Add more-itertools as a standard package

[mkoeppe]

https://pypi.org/project/more-itertools/

Then get rid of our homegrown iteration tools:

• from sage.misc.misc: uniq, _stable_uniq, subsets/powerset

Useful:

• SequenceView - e.g. gives an idiom for taking the len of an iterable

• only/strictly_n/take - #34509 comment:52

CC: @tscrim @orlitzky @slel @yyyyx4

Component: refactoring

Issue created by migration from https://trac.sagemath.org/ticket/32100

[orlitzky]

comment:3

I'll change my mind about this if it lets us delete a good bit of custom code, but looking at the given examples:

• uniq() is trivial and could be deleted anyway. Either list(set(x)) or sorted(set(x)) are idiomatic python.
• The benefit of _stable_uniq seems to be that it returns an iterator, but the only two uses of it in the tree immediately convert the result to a list. Can they be replaced by list(set(x))?
• Powerset is a nice one to have, but by itself totals nine lines of code.

[mkoeppe]

comment:5

Replying to @orlitzky:

I'll change my mind about this if it lets us delete a good bit of custom code

I agree, there needs to be a demonstrated substantial use before we add another dependency

[slel]

comment:6

One example is slicing a list into sublists
of size "ncpu" to feed a parallel function.

For that, we can each time define a custom
"list of lists" function, for instance

lol = lambda lst, sz: [lst[i:i+sz] for i in range(0, len(lst), sz)]

as found 17 times in sage/manifolds
and sage/tensor (try git grep 'lol =').

We could instead use chunked, ichunked,
sliced or isliced from more-itertools.

[orlitzky]

comment:7

lol = lambda lst, sz: [lst[i:i+sz] for i in range(0, len(lst), sz)]

Changing that line on its own won't eliminate much code, though. The pattern

lol = lambda lst, sz: [lst[i:i+sz] for i in range(0, len(lst), sz)]
ind_list = [ind for ind in other._comp] # only this line changes
ind_step = max(1, int(len(ind_list)/nproc/2))
local_list = lol(ind_list, ind_step)

is repeated over and over again. If the whole thing were factored out into a helper function, then we'd be left with changing

lol = lambda lst, sz: [lst[i:i+sz] for i in range(0, len(lst), sz)]

into

lol = lambda lst, sz: chunked(lst,sz)

which only saves us a few characters.