Simple Heap Implementation

heap.py

class Heap:
    def __init__(self):
        self.data = []
        self.length = 0

    def push(self, val):
        if len(self.data) <= self.length: # need to expand the array
            self.data.append(val)
        else:
            self.data[self.length] = val
        self.length += 1
        self.heap_up()

    def pop(self):
        if self.length > 0:
            top_val = self.data[0]
            self.data[0] = self.data[self.length-1]
            self.length -= 1
            self.heap_down()
            return top_val

    def parent_idx(self, child_idx):
        return int((child_idx - 1) / 2)

    def left_child_idx(self, parent_idx):
        idx = int(parent_idx * 2 + 1)
        if idx < self.length:
            return idx

    def right_child_idx(self, parent_idx):
        idx = int(parent_idx * 2 + 2)
        if idx < self.length:
            return idx

    def swap(self, idx1, idx2):
        self.data[idx1], self.data[idx2] = self.data[idx2], self.data[idx1]

    def heap_up(self):
        curr_idx = self.length - 1
        while curr_idx > 0 and self.data[curr_idx] < self.data[self.parent_idx(curr_idx)]:
            self.swap(curr_idx, self.parent_idx(curr_idx))
            curr_idx = self.parent_idx(curr_idx)

    def min_child_idx(self, curr_idx):
        left_idx = self.left_child_idx(curr_idx)
        right_idx = self.right_child_idx(curr_idx)
        if left_idx and right_idx: # both children exist, so choose the smaller one
            return left_idx if self.data[left_idx] < self.data[right_idx] else right_idx
        elif left_idx:
            return left_idx
        elif right_idx:
            return right_idx
        else:
            return None

    def heap_down(self):
        curr_idx = 0
        min_idx = self.min_child_idx(curr_idx)
        while curr_idx < self.length and min_idx and self.data[curr_idx] > self.data[min_idx]:
            self.swap(curr_idx, min_idx)
            curr_idx = min_idx
            min_idx = self.min_child_idx(curr_idx)