Python's Collections Module: deque

Deque, short for Double Ended Queue

It is a list-like container with fast appends and pops on either end.

Deques are a generalization of stacks and queues.

Deques support thread-safe, memory efficient appends and pops from either side of the deque with approximately the same O(1) performance in either direction.

Importing deque

from collections import deque

Creating a deque container

""" deques can be created from any iterable
using : "deque(iterable)" syntax
"""

d1 = deque([1, 2, 3, 3, 4, 5])  # from list
d2 = deque(("Kathan", 19, 'idc'))  # from tuple
d3 = deque("Followme")  # from string

# dictionary is an iterable of keys btw,
d4 = deque({"name": "R", "age": 1})  # from dict
print(d1, d2, d3, d4, sep='\n')

"""OUTPUT
deque([1, 2, 3, 3, 4, 5])
deque(['Kathan', 19, 'idc'])
deque(['F', 'o', 'l', 'l', 'o', 'w', 'm', 'e'])
deque(['name', 'age'])
"""

The maxlen argument

We can also pass an optional argument maxlen which determines the size of deque.

deque(iterable, maxlen)

📑If maxlen is not specified or is None, deques may grow to an arbitrary length.

Otherwise, the deque is bounded to the specified maximum length. Once a bounded length deque is full, when new items are added, a corresponding number of items are discarded from the opposite end.

This thing will make sense more, if we know how to update a deque after instantiation. Let's discuss those methods now,

append, appendleft, pop, popleft methods

my_deque = deque([1, 2, 3, 4, 5])

# append(y) => appends y to right side
my_deque.append(6)
print("After my_deque.append(6) =>", my_deque)

# appendleft(y) => appends y to left side
my_deque.appendleft(0)
print("After my_deque.appendleft(0) =>", my_deque)

# pop() => Remove and return an element from the right side
my_deque.pop()
print("After my_deque.pop()=>", my_deque)

# popleft() =>Remove and return an element from the left side
my_deque.popleft()
print("After my_deque.popleft()=>", my_deque)

"""OUTPUT
After my_deque.append(6) => deque([1, 2, 3, 4, 5, 6])
After my_deque.appendleft(0) => deque([0, 1, 2, 3, 4, 5, 6])
After my_deque.pop()=> deque([0, 1, 2, 3, 4, 5])
After my_deque.popleft()=> deque([1, 2, 3, 4, 5])
"""

📑 The important part here is that both append methods don't return anything, they perform inplace modifications!

extend, extendleft methods

my_deque = deque([1, 2, 3])

"""extend(iterable)
Extend the right side of the deque by 
appending elements from the iterable argument
"""
my_deque.extend([4, 5, 6])
print("After extend:", my_deque)

"""extendleft(iterable)
Extend the left side of the deque by appending 
elements from iterable.
"""
my_deque.extendleft([0, -1, -2])
print("After extendleft:",my_deque)

"""OUTPUT
After extend: deque([1, 2, 3, 4, 5, 6])
After extendleft: deque([-2, -1, 0, 1, 2, 3, 4, 5, 6])
"""

📑 If you notice in the extendleft(iterable) method, the elements are inserted in reversed order i.e. the last element of input iterable is the first in our updated deque. This is because we perform series of left appends by taking elements from start to end (from our iterable).

More methods

🔮 deque is a very powerful container, and it has loads of functionalities! I have discussed most of them here but feel free to check out the documentation for some extra goodies.

And that wraps our discussion on collections module. I hope you enjoyed the extensive coverage :) I hope you join me again for some other adventure!

Untill then, Enjoy Programming 🥳

References

https://docs.python.org/3/library/collections.html#module-collections