Queue Implemented as Single Linked List

Question

Note: I do know that Python libraries provide a Linked list and Stack. This implementation has been done to practice Python and some of the data structures and algorithms.

I have implemented Queue as a Single linked list. Feel free to make suggestions. Note: The code works.

Targeted Big O:

Search: O(n), EnQueue and DeQueue: O(1)

Methods:

en_queue(value): insert values
de_queue(): remove values
is_empty(): check to see if Queue is empty
peek_front(): look at what head points to without removing
peek_back(): look at what tail points to without removing
search(value): check to see if a value is in the Queue
length(): return size

Classes:

class Node:
 def __init__(self, value):
 self.data = value
 self.front = None
class Queue:
 def __init__(self):
 self.head = None
 self.tail = None
 self.count = 0
 def en_queue(self, value):
 new_node = Node(value)
 if self.tail is not None:
 # make the front attribute of old node point to new node
 self.tail.front = new_node
 else:
 # if first ever node in Queue both front and tail will point to it
 self.head = new_node
 self.tail = new_node
 self.count += 1
 def de_queue(self):
 if not self.is_empty():
 # point head to next node
 self.head = self.head.front
 print("sucess")
 self.count -= 1
 else:
 print("Empty QUEUE")
 def __iter__(self):
 node = self.head
 while node:
 yield node
 node = node.front
 def is_empty(self):
 if self.head is None and self.tail is None:
 return True
 else:
 return False
 def peek_front(self):
 return self.head.data
 def peek_back(self):
 return self.tail.data
 def queue_search(self, value):
 # start from the head
 p = self.head
 while p is not None:
 # make p reference to next node
 if p.front is not None:
 if p.data == value:
 print("Found value")
 return p.data
 p = p.front
 else:
 print("fail")
 return 0
 def length(self):
 return self.count

Test:

from stack_Queue import Queue
def main():
 print("-------Test Queue---------")
 print("-------Test En Queue------")
 my_queue = Queue()
 test_list = [1, 2, 3, 4, -2000000, 'a', 500000, 50]
 for i in test_list:
 my_queue.en_queue(i)
 print("-------En Queue Done-------")
 for i in my_queue:
 print(i.data)
 print("------Queue Print Done-----")
 print("------Queue Print Done-----")
 print(my_queue.peek_back())
 print(my_queue.peek_front())
 print("----------De Queue---------")
 my_queue.de_queue()
 print("--------De Queue Done------")
 for i in my_queue:
 print(i.data)
 print("-----Queue Print Done------")
 print("-----Test search-------")
 x = my_queue.queue_search('a')
 print(x)
 print("-------Full De Queue-------")
 while my_queue.length() != 0:
 my_queue.de_queue()
 print("--------De Queue Done------")
 for i in my_queue:
 print(i.data)
 print("-----Queue Print Done------")
if __name__ == "__main__":
 main()

Result:

-------Test Queue---------
-------Test En Queue------
-------En Queue Done-------
1
2
3
4
-2000000
a
500000
50
------Queue Print Done-----
------Queue Print Done-----
50
1
----------De Queue---------
sucess
--------De Queue Done------
2
3
4
-2000000
a
500000
50
-----Queue Print Done------
-----Test search-------
Found value
a
-------Full De Queue-------
sucess
sucess
sucess
sucess
sucess
sucess
sucess
--------De Queue Done------
-----Queue Print Done------
Process finished with exit code 0

• Ashwini Chaudhary: My points from my other answer will be applicable to this as well. I am not sure about posting it here again.
• BlooB: yes yes indeed and i will implement them to this as well, i just did not want to create clutter, and taught since they are a bit different to post them separate

Answer 1

In addition to the points I had mentioned in my previous answer to your Stack implementation.

Your Queue is not introspectable right now as it lacks __repr__ method. Due to this you won't be able to view its items easily. Hence you could add __repr__ to both Node and Queue class(same applicable to your Stack class in other question).

class Node:
 def __init__(self, value):
 self.data = value
 self.front = None
 def __repr__(self):
 return repr(self.data)
class EmptyQueueException(Exception):
 pass
class Queue:
 def __init__(self):
 self.head = None
 self.tail = None
 self.count = 0
 def en_queue(self, value):
 new_node = Node(value)
 if self.tail is not None:
 # make the front attribute of old node point to new node
 self.tail.front = new_node
 else:
 # if first ever node in Queue both front and tail will point to it
 self.head = new_node
 self.tail = new_node
 self.count += 1
 def de_queue(self):
 if self:
 # point head to next node
 self.head = self.head.front
 self.count -= 1
 else:
 raise EmptyQueueException()
 def peek_front(self):
 if self:
 return self.head.data
 raise EmptyQueueException()
 def peek_back(self):
 if self:
 return self.tail.data
 raise EmptyQueueException()
 def __iter__(self):
 node = self.head
 while node is not None:
 yield node
 node = node.front
 def __bool__(self):
 return not (self.head is None and self.tail is None)
 def __contains__(self, value):
 return value in (node.data for node in self)
 def __len__(self):
 return self.count
 def __repr__(self):
 return 'Queue<{nodes}>'.format(nodes=', '.join(repr(node) for node in self))

Demo:

>>> my_queue
Queue<1, 2, 3, 4, -2000000, 'a', 500000, 50>
>>> my_queue.de_queue() # Probably want to return the value as well?
>>> my_queue
Queue<2, 3, 4, -2000000, 'a', 500000, 50>
>>> my_queue.peek_back()
50
>>> 'a' in my_queue
True
>>> 'spam' in my_queue
False
>>> bool(my_queue)
True
>>> len(my_queue)
7
>>> len(Queue())
0 
>>> bool(Queue())
False
>>> Queue().peek_back()
---------------------------------------------------------------------------
EmptyQueueException Traceback (most recent call last)
...

Answer 2

In en_queue(), the pythonic way to express if self.tail is not None: is if self.tail:

• Ashwini Chaudhary: If the intention is to explicitly check against None then is not None is fine.