到另一个有10个神经元的全连接层中，每个神经元代表一个类别，用来确定图像的类别，也即图像上的数字。
卷积滤波一开始是随机挑选的，因此分类也是随机完成的。根据交叉熵（cross-entropy）来测量输入图预测值和真实类别间的错误。然后优化器用链式法则自动地将这个误差传在卷积网络中传递，更新滤波权重来提升分类质量。这个过程迭代了几千次，直到分类误差足够低。
这些特定的滤波权重和中间图像是一个优化的结果，和你执行这些代码所看到的可能会有所不同。
注意，这些在TensorFlow上的计算是在一部分图像上执行，而非单独的一张图，这使得计算更有效。也意味着在TensorFlow上实现时，这个流程图实际上会有更多的数据维度。
导入
%matplotlib inline
import matplotlib.pyplot as plt
import tensorflow as tf
import numpy as np
from sklearn.metrics import confusion_matrix
import time
from datetime import timedelta
import math
# We also need PrettyTensor.
import prettytensor as pt
使用Python3.5（Anaconda）开发，TensorFlow版本是
tf.__version__
'1.0.0-rc0'
载入数据
MNIST数据集大约12MB，如果没在文件夹中找到就会自动下载。
from tensorflow.examples.tutorials.mnist import input_data
data = input_data.read_data_sets('data/MNIST/', one_hot=True)
Extracting data/MNIST/train-images-idx3-ubyte.gz
Extracting data/MNIST/train-labels-idx1-ubyte.gz
Extracting data/MNIST/t10k-images-idx3-ubyte.gz
Extracting data/MNIST/t10k-labels-idx1-ubyte.gz
现在已经载入了MNIST数据集，它由70,000张图像和对应的标签（比如图像的类别）组成。数据集分成三份互相独立的子集。我们在教程中只用训练集和测试集。
print("Size of:")
print("- Training-set:\t\t{}".format(len(data.train.labels)))
print("- Test-set:\t\t{}".format(len(data.test.labels)))
print("- Validation-set:\t{}".format(len(data.validation.labels)))
Size of:
-Training-set: 55000
-Test-set: 10000
-Validation-set: 5000
类型标签使用One-Hot编码，这意外每个标签是长为10的向量，除了一个元素之外，其他的都为零。这个元素的索引就是类别的数字，即相应图片中画的数字。我们也需要测试数据集类别数字的整型值，用下面的方法来计算。
data.test.cls = np.argmax(data.test.labels, axis=1)
数据维度
在下面的源码中，有很多地方用到了数据维度。它们只在一个地方定义，因此我们可以在代码中使用这些数字而不是直接写数字。
# We know that MNIST images are 28 pixels in each dimension.
img_size = 28
# Images are stored in one-dimensional arrays of this length.
img_size_flat = img_size * img_size
# Tuple with height and width of images used to reshape arrays.
img_shape = (img_size, img_size)
# Number of colour channels for the images: 1 channel for gray-scale.
num_channels = 1
# Number of classes, one class for each of 10 digits.
num_classes = 10
用来绘制图片的帮助函数
这个函数用来在3x3的栅格中画9张图像，然后在每张图像下面写出真实类别和预测类别。
def plot_images(images, cls_true, cls_pred=None):
assert len(images) == len(cls_true) == 9
# Create figure with 3x3 sub-plots.
fig, axes = plt.subplots(3, 3)
fig.subplots_adjust(hspace=0.3, wspace=0.3)
for i, ax in enumerate(axes.flat):
# Plot image.
ax.imshow(images[i].reshape(img_shape), cmap='binary')
# Show true and predicted classes.
if cls_pred is None:
xlabel = "True: {0}".format(cls_true[i])
else:
xlabel = "True: {0}, Pred: {1}".format(cls_true[i], cls_pred[i])
# Show the classes as the label on the x-axis.
ax.set_xlabel(xlabel)
# Remove ticks from the plot.
ax.set_xticks([])
ax.set_yticks([])
# Ensure the plot is shown correctly with multiple plots
# in a single Notebook cell.
plt.show()
绘制几张图像来看看数据是否正确
# Get the first images from the test-set.
images = data.test.images[0:9]
# Get the true classes for those images.
cls_true = data.test.cls[0:9]
