python, heapq: difference between heappushpop() and heapreplace()

heapreplace(a, x) returns the smallest value originally in a regardless of the value of x, while, as the name suggests, heappushpop(a, x) pushes x onto a before popping the smallest value. Using your data, here’s a sequence that shows the difference:

>>> from heapq import *
>>> a = [2,7,4,0,8,12,14,13,10,3,4]
>>> heapify(a)
>>> b = a[:]
>>> heappushpop(a, -1)
-1
>>> heapreplace(b, -1)
0