Fix Python – How to split/partition a dataset into training and test datasets for, e.g., cross validation?

If you want to split the data set once in two parts, you can use numpy.random.shuffle, or numpy.random.permutation if you need to keep track of the indices (remember to fix the random seed to make everything reproducible):

import numpy
# x is your dataset
x = numpy.random.rand(100, 5)
numpy.random.shuffle(x)
training, test = x[:80,:], x[80:,:]

or

import numpy
# x is your dataset
x = numpy.random.rand(100, 5)
indices = numpy.random.permutation(x.shape[0])
training_idx, test_idx = indices[:80], indices[80:]
training, test = x[training_idx,:], x[test_idx,:]

There are many ways other ways to repeatedly partition the same data set for cross validation. Many of those are available in the sklearn library (k-fold, leave-n-out, …). sklearn also includes more advanced “stratified sampling” methods that create a partition of the data that is balanced with respect to some features, for example to make sure that there is the same proportion of positive and negative examples in the training and test set.