# Plot the images and labels using our helper-function above.
plot_images(images=images, cls_true=cls_true)
TensorFlow图
TensorFlow的全部目的就是使用一个称之为计算图（computational graph）的东西，它会比直接在Python中进行相同计算量要高效得多。TensorFlow比Numpy更高效，因为TensorFlow了解整个需要运行的计算图，然而Numpy只知道某个时间点上唯一的数学运算。
TensorFlow也能够自动地计算需要优化的变量的梯度，使得模型有更好的表现。这是由于图是简单数学表达式的结合，因此整个图的梯度可以用链式法则推导出来。
TensorFlow还能利用多核CPU和GPU，Google也为TensorFlow制造了称为TPUs（Tensor Processing Units）的特殊芯片，它比GPU更快。
一个TensorFlow图由下面几个部分组成，后面会详细描述：
- 占位符变量（Placeholder）用来改变图的输入。
- 模型变量（Model）将会被优化，使得模型表现得更好。
- 模型本质上就是一些数学函数，它根据Placeholder和模型的输入变量来计算一些输出。
- 一个cost度量用来指导变量的优化。
- 一个优化策略会更新模型的变量。
另外，TensorFlow图也包含了一些调试状态，比如用TensorBoard打印log数据，本教程不涉及这些。
占位符 （Placeholder）变量
Placeholder是作为图的输入，每次我们运行图的时候都可能会改变它们。将这个过程称为feeding placeholder变量，后面将会描述它。
首先我们为输入图像定义placeholder变量。这让我们可以改变输入到TensorFlow图中的图像。这也是一个张量（tensor），代表一个多维向量或矩阵。数据类型设置为float32，形状设为[None, img_size_flat]，None代表tensor可能保存着任意数量的图像，每张图象是一个长度为img_size_flat的向量。
x = tf.placeholder(tf.float32, shape=[None, img_size_flat], name='x')
卷积层希望x被编码为4维张量，因此我们需要将它的形状转换至[num_images, img_height, img_width, num_channels]。注意img_height == img_width == img_size，如果第一维的大小设为-1， num_images的大小也会被自动推导出来。转换运算如下：
x_image = tf.reshape(x, [-1, img_size, img_size, num_channels])
接下来我们为输入变量x中的图像所对应的真实标签定义placeholder变量。变量的形状是[None, num_classes]，这代表着它保存了任意数量的标签，每个标签是长度为num_classes的向量，本例中长度为10。
y_true = tf.placeholder(tf.float32, shape=[None, 10], name='y_true')
我们也可以为class-number提供一个placeholder，但这里用argmax来计算它。这里只是TensorFlow中的一些操作，没有执行什么运算。
y_true_cls = tf.argmax(y_true, dimension=1)
TensorFlow 实现
这一节显示了教程 #02 中直接用TensorFlow实现卷积神经网络的源代码。这份Notebook中并没有直接用到这些代码，只是为了方便和下面PrettyTensor的实现进行比较。
这里要注意的是有多少代码量以及TensorFlow保存数据、进行运算的底层细节。即使在很小的神经网络中也容易犯错。
帮助函数
在直接用TensorFlow实现时，我们创建一些在构造图时常用到的帮助函数。
这两个函数在TensorFlow图中创建新的变量并用随机值初始化。
def new_weights(shape):
return tf.Variable(tf.truncated_normal(shape, stddev=0.05))
def new_biases(length):
return tf.Variable(tf.constant(0.05, shape=[length]))
下面的帮助函数创建一个新的卷积网络。输入和输出是4维的张量（4-rank tensors）。注意TensorFlow API的底层细节，比如权重变量的大小。这里很容易犯错，可能会导致奇怪的错误信息，并且很难调试。
def new_conv_layer(input, # The previous layer.
num_input_channels, # Num. channels in prev. layer.
filter_size, # Width and height of filters.
num_filters, # Number of filters.
use_pooling=True): # Use 2x2 max-pooling.
# Shape of the filter-weights for the convolution.
# This format is determined by the TensorFlow API.
shape = [filter_size, filter_size, num_input_channels, num_filters]
# Create new weights aka. filters with the given shape.
weights = new_weights(shape=shape)
# Create new biases, one for each filter.
biases = new_biases(length=num_filters)
# Create the TensorFlow operation for convolution.
# Note the strides are set to 1 in all dimensions.
# The first and last stride must always be 1,
# because the first is for the image-number and
# the last is for the input-channel.
# But e.g. strides=[1, 2, 2, 1] would mean that the filter
# is moved 2 pixels across the x- and y-axis of the image.
# The padding is set to 'SAME' which means the input image
# is padded with zeroes so the size of the output is the same.
layer = tf.nn.conv2d(input=input,
filter=weights,
strides=[1, 1, 1, 1],
padding='SAME')
