On Thu, May 7, 2020 at 4:42 AM Kirill Balunov <kirillbalunov@gmail.com> wrote:
`chain(*iterable)` converts iterable into a tuple, concretizing it in
memory. chain.from_iterable(iterable) is lazy and goes through the elements one a time, meaning iterable can be infinite.
"meaning iterable can be infinite" - thank you, I missed this part, but to be honest I don’t remember when I do something useful with infinite iterables.
Being fuzzy about "infinite" versus "very large" here are a couple examples:
def first_ints(): ... from random import randint ... while True: ... yield range(randint(10, 20)) ... from itertools import chain nums = chain.from_iterable(first_ints()) from itertools import islice list(islice(nums, 100, 150)) [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 0, 1, 2, 3, 4, 5, 6]
In this code we want initial sequences of non-negative integers of varying lengths, repeated... because MATH. I could not concretize the infinite collection of finite sequences. Here's an example that is just "very big" instead of infinite, but would still urge for .from_iterable().
def all_man_lines(path='/usr/share/man/man1/'): ... from glob import glob ... for fname in glob(f"{path}*.gz"): ... yield gzip.open(fname) ... lines = chain.from_iterable(all_man_lines()) for line in islice(lines, 100, 105): ... print(line) ... b". ds ^ \\\\k:\\h'-(\\\\n(.wu*10/11-\\*(#H)'^\\h'|\\\\n:u'\n" b". ds , \\\\k:\\h'-(\\\\n(.wu*8/10)',\\h'|\\\\n:u'\n" b". ds ~ \\\\k:\\h'-(\\\\n(.wu-\\*(#H-.1m)'~\\h'|\\\\n:u'\n" b". ds / \\\\k:\\h'-(\\\\n(.wu*8/10-\\*(#H)'\\z\\(sl\\h'|\\\\n:u'\n" b'.\\}\n'
I do not have infinitely many man pages on my system, but I have enough of them that I don't want to open file handles to all of them at once. -- 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.