226 – Reductions on empty iterables

It’s useful to know and understand what are the results of functions like sum and all when called on empty iterables:

sum([])  # 0
math.prod([])  # 1
all([])  # True
any([])  # False

To understand these results, it can be useful to work backwards. For the built-in sum, for example, note how all of these are the same:

sum([1, 2, 3]) == 3 + sum([1, 2])  # 6

sum([1, 2])    == 2 + sum([1])     # 3

sum([1])       == 1 + sum([])      # 1

Look at the last equality. If sum([1]) is the same as 1 + sum([]), which happens to be 1, then sum([]) must be equal to 0!