# Add the biases to the results of the convolution.
# A bias-value is added to each filter-channel.
layer += biases
# Use pooling to down-sample the image resolution?
if use_pooling:
# This is 2x2 max-pooling, which means that we
# consider 2x2 windows and select the largest value
# in each window. Then we move 2 pixels to the next window.
layer = tf.nn.max_pool(value=layer,
ksize=[1, 2, 2, 1],
strides=[1, 2, 2, 1],
padding='SAME')
# Rectified Linear Unit (ReLU).
# It calculates max(x, 0) for each input pixel x.
# This adds some non-linearity to the formula and allows us
# to learn more complicated functions.
layer = tf.nn.relu(layer)
# Note that ReLU is normally executed before the pooling,
# but since relu(max_pool(x)) == max_pool(relu(x)) we can
# save 75% of the relu-operations by max-pooling first.
# We return both the resulting layer and the filter-weights
# because we will plot the weights later.
return layer, weights
下面的帮助函数将一个4维张量转换到2维，因此我们可以在卷积层之后添加一个全连接层。
def flatten_layer(layer):
# Get the shape of the input layer.
layer_shape = layer.get_shape()
# The shape of the input layer is assumed to be:
# layer_shape == [num_images, img_height, img_width, num_channels]
# The number of features is: img_height * img_width * num_channels
# We can use a function from TensorFlow to calculate this.
num_features = layer_shape[1:4].num_elements()
# Reshape the layer to [num_images, num_features].
# Note that we just set the size of the second dimension
# to num_features and the size of the first dimension to -1
# which means the size in that dimension is calculated
# so the total size of the tensor is unchanged from the reshaping.
layer_flat = tf.reshape(layer, [-1, num_features])
# The shape of the flattened layer is now:
# [num_images, img_height * img_width * num_channels]
# Return both the flattened layer and the number of features.
return layer_flat, num_features
接下来的帮助函数创建一个全连接层。
def new_fc_layer(input, # The previous layer.
num_inputs, # Num. inputs from prev. layer.
num_outputs, # Num. outputs.
use_relu=True): # Use Rectified Linear Unit (ReLU)?
# Create new weights and biases.
weights = new_weights(shape=[num_inputs, num_outputs])
biases = new_biases(length=num_outputs)
# Calculate the layer as the matrix multiplication of
# the input and weights, and then add the bias-values.
layer = tf.matmul(input, weights) + biases
# Use ReLU?
if use_relu:
layer = tf.nn.relu(layer)
return layer
构造图（Graph）
现在将会用上面的帮助函数来创建卷积神经网络。如果没有这些函数的话，代码将会又长又难以理解。
注意，我们并不会运行下面的代码。写在这里只是为了与PrettyTensor的代码进行比较。
之前的教程使用定义好的常量，因此很容易改变（变量）。比如，我们没有将 filter_size=5 当作 new_conv_layer()的参数，而是令filter_size=filter_size1，然后在其他地方定义filter_size1=5。这样子就很容易改变所有的常量。
if False: # Don't execute this! Just show it for easy comparison.
# First convolutional layer.
layer_conv1, weights_conv1 = \
new_conv_layer(input=x_image,
num_input_channels=num_channels,
filter_size=5,
num_filters=16,
use_pooling=True)
# Second convolutional layer.
layer_conv2, weights_conv2 = \
new_conv_layer(input=layer_conv1,
num_input_channels=16,
filter_size=5,
num_filters=36,
use_pooling=True)
# Flatten layer.
layer_flat, num_features = flatten_layer(layer_conv2)
# First fully-connected layer.
layer_fc1 = new_fc_layer(input=layer_flat,
num_inputs=num_features,
num_outputs=128,
use_relu=True)
# Second fully-connected layer.
layer_fc2 = new_fc_layer(input=layer_fc1,
num_inputs=128,
num_outputs=num_classes,
use_relu=False)
# Predicted class-label.
y_pred = tf.nn.softmax(layer_fc2)
# Cross-entropy for the classification of each image.
cross_entropy = \
tf.nn.softmax_cross_entropy_with_logits(logits=layer_fc2,
labels=y_true)
