sklearn 中的交叉验证
sklearn中的交叉验证（Cross-Validation）
sklearn是利用python进行机器学习中一个非常全面和好用的第三方库，用过的都说好。今天主要记录一下sklearn中关于交叉验证的各种用法，主要是对sklearn官方文档 Cross-validation: evaluating estimator performance进行讲解，英文水平好的建议读官方文档，里面的知识点很详细。
1. cross_val_score
对数据集进行指定次数的交叉验证并为每次验证效果评测
其中，score 默认是以 scoring=’f1_macro’进行评测的，余外针对分类或回归还有：
这需要from sklearn import metrics ,通过在cross_val_score 指定参数来设定评测标准；
当cv 指定为int 类型时，默认使用KFold 或StratifiedKFold 进行数据集打乱，下面会对KFold 和StratifiedKFold 进行介绍。
In [15]: from sklearn.model_selection import cross_val_score In [16]: clf = svm.SVC(kernel='linear', C=1) In [17]: scores = cross_val_score(clf, iris.data, iris.target, cv=5) In [18]: scores Out[18]: array([ 0.96666667, 1. , 0.96666667, 0.96666667, 1. ]) In [19]: scores.mean() Out[19]: 0.98000000000000009
除使用默认交叉验证方式外，可以对交叉验证方式进行指定，如验证次数，训练集测试集划分比例等
In [20]: from sklearn.model_selection import ShuffleSplit In [21]: n_samples = iris.data.shape[0] In [22]: cv = ShuffleSplit(n_splits=3, test_size=.3, random_state=0) In [23]: cross_val_score(clf, iris.data, iris.target, cv=cv) Out[23]: array([ 0.97777778, 0.97777778, 1. ])
2. cross_val_predict
cross_val_predict 与cross_val_score 很相像，不过不同于返回的是评测效果，cross_val_predict 返回的是estimator 的分类结果（或回归值），这个对于后期模型的改善很重要，可以通过该预测输出对比实际目标值，准确定位到预测出错的地方，为我们参数优化及问题排查十分的重要。
In [28]: from sklearn.model_selection import cross_val_predict
In [29]: from sklearn import metrics
In [30]: predicted = cross_val_predict(clf, iris.data, iris.target, cv=10)
In [31]: predicted
Out[31]:
array([0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 2, 2,
2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2,
2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2])
In [32]: metrics.accuracy_score(iris.target, predicted)
Out[32]: 0.96666666666666667
3. KFold
K折交叉验证，这是将数据集分成K份的官方给定方案，所谓K折就是将数据集通过K次分割，使得所有数据既在训练集出现过，又在测试集出现过，当然，每次分割中不会有重叠。相当于无放回抽样。
In [33]: from sklearn.model_selection import KFold
In [34]: X = ['a','b','c','d']
In [35]: kf = KFold(n_splits=2)
In [36]: for train, test in kf.split(X):
...: print train, test
...: print np.array(X)[train], np.array(X)[test]
...: print '\n'
...:
[2 3] [0 1]
['c' 'd'] ['a' 'b']
[0 1] [2 3]
['a' 'b'] ['c' 'd']
4. LeaveOneOut
LeaveOneOut 其实就是KFold 的一个特例，因为使用次数比较多，因此独立的定义出来，完全可以通过KFold 实现。
In [37]: from sklearn.model_selection import LeaveOneOut
In [38]: X = [1,2,3,4]
In [39]: loo = LeaveOneOut()
In [41]: for train, test in loo.split(X):
...: print train, test
...:
[1 2 3] [0]
[0 2 3] [1]
[0 1 3] [2]
[0 1 2] [3]
#使用KFold实现LeaveOneOtut
In [42]: kf = KFold(n_splits=len(X))
In [43]: for train, test in kf.split(X):
...: print train, test
...:
[1 2 3] [0]
[0 2 3] [1]
[0 1 3] [2]
[0 1 2] [3]
5. LeavePOut
这个也是KFold 的一个特例，用KFold 实现起来稍麻烦些，跟LeaveOneOut 也很像。
In [44]: from sklearn.model_selection import LeavePOut
In [45]: X = np.ones(4)
In [46]: lpo = LeavePOut(p=2)
In [47]: for train, test in lpo.split(X):
...: print train, test
...:
[2 3] [0 1]
[1 3] [0 2]
[1 2] [0 3]
[0 3] [1 2]
[0 2] [1 3]
[0 1] [2 3]
6. ShuffleSplit
ShuffleSplit 咋一看用法跟LeavePOut 很像，其实两者完全不一样，LeavePOut 是使得数据集经过数次分割后，所有的测试集出现的元素的集合即是完整的数据集，即无放回的抽样，而ShuffleSplit 则是有放回的抽样，只能说经过一个足够大的抽样次数后，保证测试集出现了完成的数据集的倍数。
In [48]: from sklearn.model_selection import ShuffleSplit
In [49]: X = np.arange(5)
In [50]: ss = ShuffleSplit(n_splits=3, test_size=.25, random_state=0)
In [51]: for train_index, test_index in ss.split(X):
...: print train_index, test_index
...:
[1 3 4] [2 0]
[1 4 3] [0 2]
[4 0 2] [1 3]
7. StratifiedKFold
对测试集合进行无放回抽样
In [52]: from sklearn.model_selection import StratifiedKFold
In [53]: X = np.ones(10)
In [54]: y = [0,0,0,0,1,1,1,1,1,1]
In [55]: skf = StratifiedKFold(n_splits=3)
In [56]: for train, test in skf.split(X,y):
...: print train, test
...:
[2 3 6 7 8 9] [0 1 4 5]
[0 1 3 4 5 8 9] [2 6 7]
[0 1 2 4 5 6 7] [3 8 9]
如果各个类的分布不均衡的话，使用micro F1score比macro F1score 比较好，显然macro F1score没有考虑各个类的数量大小
原文：https://blog.csdn.net/xiaodongxiexie/article/details/71915259
