Heap implementation

heap.py

class MinHeap:
    def __init__(self, items, key):
        """ key must be callable function """
        self.arr = list(items)
        self.key = key
        for i in range(len(self.arr) // 2 - 1, -1, - 1):
            self._heapify(i)

    def empty(self):
        return len(self.arr) == 0

    def _heapify(self, i):
        smallest = i
        for child in [2 * i + 1, 2 * i + 2]:
            if child < len(self.arr) and self.key(self.arr[child]) < self.key(self.arr[smallest]):
                smallest = child
        if smallest != i:
            self.arr[i], self.arr[smallest] = self.arr[smallest], self.arr[i]
            self._heapify(smallest)

    def pop_min(self):
        if len(self.arr) == 1:
            return self.arr.pop()
        min_val, self.arr[0] = self.arr[0], self.arr.pop(-1)
        self._heapify(0)
        return min_val

    def insert(self, val):
        self.arr.append(val)
        ind = len(self.arr) - 1
        while True:
            if ind == 0:
                break
            parent_ind = (ind - 1) // 2
            if self.key(self.arr[ind]) < self.key(self.arr[parent_ind]):
                self.arr[ind], self.arr[parent_ind] = self.arr[parent_ind], self.arr[ind]
                ind = parent_ind
            else:
                break