Python, selecting a specific file - python-3.x

Question

I need to select a specific file, after specifying the number of this file in "if"...
Instead of "MTS.txt " we need the full path to the selected file using a digit!
I have not found any information about this on the Internet. Perhaps he made up his request badly!
from email import message
from click import clear
import pywhatkit
import pyfiglet
import os
import time
directory = 'MTS/'
# files = os.listdir(directory)
f = True
while (f==True):
def main():
os.system('CLS')
# print(files)
mtstxt = pyfiglet.figlet_format("Wa Helper", font = "smslant")
print(mtstxt)
def send_message_inst():
mobile = '+7' + input(' Номер: +7')
with os.scandir(directory) as files:
for i, file in enumerate(files):
print(' ', i + 1, file.name)
nof = os.listdir(path="MTS/")
# print(len(nof))
while True:
tarif = int(input(' Вариант: '))
if tarif >= 1 and tarif <= int(len(nof)):
msg = open("MTS.txt", encoding='UTF-8')
message = msg.read()
msg.close()
break
else:
os.system('CLS')
print(mtstxt)
print(' Номер:', mobile)
with os.scandir(directory) as files:
for i, file in enumerate(files):
print(' ', i + 1, file.name)
print("\033[31m{}\033[0m".format(" Ошибка - неверный вариант!"))
pywhatkit.sendwhatmsg_instantly(phone_no=mobile, message=message)
send_message_inst()
os.system('CLS')
sndtxt = pyfiglet.figlet_format("Sent!", font = "smslant")
print(sndtxt)
time.sleep(5)
if __name__ == '__main__':
main()
else:
break