That said, the one area that has stumped me and I haven't been able to find good clean code to learn from is Feature Extraction. I was wondering if anyone here can point me in the direction of some clean code that a non-expert in the field could learn from.

I'm definitely aware that there are books / tutorials on the subject, but none of that has "made it click" for me yet. To be honest I feel like most of my real "advances" have been while looking at code (after familiarizing myself at a high level with the theory and math).

For example, the tensorflow playground source code appears to be a gold mine filled with good clean code that a novice can grok.

EDIT: If beggars can be choosers I'm most interested in seeing a practical implementation that uses a clustering algorithm (such as kmeans) to build up a set of features from image data. Such a technique is discussed in the following video.

https://www.youtube.com/watch?v=wZfVBwOO0-k