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How to Get the First and Last Elements of a Deque in Python

Learn efficient ways to access or retrieve the first and last elements of a Python deque using indexing (dq[0]/dq[-1]), popleft()/pop(), and iteration with performance and safety considerations.

A deque (double-ended queue) from Python's collections module is a versatile data structure that allows efficient insertion and removal from both ends. Accessing the first and last elements is a common operation when using deques as queues, stacks, or sliding windows. In this guide, we'll show you multiple methods to retrieve these elements and explain when to use each approach.

Quick Answer

from collections import deque

dq = deque([10, 20, 30, 40, 50])

first = dq[0]    # First element
last = dq[-1]    # Last element

print(f"First: {first}, Last: {last}")

Output:

First: 10, Last: 50

Method 1: Using Index Access [0] and [-1] (Recommended)

The simplest and most common way to access the first and last elements is using square bracket indexing. This reads the values without modifying the deque:

from collections import deque

dq = deque(['apple', 'banana', 'cherry', 'date', 'elderberry'])

print(f"Deque: {dq}")
print(f"First element: {dq[0]}")
print(f"Last element:  {dq[-1]}")
print(f"Deque after:   {dq}")  # Unchanged

Output:

Deque: deque(['apple', 'banana', 'cherry', 'date', 'elderberry'])
First element: apple
Last element:  elderberry
Deque after:   deque(['apple', 'banana', 'cherry', 'date', 'elderberry'])

Method 2: Using popleft() and pop() (Removes Elements)

The popleft() method removes and returns the first element, while pop() removes and returns the last element:

from collections import deque

dq = deque([10, 20, 30, 40, 50])

print(f"Before: {dq}")

first = dq.popleft()    # Removes and returns first element
last = dq.pop()         # Removes and returns last element

print(f"First: {first}")
print(f"Last:  {last}")
print(f"After: {dq}")   # Elements are removed

Output:

Before: deque([10, 20, 30, 40, 50])
First: 10
Last:  50
After: deque([20, 30, 40])

Handling empty deques:

from collections import deque

dq = deque()

try:
    first = dq.popleft()
except IndexError:
    print("Cannot pop from an empty deque")

Method 3: Using Iteration (For Special Cases)

If you need both the first and last elements and want to handle edge cases gracefully, you can use unpacking:

from collections import deque

dq = deque([10, 20, 30, 40, 50])

first, *middle, last = dq
print(f"First: {first}, Last: {last}")

Output:

First: 10, Last: 50

Comparison of Methods

Method Returns Modifies Deque Empty Deque Time Complexity
dq[0] / dq[-1] Element value ❌ No Raises IndexError O(1)
dq.popleft() / dq.pop() Element value ✅ Yes (removes) Raises IndexError O(1)
first, *_, last = dq Both values ❌ No Raises ValueError O(n)

Practical Examples

Using a Deque as a Queue (FIFO)

from collections import deque

tasks = deque(['task_1', 'task_2', 'task_3', 'task_4'])

print(f"Next task: {tasks[0]}")

current = tasks.popleft()
print(f"Processing: {current}")
print(f"Remaining: {tasks}")

Output:

Next task: task_1
Processing: task_1
Remaining: deque(['task_2', 'task_3', 'task_4'])

Using a Deque as a Sliding Window

from collections import deque

window = deque(maxlen=3)

for value in [10, 20, 30, 40, 50]:
    window.append(value)
    if len(window) == window.maxlen:
        print(f"Window: {list(window)} | "
              f"First: {window[0]}, Last: {window[-1]}")

Output:

Window: [10, 20, 30] | First: 10, Last: 30
Window: [20, 30, 40] | First: 20, Last: 40
Window: [30, 40, 50] | First: 30, Last: 50

Safe Access with a Helper Function

from collections import deque

def peek_first(dq, default=None):
    """Return the first element or a default if empty."""
    return dq[0] if dq else default

def peek_last(dq, default=None):
    """Return the last element or a default if empty."""
    return dq[-1] if dq else default

dq = deque([10, 20, 30])
print(f"First: {peek_first(dq)}")
print(f"Last:  {peek_last(dq)}")

empty = deque()
print(f"First: {peek_first(empty, 'N/A')}")
print(f"Last:  {peek_last(empty, 'N/A')}")

Output:

First: 10
Last:  30
First: N/A
Last:  N/A

Conclusion

  • To get the first and last elements of a deque in Python, use index access with dq[0] and dq[-1]. This is the simplest, fastest (O(1)), and safest approach because it does not modify the deque.
  • Use popleft() and pop() only when you want to remove the elements while retrieving them (for example, when processing a queue).
  • Always check that the deque is non-empty before accessing elements to avoid IndexError.
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