On Thu, May 7, 2020 at 4:42 AM Kirill Balunov <[email protected]>
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.
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