# Loss aka. cost-measure.
# This is the scalar value that must be minimized.
loss = tf.reduce_mean(cross_entropy)
PrettyTensor实现
这一节演示如何用PrettyTensor来实现一个相同的卷积神经网络。
基本思想就是用一个PrettyTensor object封装输入张量x_image，它有一个添加新卷积层的帮助函数，以此来创建整个神经网络。这有点像我们之前实现的那些帮助函数，但它更简单一些，因为PrettyTensor记录每一层的输入和输出维度等等。
x_pretty = pt.wrap(x_image)
现在我们已经将输入图像装到一个PrettyTensor的object中，再用几行代码就可以添加卷积层和全连接层。
注意，在with代码块中，pt.defaults_scope(activation_fn=tf.nn.relu)把 activation_fn=tf.nn.relu当作每个的层参数，因此这些层都用到了 Rectified Linear Units (ReLU) 。defaults_scope使我们能更方便地修改所有层的参数。
with pt.defaults_scope(activation_fn=tf.nn.relu):
y_pred, loss = x_pretty.\
conv2d(kernel=5, depth=16, name='layer_conv1').\
max_pool(kernel=2, stride=2).\
conv2d(kernel=5, depth=36, name='layer_conv2').\
max_pool(kernel=2, stride=2).\
flatten().\
fully_connected(size=128, name='layer_fc1').\
softmax_classifier(num_classes=num_classes, labels=y_true)
就是这样！现在我们用几行代码就创建了一个完全一样的卷积神经网络，如果直接用TensorFlow实现的话需要一大段非常复杂的代码。
用PrettyTensor来代替TensorFlow，我们可以清楚地看到网络的构造以及数据如何在网络中流通。这让我们可以专注于神经网络的关键思想而不是底层的实现细节。它十分简单优雅！
获取权重
不幸的是，使用PrettyTensor时并非所有的事都那么优雅。
下面，我们想要绘制出卷积层的权重。在用TensorFlow实现时，我们自己创建了变量，所以可以直接访问它们。但使用PrettyTensor构造网络时，所有的变量都是间接地由PrettyTensor创建。因此我们不得不从TensorFlow中找回变量。
我们用layer_conv1 和 layer_conv2代表两个卷积层。这也叫变量作用域（不要与上面描述的defaults_scope混淆了）。PrettyTensor会自动给它为每个层创建的变量命名，因此我们可以通过层的作用域名称和变量名来取得某一层的权重。
函数实现有点笨拙，因为我们不得不用TensorFlow函数get_variable()，它是设计给其他用途的，创建新的变量或重用现有变量。创建下面的帮助函数很简单。
def get_weights_variable(layer_name):
# Retrieve an existing variable named 'weights' in the scope
# with the given layer_name.
# This is awkward because the TensorFlow function was
# really intended for another purpose.
with tf.variable_scope(layer_name, reuse=True):
variable = tf.get_variable('weights')
return variable
借助这个帮助函数我们可以获取变量。这些是TensorFlow的objects。你需要类似的操作来获取变量的内容： contents = session.run(weights_conv1) ，下面会提到这个。
weights_conv1 = get_weights_variable(layer_name='layer_conv1')
weights_conv2 = get_weights_variable(layer_name='layer_conv2')
优化方法
PrettyTensor给我们提供了预测类型标签(y_pred)以及一个需要最小化的损失度量，用来提升神经网络分类图片的能力。
PrettyTensor的文档并没有说明它的损失度量是用cross-entropy还是其他的。但现在我们用AdamOptimizer来最小化损失。
优化过程并不是在这里执行。实际上，还没计算任何东西，我们只是往TensorFlow图中添加了优化器，以便后续操作。
optimizer = tf.train.AdamOptimizer(learning_rate=1e-4).minimize(loss)
性能度量
我们需要另外一些性能度量，来向用户展示这个过程。
首先我们从神经网络输出的y_pred中计算出预测的类别，它是一个包含10个元素的向量。类别数字是最大元素的索引。
y_pred_cls = tf.argmax(y_pred, dimension=1)
然后创建一个布尔向量，用来告诉我们每张图片的真实类别是否与预测类别相同。
correct_prediction = tf.equal(y_pred_cls, y_true_cls)
上面的计算先将布尔值向量类型转换成浮点型向量，这样子False就变成0，True变成1，然后计算这些值的平均数，以此来计算分类的准确度。
accuracy = tf.reduce_mean(tf.cast(correct_prediction, tf.float32))
运行TensorFlow
创建TensorFlow会话（session）
一旦创建了TensorFlow图，我们需要创建一个TensorFlow会话，用来运行图。
session = tf.Session()
初始化变量
我们需要在开始优化weights和biases变量之前对它们进行初始化。
session.run(tf.global_variables_initializer())
用来优化迭代的帮助函数
在训练集中有50,000张图。用这些图像计算模型的梯度会花很多时间。因此我们利用随机梯度下降的方法，它在优化器的每次迭代里只用到了一小部分的图像。
如果内存耗尽导致电脑死机或变得很慢，你应该试着减少这些数量，但同时可能还需要更优化的迭代。
train_batch_size = 64
函数执行了多次的优化迭代来逐步地提升网络层的变量。在每次迭代中，从训练集中选择一批新的数据，然后TensorFlow用这些训练样本来执行优化器。每100次迭代会打印出相关信息。
# Counter for total number of iterations performed so far.
total_iterations = 0
def optimize(num_iterations):
# Ensure we update the global variable rather than a local copy.
global total_iterations
# Start-time used for printing time-usage below.
start_time = time.time()
for i in range(total_iterations,
total_iterations + num_iterations):
