Bisect: an insertion point is not proof of a match

Useful to know

bisect_left() finds an insertion position in a sorted list. For [10, 20, 30], searching for 25 gives position 2 even though 25 is absent. [1]

To establish membership, check the boundary and value at that position. The list must already follow the same ordering. With insort(), locating the position is efficient, but shifting elements for insertion still has a cost.