[issue32142] heapq.heappop - documentation misleading or doesn't work
New submission from Scott Queen :
The documentation for heapq.heappop(heap) says:
"Pop and return the smallest item from the heap, maintaining the heap
invariant. If the heap is empty, IndexError is raised. To access the smallest
item without popping it, use heap[0]."
yet, in the following code, the resultant heap doesn't reflect the heap
invariant:
import heapq
li = [5, 7, 9, 1, 4, 3]
heapq.heapify(li)
#change a couple values in the heap
li[3] = 16
li[4] = 2
print (heapq.heappop(li))
print ("The heap after pop is : ",end="")
print (list(li))
This prints: The heap after pop is : [3, 4, 9, 16, 2]
The documentation implies to me that heapify would be called internally after
heappop, but I may be misreading. Perhaps heappop could say that the heap
invariant is maintained if the heap is properly sorted before the heappop
invocation.
--
assignee: docs@python
components: Documentation
messages: 307006
nosy: docs@python, scooter4j
priority: normal
severity: normal
status: open
title: heapq.heappop - documentation misleading or doesn't work
type: behavior
versions: Python 3.6
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[issue32142] heapq.heappop - documentation misleading or doesn't work
Scott Queen added the comment: Fair statement. "Properly sorted" was a poor choice. Seems that "if the invariant is true as a precondition, it will be true as a postcondition" is accurate and descriptive - maybe with a caution that modifying the list (heap) values by means other than the heapq functions are likely to render the invariant false (the latter phrase being for somewhat newbies like me who wouldn't realize this truth without having to learn it in the debugger). On Sun, Nov 26, 2017 at 10:51 AM, abcdef wrote: > > abcdef added the comment: > > > Perhaps heappop could say that the heap invariant is maintained if the > heap is properly sorted before the heappop invocation. > > Honestly I like this wording less. "Properly sorted" would suggest sorted > in the sense of heap.sort() [as the docs refer to this earlier], but the > array doesn't have to be sorted; it's enough if it's a heap. The function > always maintains the invariant - this means that if the invariant is true > as a precondition, it will be true as a postcondition. Maybe you have a > different idea? > > -- > nosy: +abcdef > > ___ > Python tracker > <https://bugs.python.org/issue32142> > ___ > -- ___ Python tracker <https://bugs.python.org/issue32142> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
