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031-deque-vs-list-queue-ops.md

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collections.deque vs list for Queue Operations

Problem

Python code using list as a queue or deque, calling list.pop(0) or list.insert(0, x) for FIFO operations. The metric was operation throughput. list.pop(0) is O(N) because it shifts all remaining elements left in memory, making list-based queues quadratic for N operations.

What Worked

Replacing list with collections.deque for any use case requiring efficient insertion/removal at both ends. deque is implemented as a doubly-linked list of fixed-size blocks, providing O(1) append, appendleft, pop, and popleft.

For a BFS traversal processing 5M nodes, switching from list.pop(0) to deque.popleft() reduced time from 42s to 1.2s (35x speedup).

Experiment Data

Operation list deque Speedup
Append right (1M ops) 52ms 48ms 1.1x
Pop right (1M ops) 51ms 47ms 1.1x
Pop left (1M ops) 12,400ms 47ms 264x
Insert left (1M ops) 13,100ms 48ms 273x
Random access [i] 0.05μs 0.3μs 0.17x (list wins)

Code Example

from collections import deque

# BAD: list as queue — O(N) per popleft
queue = []
queue.append(start_node)
while queue:
    node = queue.pop(0)       # O(N) — shifts all elements
    for neighbor in node.adj:
        queue.append(neighbor)

# GOOD: deque as queue — O(1) per popleft
queue = deque()
queue.append(start_node)
while queue:
    node = queue.popleft()    # O(1) — no shifting
    for neighbor in node.adj:
        queue.append(neighbor)

# Bounded deque for sliding window / recent history
recent = deque(maxlen=1000)   # automatically discards oldest when full
for event in stream:
    recent.append(event)      # O(1), old events auto-evicted

# deque also supports rotate() for circular buffer patterns
d = deque([1, 2, 3, 4, 5])
d.rotate(2)   # [4, 5, 1, 2, 3] — O(k) rotation

What Didn't Work

  • deque for random access: deque[i] is O(N) (walks the block chain), while list[i] is O(1). If you need both fast random access and fast left insertion, consider sortedcontainers.SortedList or a different data structure.
  • deque for slicing: deque doesn't support slicing (d[1:5]). Convert to list first if needed.

Environment

CPython 3.8+. collections.deque is a C extension in the stdlib. For thread-safe queues, use queue.Queue (which wraps deque with locks).