# Get a batch of training examples.
# x_batch now holds a batch of images and
# y_true_batch are the true labels for those images.
x_batch, y_true_batch = data.train.next_batch(train_batch_size)
# Put the batch into a dict with the proper names
# for placeholder variables in the TensorFlow graph.
feed_dict_train = {x: x_batch,
y_true: y_true_batch}
# Run the optimizer using this batch of training data.
# TensorFlow assigns the variables in feed_dict_train
# to the placeholder variables and then runs the optimizer.
session.run(optimizer, feed_dict=feed_dict_train)
# Print status every 100 iterations.
if i % 100 == 0:
# Calculate the accuracy on the training-set.
acc = session.run(accuracy, feed_dict=feed_dict_train)
# Message for printing.
msg = "Optimization Iteration: {0:>6}, Training Accuracy: {1:>6.1%}"
# Print it.
print(msg.format(i + 1, acc))
# Update the total number of iterations performed.
total_iterations += num_iterations
# Ending time.
end_time = time.time()
# Difference between start and end-times.
time_dif = end_time - start_time
# Print the time-usage.
print("Time usage: " + str(timedelta(seconds=int(round(time_dif)))))
用来绘制错误样本的帮助函数
函数用来绘制测试集中被误分类的样本。
def plot_example_errors(cls_pred, correct):
# This function is called from print_test_accuracy() below.
# cls_pred is an array of the predicted class-number for
# all images in the test-set.
# correct is a boolean array whether the predicted class
# is equal to the true class for each image in the test-set.
# Negate the boolean array.
incorrect = (correct == False)
# Get the images from the test-set that have been
# incorrectly classified.
images = data.test.images[incorrect]
# Get the predicted classes for those images.
cls_pred = cls_pred[incorrect]
# Get the true classes for those images.
cls_true = data.test.cls[incorrect]
# Plot the first 9 images.
plot_images(images=images[0:9],
cls_true=cls_true[0:9],
cls_pred=cls_pred[0:9])
绘制混淆（confusion）矩阵的帮助函数
def plot_confusion_matrix(cls_pred):
# This is called from print_test_accuracy() below.
# cls_pred is an array of the predicted class-number for
# all images in the test-set.
# Get the true classifications for the test-set.
cls_true = data.test.cls
# Get the confusion matrix using sklearn.
cm = confusion_matrix(y_true=cls_true,
y_pred=cls_pred)
# Print the confusion matrix as text.
print(cm)
# Plot the confusion matrix as an image.
plt.matshow(cm)
# Make various adjustments to the plot.
plt.colorbar()
tick_marks = np.arange(num_classes)
plt.xticks(tick_marks, range(num_classes))
plt.yticks(tick_marks, range(num_classes))
plt.xlabel('Predicted')
plt.ylabel('True')
# Ensure the plot is shown correctly with multiple plots
# in a single Notebook cell.
plt.show()
展示性能的帮助函数
函数用来打印测试集上的分类准确度。
为测试集上的所有图片计算分类会花费一段时间，因此我们直接用这个函数来调用上面的结果，这样就不用每次都重新计算了。
这个函数可能会占用很多电脑内存，这也是为什么将测试集分成更小的几个部分。如果你的电脑内存比较小或死机了，就要试着降低batch-size。
# Split the test-set into smaller batches of this size.
test_batch_size = 256
def print_test_accuracy(show_example_errors=False,
show_confusion_matrix=False):
# Number of images in the test-set.
num_test = len(data.test.images)
# Allocate an array for the predicted classes which
# will be calculated in batches and filled into this array.
cls_pred = np.zeros(shape=num_test, dtype=np.int)
# Now calculate the predicted classes for the batches.
# We will just iterate through all the batches.
# There might be a more clever and Pythonic way of doing this.
# The starting index for the next batch is denoted i.
i = 0
while i < num_test:
# The ending index for the next batch is denoted j.
j = min(i + test_batch_size, num_test)
# Get the images from the test-set between index i and j.
images = data.test.images[i:j, :]
# Get the associated labels.
labels = data.test.labels[i:j, :]
# Create a feed-dict with these images and labels.
feed_dict = {x: images,
y_true: labels}
# Calculate the predicted class using TensorFlow.
cls_pred[i:j] = session.run(y_pred_cls, feed_dict=feed_dict)
# Set the start-index for the next batch to the
# end-index of the current batch.
i = j
# Convenience variable for the true class-numbers of the test-set.
cls_true = data.test.cls
# Create a boolean array whether each image is correctly classified.
correct = (cls_true == cls_pred)
