set item code based on incremental alphanumeric number

next_alphanumeric_number.py

def custom_validate(self):
	# serial_no is a custom field in item group which can be any alphanumeric number
	serial_no = webnotes.conn.get_value("Item Group", self.doc.item_group, "serial_no")
	
	#increment serial_no value and assign to item_code
	next_serial_no = self.next_string(serial_no)
	self.doc.item_code = next_serial_no
	
	# update incremented serial_no in item group
	webnotes.conn.set_value("Item Group", self.doc.item_group, "serial_no", next_serial_no)
	
def next_string(self, s):
	strip_zs = s.rstrip('z')
	if strip_zs:
		return strip_zs[:-1] + chr(ord(strip_zs[-1]) + 1) + 'a' * (len(s) - len(strip_zs))
	else:
		return 'a' * (len(s) + 1)