I'm having difficulty adding to a list iteratively.
Here's a MWE:
# Given a nested list of values, or sets
sets = [[1, 2, 3], [1, 2, 4], [1, 2, 5]]
# add a value to each sublist giving the number of that set in the list.
n_sets = len(sets)
for s in range(n_sets):
(sets[s]).insert(0, s)
# Now repeat those sets reps times
reps = 4
expanded_sets = [item for item in sets for i in range(reps)]
# then assign a repetition number to each occurance of a set.
rep_list = list(range(reps)) * n_sets
for i in range(n_sets * reps):
(expanded_sets[i]).insert(0, rep_list[i])
expanded_sets
which returns
[[3, 2, 1, 0, 0, 1, 2, 3],
[3, 2, 1, 0, 0, 1, 2, 3],
[3, 2, 1, 0, 0, 1, 2, 3],
[3, 2, 1, 0, 0, 1, 2, 3],
[3, 2, 1, 0, 1, 1, 2, 4],
[3, 2, 1, 0, 1, 1, 2, 4],
[3, 2, 1, 0, 1, 1, 2, 4],
[3, 2, 1, 0, 1, 1, 2, 4],
[3, 2, 1, 0, 2, 1, 2, 5],
[3, 2, 1, 0, 2, 1, 2, 5],
[3, 2, 1, 0, 2, 1, 2, 5],
[3, 2, 1, 0, 2, 1, 2, 5]]
instead of the desired
[[0, 0, 1, 2, 3],
[1, 0, 1, 2, 3],
[2, 0, 1, 2, 3],
[3, 0, 1, 2, 3],
[0, 1, 1, 2, 4],
[1, 1, 1, 2, 4],
[2, 1, 1, 2, 4],
[3, 1, 1, 2, 4],
[0, 2, 1, 2, 5],
[1, 2, 1, 2, 5],
[2, 2, 1, 2, 5],
[3, 2, 1, 2, 5]]
Just for fun, the first loop returns an expected value of sets
[[0, 1, 2, 3], [1, 1, 2, 4], [2, 1, 2, 5]]
but after the second loop sets changed to
[[3, 2, 1, 0, 0, 1, 2, 3], [3, 2, 1, 0, 1, 1, 2, 4], [3, 2, 1, 0, 2, 1, 2, 5]]
I suspect the issue has something to do with copies and references. I've tried adding .copy() and slices in various places, but with the indexed sublists I haven't come across a combo that works. I'm running Python 3.10.6.
Thanks for looking!
Per suggested solution, [list(range(reps)) for _ in range(n_sets)] doesn't correctly replace the list(range(reps)) * n_sets, since it gives [[0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3]] instead of
the desired [0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3]. Do I need to flatten, or is there a syntax with the _ notation that gives me a single list?
Further update . . . replacing
rep_list = list(range(reps)) * n_sets
with
rep_list_nest = [list(range(reps)) for _ in range(n_sets)]
rep_list = [i for sublist in rep_list_nest for i in sublist]
gives the same undesired result for expanded_sets.
The problem is here:
expanded_sets = [item
for item in sets
for i in range(reps)]
This list now contains the same element of sets four times in a row, followed by the next element repeated four times, and so on.
Creating copies of item fixes the issue:
expanded_sets = [item.copy()
for item in sets
for i in range(reps)]
If you want a more pythonic approach, then recognize that the result is a product of two ranges, and your original sets all concatenated together:
from itertools import product
sets = [[1, 2, 3], [1, 2, 4], [1, 2, 5]]
expanded_sets = [[inner_counter, outer_counter] + sets_elem
for sets_elem, outer_counter, inner_counter in product(sets, range(len(sets)), range(4))]