Sklearn和Keras是大家做机器学习和深度学习时很熟悉的两个Python库，其中sklearn中有很多机器学习算法、数据预处理以及参数寻优的函数API，keras则可以快速实现你的神经网络结构。那么是什么缘分让sklearn和keras相遇而完美结合呢？
众所周知，神经网络算法模型有很多，通过Python和Keras或者Pytorch构建一个神经网络模型非常方便，那么要想取得一个好的模型效果，就需要对神经网络模型进行调参，单一的人工调参是非常繁琐的，往往不容易取的一个好的效果，所以可以借助sklearn来自动参数搜索，更神奇的是，sklearn中提供keras的包装器，分别为用于分类的keras.wrappers.scikit_learn.KerasClassifier和用于回归的keras.wrappers.scikit_learn.KerasRegressor。接下来我们就来看一下几个通过sklearn网格搜索GridsearchCV进行keras调参的方法。
batch_size和epochs
首先我们使用网格搜索对batch_size和epochs这两个参数进行调整，我们可以根据自己的需要设置待选参数值，在这里我们设置batch_size 为 [10, 20, 40, 60, 80, 100]且epochs 为 [10, 50, 100]。完整代码如下：
代码语言：javascript复制import numpy
from sklearn.model_selection import GridSearchCV
from keras.models import Sequential
from keras.layers import Dense
from keras.wrappers.scikit_learn import KerasClassifier
# Function to create model, required for KerasClassifier
def create_model():
# create model
model = Sequential()
model.add(Dense(12, input_dim=8, activation='relu'))
model.add(Dense(1, activation='sigmoid'))
# Compile model
model.compile(loss='binary_crossentropy', optimizer='adam', metrics=['accuracy'])
return model
# fix random seed for reproducibility
seed = 7
numpy.random.seed(seed)
# load dataset
dataset = numpy.loadtxt("pima-indians-diabetes.csv", delimiter=",")
# split into input (X) and output (Y) variables
X = dataset[:,0:8]
Y = dataset[:,8]
# create model
model = KerasClassifier(build_fn=create_model, verbose=0)
# define the grid search parameters
batch_size = [10, 20, 40, 60, 80, 100]
epochs = [10, 50, 100]
param_grid = dict(batch_size=batch_size, epochs=epochs)
grid = GridSearchCV(estimator=model, param_grid=param_grid, n_jobs=-1)
grid_result = grid.fit(X, Y)
# summarize results
print("Best: %f using %s" % (grid_result.best_score_, grid_result.best_params_))
means = grid_result.cv_results_['mean_test_score']
stds = grid_result.cv_results_['std_test_score']
params = grid_result.cv_results_['params']
for mean, stdev, param in zip(means, stds, params):
print("%f (%f) with: %r" % (mean, stdev, param))训练结果为
代码语言：javascript复制Best: 0.686198 using {'epochs': 100, 'batch_size': 20}
0.348958 (0.024774) with: {'epochs': 10, 'batch_size': 10}
0.348958 (0.024774) with: {'epochs': 50, 'batch_size': 10}
0.466146 (0.149269) with: {'epochs': 100, 'batch_size': 10}
0.647135 (0.021236) with: {'epochs': 10, 'batch_size': 20}
0.660156 (0.014616) with: {'epochs': 50, 'batch_size': 20}
0.686198 (0.024774) with: {'epochs': 100, 'batch_size': 20}
0.489583 (0.075566) with: {'epochs': 10, 'batch_size': 40}
0.652344 (0.019918) with: {'epochs': 50, 'batch_size': 40}
0.654948 (0.027866) with: {'epochs': 100, 'batch_size': 40}
0.518229 (0.032264) with: {'epochs': 10, 'batch_size': 60}
0.605469 (0.052213) with: {'epochs': 50, 'batch_size': 60}
0.665365 (0.004872) with: {'epochs': 100, 'batch_size': 60}
0.537760 (0.143537) with: {'epochs': 10, 'batch_size': 80}
0.591146 (0.094954) with: {'epochs': 50, 'batch_size': 80}
0.658854 (0.054904) with: {'epochs': 100, 'batch_size': 80}
0.402344 (0.107735) with: {'epochs': 10, 'batch_size': 100}
0.652344 (0.033299) with: {'epochs': 50, 'batch_size': 100}
0.542969 (0.157934) with: {'epochs': 100, 'batch_size': 100}我们可以看到batch_size为20和epochs为100个时达到最佳准确度68％。
优化算法的选择
神经网络中的优化算法有sgd、adam、RMSprop等，如何选择一个合适的优化算法是非常重要的。下面我们将通过一个小例子来展示如何通过网格搜索挑选优化算法。代码如下：
代码语言：javascript复制import numpy
from sklearn.model_selection import GridSearchCV
from keras.models import Sequential
from keras.layers import Dense
from keras.wrappers.scikit_learn import KerasClassifier
# Function to create model, required for KerasClassifier
def create_model(optimizer='adam'):
# create model
model = Sequential()
model.add(Dense(12, input_dim=8, activation='relu'))
model.add(Dense(1, activation='sigmoid'))
# Compile model
model.compile(loss='binary_crossentropy', optimizer=optimizer, metrics=['accuracy'])
