Use @ character to reference variable inside DataFrame.query

To evaluate conditions in a pandas dataframes I now use the df.query('column == "some_value"') syntax, rather than df[df['column']=="some_value"], which I find less readable.

With the DataFrame.query method the query takes the form of a string, so variables can't be accessed in the usual way.

Instead, to reference variables we need to use the "@" character in front of them, like so:

    countries = ['India', 'Malaysia', 'Thailand']
    df.query('Year >= 2001 & `Country Name` in @countries')

(Also, backticks are used for column names that contain spaces, like Country Name)