heapq: Improve clarity (and small speed-up) by using tuple unpacking

[rhettinger]

No description provided.

[Yossarian0916]

The names _shiftdown and _shiftup are misleading and they should be exchanged. While building a min-heap, each element is compared with its child elements so to bubble it down, not 'up'.

[tim-one]

The names should not be changed, because they're essentially arbitrary, with no compelling reason to favor one over the other - and there's no consistency about this in the literature either.

For example, when building a min-heap, the vast majority of elements that move go up in the heap, not down. It's only the single element at the top of the current sub-heap that moves down.

Which is the heart of the inherent ambiguity: something can't "go up" without something else "going down". The current names reflect the directions of the bulk of the data that moves (_siftup() generally moves children up to the their parent positions, and _siftdown() the opposite).