How to Use deque in Python: Syntax and Examples

Use collections.deque in Python to create a double-ended queue that allows fast appends and pops from both ends. Import it with from collections import deque, then create a deque with deque() and use methods like append(), appendleft(), pop(), and popleft().

Syntax

The deque is imported from the collections module. You create a deque by calling deque() optionally with an iterable. You can add items to the right with append() or to the left with appendleft(). Remove items from the right with pop() or from the left with popleft().

python

from collections import deque

d = deque([1, 2, 3])  # Create a deque with initial items

d.append(4)           # Add 4 to the right end

d.appendleft(0)       # Add 0 to the left end

right_item = d.pop()   # Remove and return item from right
left_item = d.popleft() # Remove and return item from left

Example

This example shows how to create a deque, add items to both ends, and remove items from both ends. It prints the deque after each operation to show the changes.

python

from collections import deque

d = deque([10, 20, 30])
print('Initial deque:', d)

d.append(40)
print('After append(40):', d)

d.appendleft(5)
print('After appendleft(5):', d)

right = d.pop()
print('After pop():', d, '| Popped:', right)

left = d.popleft()
print('After popleft():', d, '| Popped:', left)

Output

Initial deque: deque([10, 20, 30])
After append(40): deque([10, 20, 30, 40])
After appendleft(5): deque([5, 10, 20, 30, 40])
After pop(): deque([5, 10, 20, 30]) | Popped: 40
After popleft(): deque([10, 20, 30]) | Popped: 5

Common Pitfalls

One common mistake is using a list when you need fast appends and pops from both ends; lists are slow for popping from the front. Another is forgetting to import deque from collections. Also, using pop() or popleft() on an empty deque raises an IndexError.

Always check if the deque is not empty before popping or handle the exception.

python

from collections import deque

d = deque()

# Wrong: popping from empty deque causes error
# d.pop()  # IndexError

# Right: check before popping
if d:
    d.pop()
else:
    print('Deque is empty, cannot pop')

Output

Deque is empty, cannot pop

Quick Reference

Method	Description
deque(iterable)	Create a deque with optional initial items
append(x)	Add item x to the right end
appendleft(x)	Add item x to the left end
pop()	Remove and return item from the right end
popleft()	Remove and return item from the left end
extend(iterable)	Add multiple items to the right end
extendleft(iterable)	Add multiple items to the left end (reversed order)
clear()	Remove all items from the deque
rotate(n)	Rotate the deque n steps to the right (left if negative)

Key Takeaways

Use collections.deque for fast appends and pops from both ends of a sequence.

Remember to import deque with 'from collections import deque' before using it.

Avoid popping from an empty deque to prevent IndexError; check if it's not empty first.

deque supports useful methods like append, appendleft, pop, popleft, extend, and rotate.

deque is more efficient than lists for queue and stack operations involving both ends.