users_by_uid.py

#!/usr/bin/env python3

import re

def max_length(strings):
    max_len = 0
    for s in strings:
        if len(s) > max_len:
            max_len = len(s)
    return max_len

username = {}
with open("/etc/passwd") as f:
    for line in f:
        line = line.strip()
        fields = line.split(":")
        user, uid = fields[0], fields[2]
        username[int(uid)] = user

for uid in sorted(username, reverse=True):
    print("{:{width}} {}".format(
        username[uid], uid, width=max_length( username.values() )
    ))