module 'sklearn' has no attribute 'model_selection'

Question

Why does the code below return the error

module 'sklearn' has no attribute 'model_selection'

import sklearn
xtrain, xtest, ytrain, ytest = sklearn.model_selection.train_test_split(x,y, test_size = 0.2)

but this one works fine:

from sklearn.model_selection import train_test_split
xtrain, xtest, ytrain, ytest = train_test_split(x,y, test_size = 0.2)

note that I did declare x and y as arrays.

Answer 1

In Python, when you import a module using a statement like

 import sklearn

you're essentially importing only the top-level module, and you do not automatically gain access to all the submodules or functions contained within that module. That's why you often need to import specific submodules like

from sklearn.model_selection import train_test_split