Python: Add random number to filename (without repeating a number)

I have a folder with up to 1000 files and I want to add a random number to each of the files without repeating any number.

My files look like this:

file_a.jpg
file_b.jpg 
file_c.jpg 
file_d.jpg

and I want them to be like this:

3_file_a.jpg
1_file_b.jpg
4_file_c.jpg
2_file_d.jpg

This is my code snippet so far, but using random.sample() seems not to be the right solution. At the moment it creates a list of random variables, which are non repeating, but it creates a new list for each filename. Instead of just one non-repeating number per filename. Might be a dumb question, but I am new to python and couldn't figure it out yet how to do this properly.

import os
from os import rename
import random

os.chdir('C:/Users/......')

for f in os.listdir():  
   file_name, file_ext = os.path.splitext(f)    
   #file_name = str(random.sample(range(1000), 1)) + '_' + file_name 
   print(file_name) 
   new_name = '{}{}'.format(file_name, file_ext)    
   os.rename(f, new_name)