Python deque.maxlen: Fixed-Size Queues Explained

Automatically keep only the latest elements in a bounded deque.

What You Will Learn

deque(iterable, maxlen=n) discards old items from the opposite end when capacity is exceeded. A maxlen of zero is allowed and stores no items.

Time complexity assumes a specific input structure. Include setup work such as sorting or building a lookup table when comparing complete solutions.

Step-by-Step Example

The example below makes the behavior visible with small input values that you can change safely.

from collections import deque
recent_events = deque(maxlen=3)
for event in ['A', 'B', 'C', 'D']:
    recent_events.append(event)
print(list(recent_events))
print(recent_events.maxlen)

How the Example Works

The imports provide the standard-library tools used in the demonstration: collections.
The variable recent_events holds the result of deque(maxlen=3). Later lines use that value or inspect how it changes.
The loop demonstrates when each value is processed or when cleanup and exception handling take effect.
The print() calls make the return values or state visible so you can compare them with the explanation and check your prediction.

Expected output from the example:

['B', 'C', 'D']
3

When You Would Use This

This technique matters when a program handles larger queues, sorted records, or repeated lookups. Choose the data structure based on actual operations and the preconditions the algorithm requires. Verify boundary inputs before optimizing for speed.

Practice: Modify the Code

Store the latest five measurements without manually trimming older entries.

Verify the changed output against the behavior described above, including the relevant invalid or boundary case.

For complete parameters and version details, consult the official Python documentation.

Common Beginner Mistakes

Expecting append() to raise an overflow error after a bounded deque fills.
Assuming the operation has the same behavior for missing values, empty inputs and regular inputs.

Key Takeaways

Automatically keep only the latest elements in a bounded deque.
deque(iterable, maxlen=n) discards old items from the opposite end when capacity is exceeded.
Apply this operation only after checking the assumptions and input conditions shown in the example.