How Do I Interpret This Notation?

How and why does the notation s[s] work? I'm taking one of the micro-courses from kaggle.com and they use the notation s[s] as shown below. I have not seen that before. X_train is

Solution 1:

This is quite complicated.

X_train is a pandas data frame.

X_train.dtypes is returning a pandas Series, where the index (name of each row) is equal to the column name.

We now do == on the Series which returns a new series, with the value true or false. So it looks like:

a True b False c True

Now we get to the x[x] which says to remove the 'false' values, giving a new Series:

a True c True

Now we do .index and turn it into a list to give

['a', 'c' ]

Solution 2:

Pandas DataFrames allow you to index using boolean arrays which is how s is being used inside []. The value of the Series is True or False as you can see, so we're selecting the values of s where s is True. The purpose of this code is to get columns where the datatype is object, you can do it with the function pandas.DataFrame.select_dtypes instead:

list(X_train.select_dtypes(include=['object']).columns)