At first glance, yes. Needs a moment to think this through. But not a wat because this works exactly as the logic says. If we denote x = [] then bool(x)

At first glance, yes. Needs a moment to think this through. But not a wat because this works exactly as the logic says. If we denote x = [] then bool(x) == False # empty list is falsy bool([x]) == True # nonempty list is truthy bool([[x]]) == True # nonempty again and finally, [bool(elem) for elem in [[x]]] == [True] # all True! which is the thing `all' is interested in. It is more like a newbie mistake or careless document reading if the user thinks `all' runs through the lists and all of the nesting too.