# Calculate the number of correctly classified images.
# When summing a boolean array, False means 0 and True means 1.
correct_sum = correct.sum()
# Classification accuracy is the number of correctly classified
# images divided by the total number of images in the test-set.
acc = float(correct_sum) / num_test
# Print the accuracy.
msg = "Accuracy on Test-Set: {0:.1%} ({1} / {2})"
print(msg.format(acc, correct_sum, num_test))
# Plot some examples of mis-classifications, if desired.
if show_example_errors:
print("Example errors:")
plot_example_errors(cls_pred=cls_pred, correct=correct)
# Plot the confusion matrix, if desired.
if show_confusion_matrix:
print("Confusion Matrix:")
plot_confusion_matrix(cls_pred=cls_pred)
优化之前的性能
测试集上的准确度很低，这是由于模型只做了初始化，并没做任何优化，所以它只是对图像做随机分类。
print_test_accuracy()
Accuracy on Test-Set: 9.1% (909 / 10000)
1次迭代后的性能
做了一次优化后，此时优化器的学习率很低，性能其实并没有多大提升。
optimize(num_iterations=1)
Optimization Iteration: 1, Training Accuracy: 6.2%
Time usage: 0:00:00
print_test_accuracy()
Accuracy on Test-Set: 8.9% (892 / 10000)
100次迭代优化后的性能
100次优化迭代之后，模型显著地提升了分类的准确度。
optimize(num_iterations=99) # We already performed 1 iteration above.
Time usage: 0:00:00
print_test_accuracy(show_example_errors=True)
Accuracy on Test-Set: 83.9% (8393 / 10000)
Example errors:
1000次优化迭代后的性能
1000次优化迭代之后，模型在测试集上的准确度超过了90%。
optimize(num_iterations=900) # We performed 100 iterations above.
Optimization Iteration: 101, Training Accuracy: 93.8%
Optimization Iteration: 201, Training Accuracy: 89.1%
Optimization Iteration: 301, Training Accuracy: 85.9%
Optimization Iteration: 401, Training Accuracy: 87.5%
Optimization Iteration: 501, Training Accuracy: 92.2%
Optimization Iteration: 601, Training Accuracy: 95.3%
Optimization Iteration: 701, Training Accuracy: 95.3%
Optimization Iteration: 801, Training Accuracy: 90.6%
Optimization Iteration: 901, Training Accuracy: 98.4%
Time usage: 0:00:03
print_test_accuracy(show_example_errors=True)
Accuracy on Test-Set: 96.3% (9634 / 10000)
Example errors:
10,000次优化迭代后的性能
经过10,000次优化迭代后，测试集上的分类准确率高达99%。
optimize(num_iterations=9000) # We performed 1000 iterations above.
Optimization Iteration: 1001, Training Accuracy: 98.4%
Optimization Iteration: 1101, Training Accuracy: 95.3%
Optimization Iteration: 1201, Training Accuracy: 98.4%
Optimization Iteration: 1301, Training Accuracy: 96.9%
Optimization Iteration: 1401, Training Accuracy: 100.0%
Optimization Iteration: 1501, Training Accuracy: 95.3%
Optimization Iteration: 1601, Training Accuracy: 96.9%
Optimization Iteration: 1701, Training Accuracy: 96.9%
Optimization Iteration: 1801, Training Accuracy: 98.4%
Optimization Iteration: 1901, Training Accuracy: 96.9%
Optimization Iteration: 2001, Training Accuracy: 98.4%
Optimization Iteration: 2101, Training Accuracy: 95.3%
Optimization Iteration: 2201, Training Accuracy: 98.4%
Optimization Iteration: 2301, Training Accuracy: 98.4%
Optimization Iteration: 2401, Training Accuracy: 98.4%
Optimization Iteration: 2501, Training Accuracy: 93.8%
Optimization Iteration: 2601, Training Accuracy: 98.4%
Optimization Iteration: 2701, Training Accuracy: 98.4%
Optimization Iteration: 2801, Training Accuracy: 95.3%
Optimization Iteration: 2901, Training Accuracy: 98.4%
Optimization Iteration: 3001, Training Accuracy: 98.4%
Optimization Iteration: 3101, Training Accuracy: 100.0%
Optimization Iteration: 3201, Training Accuracy: 96.9%
Optimization Iteration: 3301, Training Accuracy: 100.0%
Optimization Iteration: 3401, Training Accuracy: 98.4%
Optimization Iteration: 3501, Training Accuracy: 96.9%
Optimization Iteration: 3601, Training Accuracy: 98.4%
Optimization Iteration: 3701, Training Accuracy: 96.9%
Optimization Iteration: 3801, Training Accuracy: 100.0%
Optimization Iteration: 3901, Training Accuracy: 98.4%
Optimization Iteration: 4001, Training Accuracy: 96.9%
Optimization Iteration: 4101, Training Accuracy: 98.4%
