> This is BARELY more plausible as a real-world case. Throwing away 28 bytes with a bunch of next() calls is completely trivial in time. A case where some implementation could conceivably save measurable time would require skipping 100s of thousands or millions of next() calls... and probably calls that actually took some work to compute to matter, even there.
Seeing as an IP packet could in theory be as large 64K `stream.advance(total_length - 32 - len(header))` could be skipping ~65,000 next calls. (Although in practice packets are very unlikely to exceed 1,500 bytes because of the ethernet standard.) In any event avoiding ~1500 next calls per packet is hardly insignificant if you want to process more than handful of packet. Now a better rejection to my example is that this sort of code does not belong in python and should be in a systems language, an argument I would agree with. -- Caleb Donovick On Wed, Oct 7, 2020 at 3:38 PM David Mertz <me...@gnosis.cx> wrote: > On Wed, Oct 7, 2020 at 6:24 PM Caleb Donovick <donov...@cs.stanford.edu> > wrote: > >> Itertools.count was an example (hence the use of "e.g.") of an iterator >> which can be efficiently >> advanced without producing intermediate state. Clearly anyone can advance >> it manually. >> My point is that an iterator may have an efficient way to calculate its >> state some point in the future >> without needing to calculate the intermediate state. >> > > Yes, this is technically an example. But this doesn't get us any closer > to a real-world use case. If you want an iterator than counts from N, the > spelling `count(N)` exists now. If you want to starting counting N > elements later than wherever you are now, I guess do: > > new_count = counter(next(old_cound) + N) > > For example the fibonacci sequence has a closed >> form formula for the nth element and hence could be advanced efficiently. >> > > Sure. And even more relevantly, if you want the Nth Fibonacci you can > write a closed-form function `nth_fib()` to get it. This is toy examples > where *theoretically* a new magic method could be used, but it's not close > to a use case that would motivate changing the language. > > ``` >> def get_tcp_headers(stream: Iterator[Byte]): >> while stream: >> # Move to the total length field of the IP header >> stream.advance(2) >> # record the total length (two bytes) >> total_length = ... >> # skip the rest of IP header >> stream.advance(28) >> # record the TCP header >> header = ... >> yield header >> stream.advance(total_length - 32 - len(header)) >> ``` >> > > This is BARELY more plausible as a real-world case. Throwing away 28 > bytes with a bunch of next() calls is completely trivial in time. A case > where some implementation could conceivably save measurable time would > require skipping 100s of thousands or millions of next() calls... and > probably calls that actually took some work to compute to matter, even > there. > > What you'd need to motivate the new API is a case where you might skip a > million items in an iterator, and yet the million-and-first item is > computable without computing all the others. Ideally something where each > of those million calls does something more than just copy a byte from a > kernel buffer. > > I don't know that such a use case does not exist, but nothing comes to my > mind, and no one has suggested one in this thread. Otherwise, > itertools.islice() completely covers the situation already. > > -- > The dead increasingly dominate and strangle both the living and the > not-yet born. Vampiric capital and undead corporate persons abuse > the lives and control the thoughts of homo faber. Ideas, once born, > become abortifacients against new conceptions. >
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