Adding string after every 3rd charater [SOLVED]

[AlphaInc]

Hello everybody,

I want to get the filesize of a folder and store it as a variable. So far I've manged to do so:

import os

def get_size(start_path = '.'):
    total_size = 0
    for dirpath, dirnames, filenames in os.walk(start_path):
        for f in filenames:
            fp = os.path.join(dirpath, f)
            if not os.path.islink(fp):
                total_size += os.path.getsize(fp)
    return total_size

line = get_size()
print(line)

Now I want to expand on that so that, going from the end to beginning, after every 3 digits it adds a point.
For example, instead of 1515124421 I want it to look like 1.515.124.421

How can I do so?