https://www.jianshu.com/p/55b9f2ea283b
Simple Grid Search：简单的网格搜索
from sklearn.datasets import load_iris
from sklearn.svm import SVC
from sklearn.model_selection import train_test_split
iris = load_iris()
X_train,X_test,y_train,y_test = train_test_split(iris.data,iris.target,random_state=0)
print("Size of training set:{} size of testing set:{}".format(X_train.shape[0],X_test.shape[0]))
#### grid search start
best_score = 0
for gamma in [0.001,0.01,0.1,1,10,100]:
for C in [0.001,0.01,0.1,1,10,100]:
svm = SVC(gamma=gamma,C=C)#对于每种参数可能的组合，进行一次训练；
svm.fit(X_train,y_train)
score = svm.score(X_test,y_test)
if score > best_score:#找到表现最好的参数
best_score = score
best_parameters = {'gamma':gamma,'C':C}
#### grid search end
print("Best score:{:.2f}".format(best_score))
print("Best parameters:{}".format(best_parameters))
存在的问题：
原始数据集划分成训练集和测试集以后，其中测试集除了用作调整参数，也用来测量模型的好坏；这样做导致最终的评分结果比实际效果要好。（因为测试集在调参过程中，送到了模型里，而我们的目的是将训练模型应用在unseen data上）
解决方法：
对训练集再进行一次划分，分成训练集和验证集，这样划分的结果就是：原始数据划分为3份，分别为：训练集、验证集和测试集；其中训练集用来模型训练，验证集用来调整参数，而测试集用来衡量模型表现好坏。
image.png
X_trainval,X_test,y_trainval,y_test = train_test_split(iris.data,iris.target,random_state=0)
X_train,X_val,y_train,y_val = train_test_split(X_trainval,y_trainval,random_state=1)
print("Size of training set:{} size of validation set:{} size of teseting set:{}".format(X_train.shape[0],X_val.shape[0],X_test.shape[0]))
best_score = 0.0
for gamma in [0.001,0.01,0.1,1,10,100]:
for C in [0.001,0.01,0.1,1,10,100]:
svm = SVC(gamma=gamma,C=C)
svm.fit(X_train,y_train)
score = svm.score(X_val,y_val)
if score > best_score:
best_score = score
best_parameters = {'gamma':gamma,'C':C}
svm = SVC(**best_parameters) #使用最佳参数，构建新的模型
svm.fit(X_trainval,y_trainval) #使用训练集和验证集进行训练，more data always results in good performance.
test_score = svm.score(X_test,y_test) # evaluation模型评估
print("Best score on validation set:{:.2f}".format(best_score))
print("Best parameters:{}".format(best_parameters))
print("Best score on test set:{:.2f}".format(test_score))
Grid Search with Cross Validation 网格搜索与交叉验证
from sklearn.model_selection import cross_val_score
best_score = 0.0
for gamma in [0.001,0.01,0.1,1,10,100]:
for C in [0.001,0.01,0.1,1,10,100]:
svm = SVC(gamma=gamma,C=C)
scores = cross_val_score(svm,X_trainval,y_trainval,cv=5) #5折交叉验证
score = scores.mean() #取平均数
if score > best_score:
best_score = score
best_parameters = {"gamma":gamma,"C":C}
svm = SVC(**best_parameters)
svm.fit(X_trainval,y_trainval)
test_score = svm.score(X_test,y_test)
print("Best score on validation set:{:.2f}".format(best_score))
print("Best parameters:{}".format(best_parameters))
print("Score on testing set:{:.2f}".format(test_score))
交叉验证经常与网格搜索进行结合，作为参数评价的一种方法，这种方法叫做grid search with cross validation。sklearn因此设计了一个这样的类GridSearchCV，这个类实现了fit，predict，score等方法，被当做了一个estimator，使用fit方法，该过程中：（1）搜索到最佳参数；（2）实例化了一个最佳参数的estimator；
from sklearn.model_selection import GridSearchCV
#把要调整的参数以及其候选值 列出来；
param_grid = {"gamma":[0.001,0.01,0.1,1,10,100],
"C":[0.001,0.01,0.1,1,10,100]}
# 我们需要为每个参数（C和gamma）指定它在管道中所属的步骤。
# 为管道定义参数网格的语法是为每个参数指定步骤名称，后面加上__（双下划线），
# 然后是参数名称（svm__C和svm_gamma）
grid_search = GridSearchCV(SVC(),param_grid,cv=5)
#实例化一个GridSearchCV类，如果要多步处理则改用Pipeline
X_train,X_test,y_train,y_test = train_test_split(iris.data,iris.target,random_state=10)
grid_search.fit(X_train,y_train) #训练，找到最优的参数，同时使用最优的参数实例化一个新的SVC estimator。
>>>print("Parameters:{}".format(param_grid))
Parameters:{'gamma': [0.001, 0.01, 0.1, 1, 10, 100], 'C': [0.001, 0.01, 0.1, 1, 10, 100]}
>>>print("Test set score:{:.2f}".format(grid_search.score(X_test,y_test)))
Test set score:0.97
>>>print("Best parameters:{}".format(grid_search.best_params_))
Best parameters:{'C': 10, 'gamma': 0.1}
>>>print("Best score on train set:{:.2f}".format(grid_search.best_score_))
Best score on train set:0.98
Grid Search 调参方法存在的共性弊端就是：耗时；参数越多，候选值越多，耗费时间越长。所以，一般情况下，先定一个大范围，然后再细化。
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