In python an empty string/list/set/tuple is considered falsy, similar to False, None or 0 (read more here: https://docs.python.org/2/library/stdtypes.html#truth-value-testing)

So if numbers: means that if the numbers list is not empty, then...

if x is the same as if len(x) > 0 or if x > 0 or if x == True etc.