Looping through a list of pandas dataframes

Question

Two quick pandas questions for you.

1. I have a list of dataframes I would like to apply a filter to.

   countries = [us, uk, france]
   for df in co         
   
   
           

Answer 1

For why

for df in countries:
    df["Continent"] = "Europe"

modifies countries, while

for df in countries:
    df = df[(df["Send Date"] > '2016-11-01') & (df["Send Date"] < '2016-11-30')] 

does not, see why should I make a copy of a data frame in pandas. df is a reference to the actual DataFrame in countries, and not the actual DataFrame itself, but modifications to a reference affect the original DataFrame as well. Declaring a new column is a modification. However, taking a subset is not a modification. It is just changing what the reference is referring to in the original DataFrame.