[python/en] naming conventions and underscores

I got a bit confused about the usage of underscores so I looked it up. Made a few tweaks and added some comments based on what I found here: http://legacy.python.org/dev/peps/pep-0008/#descriptive-naming-styles.
Perhaps a tutorial like this is not the best place to bring up naming conventions, but then we should avoid cases where we have to use such naming conventions instead.
`underscore` => `underscores`