Priority Queue with update functionality

[PythonNewbee]

I'm trying to implement a priority queue which allows me to update details of items already in queue.
I found a simple implementation in the Python documentation, which I'm working through to understand (https://docs.python.org/2/library/heapq....tion-notes).
However, there's a portion of this example which the understanding of escapes me; and I'm hoping someone can help me clarify?
The specific example is:

import itertools
import heapq

pq = []                         # list of entries arranged in a heap
entry_finder = {}               # mapping of tasks to entries
REMOVED = '<removed-task>'      # placeholder for a removed task
counter = itertools.count()     # unique sequence count

def add_task(task, priority=0):
    'Add a new task or update the priority of an existing task'
    if task in entry_finder:
        remove_task(task)
    count = next(counter)
    entry = [priority, count, task]
    entry_finder[task] = entry
    heappush(pq, entry)

def remove_task(task):
    'Mark an existing task as REMOVED.  Raise KeyError if not found.'
    entry = entry_finder.pop(task)
    entry[-1] = REMOVED

def pop_task():
    'Remove and return the lowest priority task. Raise KeyError if empty.'
    while pq:
        priority, count, task = heappop(pq)
        if task is not REMOVED:
            del entry_finder[task]
            return task
    raise KeyError('pop from an empty priority queue')

#Testing the above:
add_task('Planning')
add_task('Design',1)
add_task('Building',2)
print(pq)
#output is: [[0, 0, 'Planning'], [1, 1, 'Design'], [2, 2, 'Building']]
add_task('Design',1) #this should update an existing entry
print(pq)
'''
output:
[[0, 0, 'Planning'],
 [1, 1, '<removed-task>'],
 [2, 2, 'Building'],
 [1, 3, 'Design']]
'''

When I work through the example I'm struggling to understand how the second item in the queue gets updated when the second add_task('Design',1) is executed.
The remove_task(task) pops the item in the dictionary to a variable, and the last components of the variable's content is set to REMOVED.
However, that variable only lives in the scope of the remove_task() function, and nowhere does the add_task() or remove_task() functions update the queue when the item is marked for removal.
So I do not understand how this line: [1, 1, '<removed-task>'], comes into existence when printing the queue.
I only see reference to the dictionalry "entry_finder", but nowhere to I see code which updates the item in the queue to "<removed-task>", so how did it happen?

Can anyone explain?

Much appreciated

[Larz60+]

The document that you refer to is for python 2 which hasen't been supported since Jan 1, 2020

you can use: https://docs.python.org/3/library/queue.html which has a bulit-in priority queue

Are you using version 2.something?

[PythonNewbee]

I'm trying to implement a priority queue which allows me to update details of items already in queue.
I found a simple implementation in the Python documentation, which I'm working through to understand (https://docs.python.org/2/library/heapq....tion-notes).
However, there's a portion of this example which the understanding of escapes me; and I'm hoping someone can help me clarify?
The specific example is:

import itertools
import heapq

pq = []                         # list of entries arranged in a heap
entry_finder = {}               # mapping of tasks to entries
REMOVED = '<removed-task>'      # placeholder for a removed task
counter = itertools.count()     # unique sequence count

def add_task(task, priority=0):
    'Add a new task or update the priority of an existing task'
    if task in entry_finder:
        remove_task(task)
    count = next(counter)
    entry = [priority, count, task]
    entry_finder[task] = entry
    heappush(pq, entry)

def remove_task(task):
    'Mark an existing task as REMOVED.  Raise KeyError if not found.'
    entry = entry_finder.pop(task)
    entry[-1] = REMOVED

def pop_task():
    'Remove and return the lowest priority task. Raise KeyError if empty.'
    while pq:
        priority, count, task = heappop(pq)
        if task is not REMOVED:
            del entry_finder[task]
            return task
    raise KeyError('pop from an empty priority queue')

#Testing the above:
add_task('Planning')
add_task('Design',1)
add_task('Building',2)
print(pq)
#output is: [[0, 0, 'Planning'], [1, 1, 'Design'], [2, 2, 'Building']]
add_task('Design',1) #this should update an existing entry
print(pq)
'''
output:
[[0, 0, 'Planning'],
 [1, 1, '<removed-task>'],
 [2, 2, 'Building'],
 [1, 3, 'Design']]
'''

When I work through the example I'm struggling to understand how the second item in the queue gets updated when the second add_task('Design',1) is executed.
The remove_task(task) pops the item in the dictionary to a variable, and the last components of the variable's content is set to REMOVED.
However, that variable only lives in the scope of the remove_task() function, and nowhere does the add_task() or remove_task() functions update the queue when the item is marked for removal.
So I do not understand how this line: [1, 1, '<removed-task>'], comes into existence when printing the queue.
I only see reference to the dictionalry "entry_finder", but nowhere to I see code which updates the item in the queue to "<removed-task>", so how did it happen?

Can anyone explain?

Much appreciated

I think I have the answer. Is it that the queue and entry_finder entries are pointers to the original variables I declared, therefore when "remove_task()" pops the entry the new variable "entry" is another pointer to the original. If so, then setting entry[-1] actually changes the original variable and therefore the queue is updated because it contains a pointer to the variable?

[PythonNewbee]

The document that you refer to is for python 2 which hasen't been supported since Jan 1, 2020

you can use: https://docs.python.org/3/library/queue.html which has a bulit-in priority queue

Are you using version 2.something?

No, using version 3.8.
I will check out the document you mentioned, thank you

[Yoriz]

In the function add_task a list is created entry = [priority, count, task]
A pointer to the same list is added to entry_finder and pqin the lines

entry_finder[task] = entry
heapq.heappush(pq, entry)

In the function remove_task in the lines

entry = entry_finder.pop(task)
entry[-1] = REMOVED

It pops off the pointer to a list from entry_finder and then alters the last item in that list because that same list is also in pq it will also show the alteration.