Optimization Iteration: 4201, Training Accuracy: 100.0%
Optimization Iteration: 4301, Training Accuracy: 100.0%
Optimization Iteration: 4401, Training Accuracy: 100.0%
Optimization Iteration: 4501, Training Accuracy: 100.0%
Optimization Iteration: 4601, Training Accuracy: 98.4%
Optimization Iteration: 4701, Training Accuracy: 96.9%
Optimization Iteration: 4801, Training Accuracy: 95.3%
Optimization Iteration: 4901, Training Accuracy: 100.0%
Optimization Iteration: 5001, Training Accuracy: 96.9%
Optimization Iteration: 5101, Training Accuracy: 100.0%
Optimization Iteration: 5201, Training Accuracy: 98.4%
Optimization Iteration: 5301, Training Accuracy: 98.4%
Optimization Iteration: 5401, Training Accuracy: 100.0%
Optimization Iteration: 5501, Training Accuracy: 98.4%
Optimization Iteration: 5601, Training Accuracy: 96.9%
Optimization Iteration: 5701, Training Accuracy: 100.0%
Optimization Iteration: 5801, Training Accuracy: 96.9%
Optimization Iteration: 5901, Training Accuracy: 100.0%
Optimization Iteration: 6001, Training Accuracy: 98.4%
Optimization Iteration: 6101, Training Accuracy: 98.4%
Optimization Iteration: 6201, Training Accuracy: 98.4%
Optimization Iteration: 6301, Training Accuracy: 98.4%
Optimization Iteration: 6401, Training Accuracy: 100.0%
Optimization Iteration: 6501, Training Accuracy: 100.0%
Optimization Iteration: 6601, Training Accuracy: 100.0%
Optimization Iteration: 6701, Training Accuracy: 100.0%
Optimization Iteration: 6801, Training Accuracy: 96.9%
Optimization Iteration: 6901, Training Accuracy: 100.0%
Optimization Iteration: 7001, Training Accuracy: 100.0%
Optimization Iteration: 7101, Training Accuracy: 100.0%
Optimization Iteration: 7201, Training Accuracy: 100.0%
Optimization Iteration: 7301, Training Accuracy: 96.9%
Optimization Iteration: 7401, Training Accuracy: 100.0%
Optimization Iteration: 7501, Training Accuracy: 100.0%
Optimization Iteration: 7601, Training Accuracy: 96.9%
Optimization Iteration: 7701, Training Accuracy: 100.0%
Optimization Iteration: 7801, Training Accuracy: 100.0%
Optimization Iteration: 7901, Training Accuracy: 100.0%
Optimization Iteration: 8001, Training Accuracy: 98.4%
Optimization Iteration: 8101, Training Accuracy: 100.0%
Optimization Iteration: 8201, Training Accuracy: 100.0%
Optimization Iteration: 8301, Training Accuracy: 100.0%
Optimization Iteration: 8401, Training Accuracy: 100.0%
Optimization Iteration: 8501, Training Accuracy: 98.4%
Optimization Iteration: 8601, Training Accuracy: 100.0%
Optimization Iteration: 8701, Training Accuracy: 100.0%
Optimization Iteration: 8801, Training Accuracy: 100.0%
Optimization Iteration: 8901, Training Accuracy: 100.0%
Optimization Iteration: 9001, Training Accuracy: 98.4%
Optimization Iteration: 9101, Training Accuracy: 98.4%
Optimization Iteration: 9201, Training Accuracy: 100.0%
Optimization Iteration: 9301, Training Accuracy: 100.0%
Optimization Iteration: 9401, Training Accuracy: 98.4%
Optimization Iteration: 9501, Training Accuracy: 100.0%
Optimization Iteration: 9601, Training Accuracy: 100.0%
Optimization Iteration: 9701, Training Accuracy: 100.0%
Optimization Iteration: 9801, Training Accuracy: 98.4%
Optimization Iteration: 9901, Training Accuracy: 100.0%
Time usage: 0:00:27
print_test_accuracy(show_example_errors=True,
show_confusion_matrix=True)
Accuracy on Test-Set: 98.8% (9881 / 10000)
Example errors:
Confusion Matrix:
[[ 975 0 0 0 0 0 1 1 3 0]
[ 0 1127 2 0 0 0 1 2 3 0]
[ 2 2 1019 1 1 0 1 2 4 0]
[ 0 0 0 1005 0 1 0 1 3 0]
[ 0 0 0 0 977 0 1 0 1 3]
[ 2 0 0 13 0 870 1 0 6 0]
[ 5 2 0 0 1 3 943 0 4 0]
[ 0 2 8 2 1 0 0 1007 1 7]
[ 2 0 2 3 1 1 0 0 964 1]
[ 0 2 0 4 5 1 0 1 2 994]]
权重和层的可视化
当我们直接用TensorFlow来实现卷积神经网络时，可以很容易地画出卷积权重和不同层的输出图像。当使用PrettyTensor的时候，我们也可以通过上面提到过的方法取得权重，但我们无法简单得到卷积层的输出（图像）。因此下面只绘制了权重。
绘制卷积权重的帮助函数
def plot_conv_weights(weights, input_channel=0):