return model
# fix random seed for reproducibility
seed = 7
numpy.random.seed(seed)
# load dataset
dataset = numpy.loadtxt("pima-indians-diabetes.csv", delimiter=",")
# split into input (X) and output (Y) variables
X = dataset[:,0:8]
Y = dataset[:,8]
# create model
model = KerasClassifier(build_fn=create_model, epochs=100, batch_size=10, verbose=0)
# define the grid search parameters
optimizer = ['SGD', 'RMSprop', 'Adagrad', 'Adadelta', 'Adam', 'Adamax', 'Nadam']
param_grid = dict(optimizer=optimizer)
grid = GridSearchCV(estimator=model, param_grid=param_grid, n_jobs=-1)
grid_result = grid.fit(X, Y)
# summarize results
print("Best: %f using %s" % (grid_result.best_score_, grid_result.best_params_))
means = grid_result.cv_results_['mean_test_score']
stds = grid_result.cv_results_['std_test_score']
params = grid_result.cv_results_['params']
for mean, stdev, param in zip(means, stds, params):
print("%f (%f) with: %r" % (mean, stdev, param))输出结果为
代码语言：javascript复制Best: 0.704427 using {'optimizer': 'Adam'}
0.348958 (0.024774) with: {'optimizer': 'SGD'}
0.348958 (0.024774) with: {'optimizer': 'RMSprop'}
0.471354 (0.156586) with: {'optimizer': 'Adagrad'}
0.669271 (0.029635) with: {'optimizer': 'Adadelta'}
0.704427 (0.031466) with: {'optimizer': 'Adam'}
0.682292 (0.016367) with: {'optimizer': 'Adamax'}
0.703125 (0.003189) with: {'optimizer': 'Nadam'}可见，针对不同的问题，不同的优化函数取得的结果是不一样的，从本例结果可以看到adam算法取得最优结果。
挑选激活函数
激活函数是神经网络的重要部分，比较常用的激活函数有sigmod、relu以及softmax。不同的激活函数又其各自的适应范围，下面我们就来看一下如何通过网格搜索获得最优激活函数。完整代码如下：
代码语言：javascript复制import numpy
from sklearn.model_selection import GridSearchCV
from keras.models import Sequential
from keras.layers import Dense
from keras.wrappers.scikit_learn import KerasClassifier
# Function to create model, required for KerasClassifier
def create_model(activation='relu'):
# create model
model = Sequential()
model.add(Dense(12, input_dim=8, kernel_initializer='uniform', activation=activation))
model.add(Dense(1, kernel_initializer='uniform', activation='sigmoid'))
# Compile model
model.compile(loss='binary_crossentropy', optimizer='adam', metrics=['accuracy'])
return model
# fix random seed for reproducibility
seed = 7
numpy.random.seed(seed)
# load dataset
dataset = numpy.loadtxt("pima-indians-diabetes.csv", delimiter=",")
# split into input (X) and output (Y) variables
X = dataset[:,0:8]
Y = dataset[:,8]
# create model
model = KerasClassifier(build_fn=create_model, epochs=100, batch_size=10, verbose=0)
# define the grid search parameters
activation = ['softmax', 'softplus', 'softsign', 'relu', 'tanh', 'sigmoid', 'hard_sigmoid', 'linear']
param_grid = dict(activation=activation)
grid = GridSearchCV(estimator=model, param_grid=param_grid, n_jobs=-1)
grid_result = grid.fit(X, Y)
# summarize results
print("Best: %f using %s" % (grid_result.best_score_, grid_result.best_params_))
means = grid_result.cv_results_['mean_test_score']
stds = grid_result.cv_results_['std_test_score']
params = grid_result.cv_results_['params']
for mean, stdev, param in zip(means, stds, params):
print("%f (%f) with: %r" % (mean, stdev, param))输出结果为：
代码语言：javascript复制Best: 0.722656 using {'activation': 'linear'}
0.649740 (0.009744) with: {'activation': 'softmax'}
0.720052 (0.032106) with: {'activation': 'softplus'}
0.688802 (0.019225) with: {'activation': 'softsign'}
0.720052 (0.018136) with: {'activation': 'relu'}
0.691406 (0.019401) with: {'activation': 'tanh'}
0.680990 (0.009207) with: {'activation': 'sigmoid'}
0.691406 (0.014616) with: {'activation': 'hard_sigmoid'}
0.722656 (0.003189) with: {'activation': 'linear'}由结果可以看到，线性激活函数取得了最优的结果，并不是我们常用的sigmod和relu。到这里大家应该知道如何通过sklearn中的网格搜索来对神经网络调参，本例程只列出来几个神经网络的参数，还有学习率以及神经元数量等参数的调整方法一样，只需要将待选参数输入进去就可以等待运行结果。大家如果有兴趣可以找一个数据集然后按照例程中的程序自己运行一遍，你将会有不一样的收获。数据集下载可以去UCL。
