You don't necessarily know which functions are going to be chained together though. If I write (my Python's a bit rusty):

    def f(x):
      return map(someOtherFunction, map(someFunction, x))

how can I know whether someOtherFunction interferes with someFunction if they run at the same time? The only way to know whether the maps are strict or lazy is knowing what x is, but x might come from some a different file or a different project even, so there's no way to know whether this function only works (and is only expected to work) for strict x or not. Actually even knowing what x is isn't enough, because even if x is strict, f can end up being called in a lazy way if I later do:

    (y for x in someGeneratorThatGeneratesLists for y in f(x))