how do I get the PID list and USERNAME of each of those PIDs in LINUX using python?

Question

I am trying to get the PID's and their respective usernames for displaying something similar to the top command in linux. I managed to get the list of PIDs but I am unable to fetch any username for the same. PID from /proc and username is in the /proc/$PID/environ file which while i open to see, has nothing in it for all the PIDs. Kindly suggest how do I go about this..