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Digital · Guided lesson

Implement a fair FIFO queue

About 19 minutes · 4 reading + 15 practice and checks
Time includes reading, practice, and questions. Extra videos or further study are optional.
See this course’s project requirements

A runnable fictional help-desk simulator with an undo stack, FIFO queue and small dependency graph, plus empty and cyclic case tests.

Keep your working files in your own notebook or software. Save a private description, checklist, and work link in My Proof.

What to include

  • An operation-to-structure map with three distinct retrieval rules.
  • An undo trace with last-in-first-out and empty-state handling.
  • A FIFO record with clear empty behavior.
  • A four-node dependency plan ending with archive.
  • A cycle diagnosis and justified repair record.
  • A runnable stack/queue/dependency demonstration with boundary evidence.

Review and revise

Use these criteria to check your work and choose what to improve.

  • The implementation or declared simulation follows the supplied contract and retains inspectable original inputs.
  • Normal, boundary and rejected cases are recorded with expected versus observed results.
  • The changed case has a correct explanation and does not rely on hidden state.
  • The project includes source, outputs, instructions and honest tool, privacy and verification limits.
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Before you begin

The idea in everyday life

Queue starts[A, B], then C arrives. No priority exceptions are permitted in this practice.

The idea

Use collections.deque for a queue whose front is removed often. append adds at the right, popleft removes at the left. A priority queue is a different policy and should not be mislabeled FIFO.

Follow along

Implement a fair FIFO queue — original worked practice

Use collections.deque for a queue whose front is removed often. append adds at the right, popleft removes at the left. A priority queue is a different policy and should not be mislabeled FIFO.

Queue starts[A, B], then C arrives. No priority exceptions are permitted in this practice.

  1. 1. Step 1

    Create deque([A, B]).

    You should see: Front A, back B.

    This keeps the rule and its observable result together.

  2. 2. Step 2

    Append C.

    You should see: The order is A, B, C.

    This keeps the rule and its observable result together.

  3. 3. Step 3

    Call popleft and record both returned and remaining values.

    You should see: Returned A; remaining B, C.

    This keeps the rule and its observable result together.

  4. 4. Step 4

    Guard an empty queue and write the service-order log.

    You should see: A FIFO record with clear empty behavior.

    This keeps the rule and its observable result together.

Now change one thing

After serving A, append D. Enter the next ticket ID.

Your work

What to finish

A FIFO record with clear empty behavior.

Common mistake

Watch for this

Appending urgent work at the front while still claiming the same first-in-first-out policy.

How can I tell I understand it?

Explain and demonstrate the required distinction: Use collections.deque for a queue whose front is removed often. append adds at the right, popleft removes at the left. A priority queue is a different policy and should not be mislabeled FIFO. Show a changed input and the expected result without weakening this contract.

Try 3 questions

Quick knowledge check

Answer all 3 questions. Get at least 2 right and the lesson completes automatically.

Not yet passed

You’ll get an explanation for each answer. Signed in? Your results save to your account.

1Which call serves the front of this deque?
2After A is served and D appended, which order remains?
3Why not silently serve urgent C first?
0 of 3 answeredApply the lesson to a short question, then review the explanation.
3
Finish this lesson

Pass the check. We’ll save the progress.

Answer all 3 questions above and get at least 2 right. You can review the explanations and try again.

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Sources and further reading

Explore the references behind this lesson.

docs.python.org — collections.html
Source & access notes · checked 2026-10-01

Relevant documented module/API behavior inspected during expansion authoring; actual execution evidence is recorded separately. No grant eligibility is established.

Access: Official page retrieved with the web reader on 2026-10-01. Documentation reading does not certify execution in every native tool or browser.

Reuse: Original local practice files, fictional data, worked examples and questions. Concepts paraphrased and linked; source exercises are not copied.

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