how can I generate a password in hashlib

[buran]

And here is what you really need to do

import csv
import hashlib

# create rainbow table
rainbow_table = {}
for i in range(1000, 10000): # note you need upper end to be 10000 in odrder to include 9999
    my_hash = hashlib.sha256(str(i).encode()).hexdigest()
    rainbow_table[my_hash] = i

# read the input file
with open ('code.csv', 'r') as in_file:
    rdr=csv.reader(in_file)
    data = [[name, rainbow_table.get(my_hash, '')] for name, my_hash in rdr]

# print data to see what you've got
print(data)

with open('decoded.csv', 'w', newline='') as out_file:
        wrtr = csv.writer(out_file)
        wrtr.writerows(data)

if you don't want to process the entire file in memory before write to output file eventually you may

import csv
import hashlib

# create rainbow table
rainbow_table = {}
for i in range(1000, 10000): # note you need upper end to be 10000 in odrder to include 9999
    my_hash = hashlib.sha256(str(i).encode()).hexdigest()
    rainbow_table[my_hash] = i

# read the input file, write to output
with open ('code.csv', 'r') as in_file, open('decoded.csv', 'w', newline='') as out_file:
    rdr=csv.reader(in_file)
    wrtr = csv.writer(out_file)
    for name, my_hash in rdr:
        data = [name, rainbow_table.get(my_hash, '')]
        wrtr.writerow(data)