When scale the data, why the train dataset use 'fit' and 'transform', but the test dataset only use 'transform'?

Question

When scale the data, why the train dataset use \'fit\' and \'transform\', but the test dataset only use \'transform\'?

SAMPLE_COUNT = 5000
TEST_COUNT = 20000
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Answer 1

we use fit() or fit_transform() in order to learn (to train the model) on the train data set. transform() can be used on the trained model against the test data set.