TypeError:unhashable type: 'list'

Question

Hi Every one,
I just started the python course and would like to make a table containing the "unique cities USA" from the given .csv file. I want the table to contain the STATE_NAME, unique cities, etc. I wrote the loop down, but when i Run it, it came the "TypeError:unhashable type: 'list'" highlighting the " return set(columns)". Can any one help me, please?

import csv
def unique_cities_USA(input_csv_file,top,header_name):
f=open(r'H:\paython\USCounties.csv','r')
reader =csv.reader(f)
header = next(reader)
print(header)
header_var = header.index(header_name)
rows=[]
while(reader.line_num<top):
rows.append(next(reader))
columns=[]
for row in rows:
columns.append(rows[header_var])
f.close()
return set(columns)
csv_path = r"H:\paython\USCounties.csv"
print (unique_cities_USA(input_csv_file =csv_path,top=100,header_name='STATE_NAME'))

Answer 1

Usually because you entered mutable object (like list )n the set function