Cross-Platform Python library to listen to the system applications events

Question

I am searching for the best-known library to use/implement a listener over the system applications in Python

For example,

1. print "This is a new application" whenever a new application (notepad let's say) starts [Event]
2. print "appicationX is closed" whenever a running application (notepad let's say) closed [Event]
3. I want to print "User is now focused on AppX" whenever the user jump from one application to another (jumped from notepad to chrome) [Event]

Through my search, I found this Python code which makes use of the ctypes library to do something similar to what I am searching for

It has two problems

1. it works on windows only
2. it doesn't capture on-open / on-close events -- it just captures the on-change events

I am looking for a library to use that can overcome those two