Question

In Python, the built-in functions all and 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

How about some analogies…

You have a sock drawer, but it is currently empty. Does it contain any black sock? No – you don’t have any socks at all so you certainly don’t have a black one. Clearly any([]) must return false – if it returned true this would be counter-intuitive.

The case for all([]) is slightly more difficult. See the Wikipedia article on vacuous truth. Another analogy: If there are no people in a room then everyone in that room can speak French.

Mathematically all([]) can be written:

where the set A is empty.

There is considerable debate about whether vacuous statements should be considered true or not, but from a logical viewpoint it makes the most sense:

The main argument that all vacuously true statements are true is as follows: As explained in the article on logical conditionals, the axioms of propositional logic entail that if P is false, then P => Q is true. That is, if we accept those axioms, we must accept that vacuously true statements are indeed true.

Also from the article:

There seems to be no direct reason to pick true; it’s just that things blow up in our face if we don’t.

Defining a “vacuously true” statement to return false in Python would violate the principle of least astonishment.