Adding and removing data
In the previous sections, we frequently selected a subset of the columns, but if columns/rows aren't useful to us, we should just get rid of them. We also frequently selected data based on the value of the mag column; however, if we had made a new column holding the Boolean values for later selection, we would have only needed to calculate the mask once. Very rarely will we get data where we neither want to add nor remove something.
Before we begin adding and removing data, it's important to understand that while most methods will return a new DataFrame object, some will be in-place and change our data. If we write a function where we pass in a dataframe and change it, it will change our original dataframe as well. Should we find ourselves in a situation where we don't want to change the original data, but rather want to return a new copy of the data that has been modified, we must be sure to copy our dataframe before making any changes:
