I am trying to build a heap with a custom sort predicate. Since the values going into it are of 'user-defined' type, I cannot modify their built-in comparison predicate.
Is there a way to do something like:
h = heapq.heapify([...], key=my_lt_pred)
h = heapq.heappush(h, key=my_lt_pred)
According to the heapq documentation, the way to customize the heap order is to have each element on the heap to be a tuple, with the first tuple element being one that accepts normal Python comparisons.
The functions in the heapq module are a bit cumbersome (since they are not object-oriented), and always require our heap object (a heapified list) to be explicitly passed as the first parameter. We can kill two birds with one stone by creating a very simple wrapper class that will allow us to specify a
key function, and present the heap as an object.
The class below keeps an internal list, where each element is a tuple, the first member of which is a key, calculated at element insertion time using the
key parameter, passed at Heap instantiation:
# -*- coding: utf-8 -*- import heapq class MyHeap(object): def __init__(self, initial=None, key=lambda x:x): self.key = key if initial: self._data = [(key(item), item) for item in initial] heapq.heapify(self._data) else: self._data =  def push(self, item): heapq.heappush(self._data, (self.key(item), item)) def pop(self): return heapq.heappop(self._data)