Reason for "all" and "any" result on empty lists?

Question

In Python, the built-in functions [`all`](http://docs.python.org/library/functions.html#all) and [`any`](http://docs.python.org/library/functions.html#any) return `True` and `False` respectively for empty iterables. I realise that if it were the other way around, this question could still be asked. But I'd like to know why *that specific* behaviour was chosen. Was it arbitrary, ie. could it just as easily have been the other way, or is there an underlying reason? (The reason I ask is simply because I never remember which is which, and if I knew the rationale behind it then I might. Also, curiosity.)

Answer 1

This is called "vacuous truth" in math. Let's consider another way to think about the `all` function: it looks for exceptions. It looks over the list, trying to find any value that is False. If there are no elements, there are no False values. So it returns True.

The same goes for `any`. It looks over the list, trying to find any values that are True. If there are no elements, there are no True values. So it returns False.