[Raymond Hettinger

... Q. How readable is the proposed code? A. Look at the following code and ask yourself what it does:

accumulate(range(4, 6), operator.mul, start=6)

Now test your understanding:

How many values are emitted?

3

What is the first value emitted?

6

Are the two sixes related?

No.

What is this code trying to accomplish?

It's quite obviously trying to bias the reader against the proposal by presenting a senseless example ;-) Assuming there's any real reason to write that code at all, a better question is whether it's more comprehensible than accumulate(itertools.chain([6], range(4, 6)), operator.mul)

... Q. What would this look like in real code? A. We have almost no real-world examples, but here is one from a StackExchange post:

def wsieve(): # wheel-sieve, by Will Ness. ideone.com/mqO25A->0hIE89 ...

By sheer coincidence, I happened to write another yesterday. This is from a program looking for the smallest integers that yield new records for Collatz sequence lengths. The details don't matter, except that - like Will Ness's wheel sieve code - it needs to generate an unbounded increasing sequence of integers with a periodic, but complicated, sequence of deltas, starting at a more-or-less arbitrary point. def buildtab(SHIFT, LIMIT): ... # Candidates are of the form i*LIMIT + j, for i >= 1 and j in # goodix. However, a new record can't be set for a number of # the form 3k+2: that's two steps after 2k+1, so the smaller # 2k+1 has a glide 2 longer. We want to arrange to never try # numbers of the form 3k+2 to begin with. base = 0 ix2 = [] for i in range(3): base += LIMIT for ix in goodix: num = base + ix if num % 3 != 2: ix2.append(num) ix2.append(ix2[0] + 3 * LIMIT) assert len(ix2) == 2 * len(goodix) + 1 del goodix deltas = tuple(ix2[i] - ix2[i-1] for i in range(1, len(ix2))) return tuple(result), ix2[0], deltas A note on "complicated": the tuple of deltas here can contain millions of integers, and that's the smallest length at which it becomes periodic. Later: def coll(SHIFT=24): ... from itertools import accumulate, chain, cycle ... LIMIT = 1 << SHIFT ... abc, first, deltas = buildtab(SHIFT, LIMIT) ... for num in accumulate(chain([first], cycle(deltas))): assert num % 3 != 2 As in Will's code, it would be more readable as: for num in accumulate(cycle(deltas), start=first): That says what it does pretty clearly, whereas deducing the behavior from "OK, it's chaining together a singleton list and a cycle, because ...?" is a bit of a head scratcher at first. That said, if the need came up often, as you noted it's dead easy to write a helper function to encapsulate the "head scratcher" part, and with no significant loss of efficiency. So I'd be -0 overall, _except_ that "chain together a singleton list and a cycle" is so obscure on the face of it than I'm not sure most programmers who wanted the functionality of `start=` would ever think of it. I'm not sure that I would have, except that I studied Ness's wheel sieve code a long time ago and the idea stuck. So that makes me +0.4.