A dataframe column is a pd.Series. So, I don’t quite get your question. What is it you want to do (what’s the purpose of the flattening/multi-index creation)?

I want to represent a 3-dimensional dataset by a typical (2-dimensional) dataframe. You are right, a single dataframe column would work. Thank you so much Hans!

Just to clarify, the problem I have is that I have zoo 1, zoo 2, zoo 3, … For each zoo, I have a cat and a dog, and I have their data (weight, height, blood_pressure). Since it’s hard to visualize the 3-dimensional data, I try to convert the dataframe for each zoo to a column, so that I may show all data in a typical 2-dimensional dataframe.