I am using IDF estimator/model (TF-IDF) to convert text features into vectors. Currently, I fit IDF model on all sample data and then transform them. I read somewhere that I should split my data into training and test before fitting IDF model; Fit IDF only on training data and then use same transformer to transform training and test data. 
This raise more questions:
1) Why would you do that? What exactly do IDF learn during fitting process that it can reuse to transform any new dataset. Perhaps idea is to keep same value for |D| and DF|t, D| while use new TF|t, D| ?
2) If not then fitting and transforming seems redundant for IDF model



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