fit_transform with the training data and transform with the testing
If you use fit only on the training and transform on the test data, you won't get the correct result.
When using fit_transform on the training data, it means that the machine is learning from the parameters in the feature space and also transforming (scaling) the training data. On the other hand, you should only use transform on the test data to scale it according to the parameters learned from the training data.
As the title says, I am using fit_transform with the CountVectorizer on the training data .. and then I am using tranform only with the testing data ... will this gives me the same as using fit only on the training and tranform only on the testing data ?