There is a df_new Pandas dataframe: year date route operator group_name bus_garage district injury_result incident_type victim_category victim_sex victim_age 0 2015 01.01.2015 1 London General Go-Ahead Garage Not Available Southwark Injuries treated on scene Onboard Injuries Passenger Male Child 1 2015 01.01.2015 4 Metroline Metroline Garage Not Available Islington Injuries treated on scene Onboard Injuries Passenger Male Unknown 2 2015 01.01.2015 5 East London Stagecoach Garage Not Available Havering Taken to Hospital – Reported Serious Injury or... Onboard Injuries Passenger Male Elderly Calculate the average number of "date" string values for all "operator"

def mean_str_len(data, column1, column2):
    # data - a dataframe
    # column1 - a string, the name of a column
    # column2 - a string, the name of another column
    # YOUR CODE HERE
    ...