Why is are my rows not being written to the CSV file?

Question

With the code below the CSV file aaab.csv is created but nothing is written to it. The print(row) gives me the correct information. What am I doing wrong? I have tried writer.close() but that gives me an error.

import csv
from csv import writer
from csv import reader
dict1 = {}
dict2 = {}
duplist = []
notinlist2 = []
counta = 0
countb = 0
with open('/Users/list2.csv') as file_handler:
    reader = csv.reader(file_handler)
    next(reader, None) # skip header
    for row in reader:
        key = row[1]
        counta += 1
        #print(key)
        if key in dict1.keys():
            duplist.append(key)
            
        else:
            dict1[row[1]] = counta


reader = csv.reader(open('/Users/list1.csv','r'))
writer = csv.writer(open('/Users/aaab.csv', 'w',encoding='UTF8'))
next(reader, None) # skip header
headers = ['MVA','REG','No']
writer.writerow(headers)
for row in reader:
    key = row[1]
    
    if key in dict1.keys():
        a = dict1.get(row[1])
        countb += 1
        row.append(a)
        writer.writerow(row)
        print(row)
        
                
    else:
        notinlist2.append(key)
        writer.writerow(row)
        print(row)

    
print(len(dict1))
print(duplist)
print(notinlist2)
print(countb)

Here is a sample CSV

MVA    ,REG 
8144802,BK36HNGP 
5296130,BN33KWGP 
3254005,BT50XBGP