title: Event Timers in Python 001 heapq
tags: python timer priority-queue heapq

We use `heapq` to make a priority queue:
```py
# using heapq from the standard library

import heapq

class PriorityQueue:
  def __init__(self):
    self._queue = []

  def push(self, item, priority):
    heapq.heappush(self._queue, (priority, item))

  def pop(self):
    if not self.is_empty():
      return heapq.heappop(self._queue)[1]
    else:
      raise IndexError("pop from an empty priority queue")

  def is_empty(self):
    return len(self._queue) == 0

# example usage
priority_queue = PriorityQueue()
priority_queue.push("Task 1", 3)
priority_queue.push("Task 2", 1)
priority_queue.push("Task 3", 2)

while not priority_queue.is_empty():
  print(priority_queue.pop())
```

For a second based timer:
```py
# priority queue based timer

import heapq
import time

class Timer:
  def __init__(self):
    self._queue = []

  def schedule(self, callback, delay):
    execution_time = time.time() + delay
    heapq.heappush(self._queue, (execution_time, callback))

  def run_pending(self):
    current_time = time.time()
    while self._queue and self._queue[0][0] <= current_time:
      _, callback = heapq.heappop(self._queue)
      callback()

  def is_empty(self):
    return len(self._queue) == 0

def callback1():
  print("Callback 1 executed")

def callback2():
  print("Callback 2 executed")

timer = Timer()

timer.schedule(callback1, 2)
timer.schedule(callback2, 7)

while not timer.is_empty():
  timer.run_pending()
  time.sleep(1)
```

For a millisecond based timer:
```py
# priority queue based timer

import heapq
import time

class Timer:
  def __init__(self):
    self._queue = []

  def schedule(self, callback, delay):
    ms = time.time_ns()/10**6
    execution_time = ms + delay
    heapq.heappush(self._queue, (execution_time, callback))

  def run_pending(self):
    ms = time.time_ns()/10**6
    while self._queue and self._queue[0][0] <= ms:
      _, callback = heapq.heappop(self._queue)
      callback()

  def is_empty(self):
    return len(self._queue) == 0

def callback1():
  print("Callback 1 executed")

def callback2():
  print("Callback 2 executed")

timer = Timer()

timer.schedule(callback1, 200)
timer.schedule(callback2, 700)
timer.schedule(callback2, 2700)

while not timer.is_empty():
  timer.run_pending()
  time.sleep(1/1000)
```

Note that I don't know how accurate these are.