Complete NEWB and openpyxl project

[buran]

Ok, so groups is a list. And group is a collection of lists based on and named after the last character of the last element.

So ultimately groups becomes a list of lists?

I am afraid not. There are different data structures (containers) in python - these are basic ones, but there are also others, e.g. - in the collections module.

groups is a defaultdict - in simple terms that's a dict (dictionary) with default factor value - in our case - [empty] list.
dict (and also the defaultdict) is a mapping type, i.e. it has key:value pairs. keys are unique. in our case, with the defaultdict we have lists as values.