Error in the coding : TypeError : list indices must be integer, not str

Code is importing another file, which is working perfectly.

But, there is a problem in the line where I try to import the csv file, with a column called 'account key', returning the TypeError above.

import file_import as fi

Function for collectively finding data necessary from a csv file.

def unique_students(csv_file):
    unique_students_list = set()
    for information in csv_file:
        unique_students_list.add(csv_file["account_key"])
    return len(unique_students_list)

#enrollment_num_rows = len(fi.enrollments)  
#engagement_num_rows = len(fi.daily_engagement)           
#submission_num_rows = len(fi.project_submissions)    

#enrollment_num_unique_students = unique_students(fi.enrollments)
#engagement_num_unique_students = unique_students(fi.daily_engagement)
#submission_num_unique_students = unique_students(fi.project_submissions)