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.