Sometimes, it's really hard to understand what happens inside a function or even a whole module of Python's Standard library. For example, the subprocess module contains a very tricky Popep class. I tried to use the the module to communicate with a MATLAB subprocess shell (e.g. send MATLAB commands to subprocess and read the output). Unfortunately I failed and was just able to pass a MATLAB script via command-line arguments. Yet, I learnt much about the Popen.communicate() method and I'd like to share this knowledge with you.