How to combine and separate test and train data for data cleaning?
There are several methods to choose from. If you insist on concatenating the two dataframes, then first add a new column to each DataFrame called source. Make the value for test.csv 'test' and likewise for the training set.
When you have finished cleaning the combined df, then use the source column to split the data again.
An alternative method is to record all the operations you perform on the training set and simply repeat for the test set. This won't work it you normalise values based on the population.