# Assume weights are TensorFlow ops for 4-dim variables
# e.g. weights_conv1 or weights_conv2.
# Retrieve the values of the weight-variables from TensorFlow.
# A feed-dict is not necessary because nothing is calculated.
w = session.run(weights)
# Get the lowest and highest values for the weights.
# This is used to correct the colour intensity across
# the images so they can be compared with each other.
w_min = np.min(w)
w_max = np.max(w)
# Number of filters used in the conv. layer.
num_filters = w.shape[3]
# Number of grids to plot.
# Rounded-up, square-root of the number of filters.
num_grids = math.ceil(math.sqrt(num_filters))
# Create figure with a grid of sub-plots.
fig, axes = plt.subplots(num_grids, num_grids)
# Plot all the filter-weights.
for i, ax in enumerate(axes.flat):
# Only plot the valid filter-weights.
if i卷积层 1
现在绘制第一个卷积层的滤波权重。
其中正值权重是红色的，负值为蓝色。
plot_conv_weights(weights=weights_conv1)
卷积层 2
现在绘制第二个卷积层的滤波权重。
第一个卷积层有16个输出通道，代表着第二个卷基层有16个输入。第二个卷积层的每个输入通道也有一些权重滤波。我们先绘制第一个通道的权重滤波。
同样的，正值是红色，负值是蓝色。
plot_conv_weights(weights=weights_conv2, input_channel=0)
第二个卷积层共有16个输入通道，我们可以同样地画出15张其他滤波权重图像。这里我们再画一下第二个通道的图像
plot_conv_weights(weights=weights_conv2, input_channel=1)
关闭TensorFlow会话
现在我们已经用TensorFlow完成了任务，关闭session，释放资源。
总结
相比直接使用TensorFlow，PrettyTensor可以用更简单的代码来实现神经网络。这使你能够专注于自己的想法而不是底层的实现细节。它让代码更易于理解，也减少犯错的可能。
然而，PrettyTensor中有一些矛盾和笨拙的设计，它的文档简短而又令人疑惑，也不易于学习。希望未来会有所改进（本文写于2016七月）。
还有一些PrettyTensor的替代品，包括TFLearn和Keras。
练习
下面使一些可能会让你提升TensorFlow技能的一些建议练习。为了学习如何更合适地使用TensorFlow，实践经验是很重要的。
在你对这个Notebook进行修改之前，可能需要先备份一下。
- 将所有层的激活函数改成sigmod。
- 在一些层中使用sigmod，一些层中使用relu。这里能用defaults_scope吗？
- 在所有层里使用12loss。然后试着只在某些层里使用这个。
- 用PrettyTensor的reshape函数代替TensorFlow的。其中某一个会更好吗？
- 在全连接层后面添加一个dropout-layer。如果你在训练和测试的时候想要一个不同的keep_prob，就需要在feed-dict中设置一个placeholder变量。
- 用stride=2来代替 2x2 max-pooling层。分类准确率会有所不同么？你多次优化它们之后呢？差异是随机的，你如何度量是否真实存在差异呢？在卷积层中使用max-pooling和stride的优缺点是什么？
- 改变层的参数，比如kernel、depth、size等等。耗费的时间以及分类准确率有什么差别？
- 添加或删除某些卷积层和全连接层。
- 你设计的表现良好的最简网络是什么？
- 取回卷积层的bias-values并打印出来。参考一下get_weights_variable()的实现。
- 不看源码，自己重写程序。
- 向朋友解释程序如何工作。