for bar_vec in bar_mat:
      if bar_vec != []:
          for y in bar_vec:
              if baz(y):
                  for x in y:
                      foo(x)

Seeing it written out here, the example looks nice. In real code with more complexity, it takes longer for me to write it, and it usually takes up a lot more space than this. It bugs me how much space it takes up for something that is one idea in my head (Foo all the baz things in bar_vec).

But if there is anyone coming after me, I would write it in this style for their sake, because this style is easier to change.

At least in my code, these nested loop thingies are rarely unique. If I have to do it once I probably have to do it many times. If I'm being mindful, I stop repeating myself and just write a generator so that I can invoke it like:

  [foo(x) for x in nonempty(bar_mat)]

Mostly this is because I live in fear of ruff's complexity warning, C901.