NumesSanguis NumesSanguis - 4 months ago 22
Python Question

sklearn Kfold acces single fold instead of for loop

After using cross_validation.KFold(n, n_folds=folds) I would like to access the indexes for training and testing of single fold, instead of going through all the folds.

So let's take the example code:

from sklearn import cross_validation
X = np.array([[1, 2], [3, 4], [1, 2], [3, 4]])
y = np.array([1, 2, 3, 4])
kf = cross_validation.KFold(4, n_folds=2)

>>> print(kf)
sklearn.cross_validation.KFold(n=4, n_folds=2, shuffle=False,
random_state=None)
>>> for train_index, test_index in kf:


I would like to access the first fold in kf like this (instead of for loop):

train_index, test_index in kf[0]


This should return just the first fold, but instead I get the error: "TypeError: 'KFold' object does not support indexing"

What I want as output:

>>> train_index, test_index in kf[0]
>>> print("TRAIN:", train_index, "TEST:", test_index)
TRAIN: [2 3] TEST: [0 1]


Link: http://scikit-learn.org/stable/modules/generated/sklearn.cross_validation.KFold.html

Question



How do I retrieve the indexes for train and test for only a single fold, without going through the whole for loop?

Answer

You are on the right track. All you need to do now is:

kf = cross_validation.KFold(4, n_folds=2)
mylist = list(kf)
train, test = mylist[0]

kf is actually a generator, which doesn't compute the train-test split until it is needed. This improves memory usage, as you are not storing items you don't need. Making a list of the KFold object forces it to make all values available.

Here are two great SO question that explain what generators are: one and two