In this video we will see how cProfile (default Python library) can help you to get run-times from your Python program.

Queue vs Python lists

In this video we will compare the performance of a simple Queue implemented directly into Python (no optimisations) with the default Python list.

Can it compare with it on performance?

This is where time complexity analysis come into the picture. A Queue insert and deletion is O(1) time complexity. A Python list used as a queue has O(n) time complexity.

But does the performance and run-time show the same? Here we compare the run-time by using cProfile in Python.

It is like a stack of plates. The last one you put on the top is the first one you take.

How can you implement them in Python? Well, we are in luck, you can use a Stack, and if done correctly, you will have the same performance as an actual Stack implementation will have.

But first, how can you do it wrong?

Well, you might think that the first element of the list is the top of your stack, hence in you will insert the elements on the first position, and, hence, remove them from the first position as well.

# Create a list as a stack
s = []
# Insert into the first position.
element = 7
s.insert(0, element)
# Remove from the first position.
s.pop(0)

Sounds about right?

Let’s test that and compare it with a different approach. To add the newest element to the end of the list, and, hence, remove them from the end of the list.

# Create a list and use it as stack
s = []
# Insert element in last postion
element = 7
s.append(element)
# Remove from the last position
s.pop()

Let’s check the performance of those two approaches.

Comparing the performance of the two approaches

How do you compare. You can use cProfile library. It is easy to use and informative results

See the sample code below, which compares the two approaches by create a stack each and inserting n elements to it and removing them afterwards.

import cProfile
def profile_list_as_queue_wrong(n):
s = []
for i in range(n):
s.insert(0, i)
while len(s) > 0:
s.pop(0)
def profile_list_as_queue_correct(n):
s = []
for i in range(n):
s.append(i)
while len(s) > 0:
s.pop()
def profile(n):
profile_list_as_queue_wrong(n)
profile_list_as_queue_correct(n)
cProfile.run("profile(100000)")

And you notice that the append and pop (last element) are O(1), which means constant time. Constant time, means that the operations are independent on the size of the lists. That means the correct implementation gives O(n) time complexity.

On the other hand, the insert and pop(0) have linear performance. That basically means that we with the wrong implementation end up with O(n^2) time complexity.

We all know what a queue is. You go to the grocery store and get spinach, strawberry and bananas for your shake. Then you see a long line of people in front of the register. That line is a queue.

The same holds in programming. You create queues to process data or input of any kind.

How to implement a Queue in Python

It is easier than you think.

First you create a Node class to represent each node in a queue. A node is an abstraction to represent a point to the next node and the actual element.

class Node:
def __init__(self, element=None, next_node=None):
self.element = element
self.next_node = next_node

Then you create the class for the Queue.

class Queue:
def __init__(self):
self.head = None
self.tail = None
def enqueue(self, element):
if self.head is None:
self.head = self.tail = Node(element)
else:
n = Node(element, self.tail)
self.tail.next_node = n
self.tail = n
def dequeue(self):
element = self.head.element
if self.tail == self.head:
self.tail = self.head = None
else:
self.head = self.head.next_node
return element
def is_empty(self):
return self.head is None

How does it work. Let’s make a simple example.

q = Queue()
for i in range(10):
q.enqueue(i)
while not q.is_empty():
print(q.dequeue())

Which will output.

0
1
2
3
4
5
6
7
8
9

Yes! You guessed it.

How do we test performance?

I like to use the cProfile library. It is easy to use and gives informative results.

So how do you test performance? You simply import the cProfile library and use the cProfile.run(…) call.

You also need to do some operations to see how your Queue performs. See the code as an example.

import cProfile
def profile_queue(n):
q = Queue()
for i in range(n):
q.enqueue(i)
while not q.is_empty():
q.dequeue()
def profile(n):
profile_queue(n)
cProfile.run("profile(100000)")

The interesting line is line 9, which tells us how much time is spend in the call to profile_queue.

But is the result good?

We need to compare it to other implementations.

Performance testing the Queue with a Python list

Python lists are used for anything. Can we use a Python list as a Queue. Of course. Let’s try to implement that and compare it to our Queue.

import cProfile
def profile_queue(n):
q = Queue()
for i in range(n):
q.enqueue(i)
while not q.is_empty():
q.dequeue()
def profile_list_as_queue(n):
q = []
for i in range(n):
q.insert(0,i)
while len(q) > 0:
q.pop()
def profile(n):
profile_queue(n)
profile_list_as_queue(n)
cProfile.run("profile(100000)")