Need to generate serial number - python-2.7

Question

I have requirement to generate 4 character serial numbers like below in Python or Shell Scripting.
Serial number should start from 0001, 0002..... when reached 999 it should generate A001,A002....A999, then B001, so on.
I tried below code in Python, but its not fully working, After few number it starts generating 5 characters..
def excel_format(num):
res = ""
while num:
mod = (num - 1) % 26
res = chr(65 + mod) + res
num = (num - mod) // 26
return res
def full_format(num, d=3):
set_flag = 0
chars = num // (10**d-1) + 1 # this becomes A..ZZZ
if len(excel_format(chars)) >= 2:
set_flag = 1
if len(excel_format(chars)) > 2:
set_flag = 2
if set_flag == 1:
d = 2
chars = num // (10 ** d - 1) + 1 # this becomes A..ZZZ
digit = num % (10**d-1) + 1 # this becomes 001..999
return excel_format(chars) + "{:0{}d}".format(digit, d)
if __name__ == '__main__':
for i in range(1,10001):
unique_code = full_format(j, d=3)
print('Unique Code is =>', unique_code)

Answer 1

This Python code will generate the required 4-character (Unique_code) serial numbers:
#!/usr/bin/python3
import re
for i in range(1,10000):
if (i < 1000):
print ("i =", str(i).zfill(4))
else:
m = re.findall(r'(\d)(\d\d\d)', str(i))
code = 64+int(m[0][0])
print ("i =",i, "Unique_code =", chr(code) + m[0][1])
Excerpts from output:
i = 0001
i = 0002
...
i = 0999
i = 1000 Unique_code = A000
i = 1001 Unique_code = A001
i = 1002 Unique_code = A002
...
i = 1997 Unique_code = A997
i = 1998 Unique_code = A998
i = 1999 Unique_code = A999
i = 2000 Unique_code = B000
i = 2001 Unique_code = B001
i = 2002 Unique_code = B002
i = 2003 Unique_code = B003
...
i = 9997 Unique_code = I997
i = 9998 Unique_code = I998
i = 9999 Unique_code = I999

Answer 2

Not familiar enough with Python but since you've flagged ksh
#!/bin/ksh
typeset -Z3 sn
Letter=( 0 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z )
Index=0
while [[ $Index -lt 28 ]]; do
sn=0
while [[ $sn -lt 1000 ]]; do
print ${Letter[$Index]}$sn
((sn++))
done
((Index++))
done