What is the difference between pd concat frames and pd concat frames key

Question

I came across two command while I was finding out how to concatenate Pandas dataframe: pd.concat(frames) and pd.concat(frames, key[]). Can someone tell me what is the difference between these two?

Answer 1

Both the codes are used to concatenate the dataframes. 

pd.concatenate(frames)

This code will take dataframes in the frames list and concatenate them forming a new dataframe.

pd.concatenate(frames, key[])

This code will also concatenate the dataframes but the difference is that this code will add certain keys to the newly formed database to identify those dataframes. This is how the data is stored:

Here, x is the key to dataframe df1, y is the key to dataframe df2, z is the key to dataframe df3.