Python: How to set up a __str__ function in a PriorityQueue

Question

I have made a priority queue whose functions use my binary heaps functions. However, in my testing file I am trying to print out my Queue to look something like this.

Your queue looks like this: 15 -> 45 -> 10 -> 100

Somewhere like that, however it keeps printing out where the queue is stored rather than the items in the queue, an example of this:

<PriorityQueue.PriorityQueue object at 0x01E95530>

I read up on the pythonDocs and concluded I need a str function. However, I am having trouble creating it Could anyone help me out here on how it would look like? Thanks a lot. Here is my whole amount of code.

class Heap(object):

    def __init__(self, items=None):

        '''Post: A heap is created with specified items.'''

        self.heap = [None]
        if items is None:
            self.heap_size = 0
        else:
            self.heap += items
            self.heap_size = len(items)
            self._build_heap()

    def size(self):

        '''Post: Returns the number of items in the heap.'''

        return self.heap_size

    def _heapify(self, position):

        '''Pre: Items from 0 to position - 1 satisfy the Heap property.
           Post: Heap Property is satisfied for the entire heap.'''

        item = self.heap[position]
        while position * 2 <= self.heap_size:
            child = position * 2
            # If the right child, determine the maximum of two children.
            if (child != self.heap_size and self.heap[child+1] > self.heap[child]):
                child += 1
            if self.heap[child] > item:
                self.heap[position] = self.heap[child]
                position = child
            else:
                break
        self.heap[position] = item

    def delete_max(self):

        '''Pre: Heap property is satisfied
           Post: Maximum element in heap is removed and returned. '''

        if self.heap_size > 0:
            max_item = self.heap[1]
            self.heap[1] = self.heap[self.heap_size]
            self.heap_size -= 1
            self.heap.pop()
            if self.heap_size > 0:
                self._heapify(1)
            return max_item

    def insert(self, item):

        '''Pre: Heap Property is Satisfied.
           Post: Item is inserted in proper location in heap.'''

        self.heap_size += 1
        # extend the length of the list.
        self.heap.append(None)
        position = self.heap_size
        parent = position // 2
        while parent > 0 and self.heap[parent] < item:
            # Move the item down.
            self.heap[position] = self.heap[parent]
            position = parent
            parent = position // 2
        # Puts the new item in the correct spot.
        self.heap[position] = item

    def _build_heap(self):

        ''' Pre: Self.heap has values in 1 to self.heap_size
           Post: Heap property is satisfied for entire heap. '''

        # 1 through self.heap_size.

        for i in range(self.heap_size // 2, 0, -1): # Stops at 1.
            self._heapify(i)

    def heapsort(self):

        '''Pre: Heap Property is satisfied.
           Post: Items are sorted in self.heap[1:self.sorted_size].'''

        sorted_size = self.heap_size

        for i in range(0, sorted_size -1):
            # Since delete_max calls pop to remove an item, we need to append a dummy value to avoid an illegal index.
            self.heap.append(None)
            item = self.delete_max()
            self.heap[sorted_size - i] = item

Above is my Heap class where my PriorityQueue takes from: Below is my PQ class and my test file.

#PriorityQueue.py
from MyHeap import Heap


class PriorityQueue(object):

    def __init__(self):
        self.heap = Heap()

    def enqueue(self, priority, item):
        '''Post: Item is inserted with specified priority in the PQ.'''
        self.heap.insert((priority, item))
        return item

    def first(self):
        '''Post: Returns but does not remove the highest priority item from the PQ.'''
        return self.heap.size()


    def dequeue(self):
        '''Post: Removes and returns the highest priority item from the PQ.'''
        if self.heap.size() is None:
            raise ValueError("Error your queue is empty.")
        x = self.first()
        self.heap.delete_max()
        return x

    def size(self):
        '''Post: Returns the number of items in the PQ.'''
        return self.heap.size()

---

from PriorityQueue import PriorityQueue

PQ = PriorityQueue()


print(PQ.enqueue(1, 10))
print(PQ.enqueue(2, 5))
print(PQ.enqueue(3, 90))
print PQ
print(PQ.size())

EDIT: I tried to do the following:

 def __str__(self):
        return str(self.heap)

This only prints out this:

<MyHeap.Heap object at 0x01E255F0>

Answer 1

Ok, the idea of __str__ is to return some sort of string that represents the object in a human-readable way. You have to construct the string and then whatever you return will be printed instead of

<PriorityQueue.PriorityQueue object at 0x01E95530>

In your case, we have to return the items of self.heap.heap, separated by ->. This would work to get the output like you described:

def __str__(self):
    if self.size():
        heap_items = [str(i) for i in self.heap.heap if i]
        heap_items_str = ' -> '.join(heap_items)
        return "Your queue looks like this: {}".format(heap_items_str)
    else:
        return "Your queue is empty."

Note that we're using self.heap.heap, not self.heap because self in this case is a PriorityQueue instance, and PriorityQueue has a .heap property that contains a Heap. It's this Heap that we actually want to call .heap on, which in-turn gives us the list we're going for.

Answer 2

EDIT: I tried to do the following:

def __str__(self):
    return str(self.heap)

Instead, try this:

def __str__(self):
    return self.heap.__str__()