Tuples (datetime, description) all are sorted by the date except one entry (2021-03-09) which is out of order:

        Expected order:             Actual order:
        2021-03-04 Event E          2021-03-04 Event E
        2021-03-07 Event B          2021-03-07 Event B
        2021-03-08 Event C          2021-03-08 Event C
        2021-03-09 Event A          2021-03-11 Event D
        2021-03-11 Event D          2021-03-09 Event A

In REPL it can be replicated by pasting the following code:

import heapq
from datetime import datetime

event_a = (datetime.strptime('2021-03-09', '%Y-%m-%d'), "Event A")
event_b = (datetime.strptime('2021-03-07', '%Y-%m-%d'), "Event B")
event_c = (datetime.strptime('2021-03-08', '%Y-%m-%d'), "Event C")
event_d = (datetime.strptime('2021-03-11', '%Y-%m-%d'), "Event D")
event_e = (datetime.strptime('2021-03-04', '%Y-%m-%d'), "Event E")

events = []
heapq.heappush(events, event_a)
heapq.heappush(events, event_b)
heapq.heappush(events, event_c)
heapq.heappush(events, event_d)
heapq.heappush(events, event_e)

expected_list = [event_e, event_b, event_c, event_a, event_d]

assert events == expected_list

Heaps are not sorted lists! It is true that a sorted list is a heap, but heaps are not necessarily sorted.

Here is another heap which is not sorted:

>>> L = []
>>> for n in (9, 7, 8, 11, 4):
...     heapq.heappush(L, n)
... 
>>> L
[4, 7, 8, 11, 9]


https://en.wikipedia.org/wiki/Heap_(data_structure)


Also:

>>> L = [9, 8, 7, 2, 3, 5, 4, 1, 0, 6]
>>> heapq.heapify(L)
>>> L
[0, 1, 4, 2, 3, 5, 7, 8, 9, 6]


If we change the order of the initial values, the heap changes too:


>>> L = [9, 8, 7, 2, 3, 5, 4, 1, 0, 6]
>>> L.reverse()
>>> heapq.heapify(L)
>>> L
[0, 4, 1, 6, 5, 3, 2, 7, 8, 9]