Nested Data

Represent structured information with containers inside containers.

• Level: Beginner
• Estimated time: 25–40 minutes
• You will learn: Represent structured information with containers inside containers.
• Practice in: Google Colab, Jupyter, or VS Code

Open this lesson in Google Colab

Questions

• What problem does Nested Data help us solve in a small Python program?
• What should we predict before running the example?
• What value, output, or error should we inspect after changing one line?

Objectives

• Run a complete example for nested data in Colab.
• Explain the example line by line using plain language.
• Change one part of the code and predict the result before running it.
• Recognize one common mistake and use the error message as evidence.

Hands-on episode: Nested Data

Nested data means containers inside containers. Read it one level at a time: outer collection first, then inner collection, then the specific value.

We will learn this by running code, not by memorizing a definition first. Open the Colab notebook from the button above, find this section, and run each cell in order. Keep a small note beside the notebook with three columns: prediction, actual result, and what changed.

Example 1.1

Predict the output by naming each level before reading the next one.

students = [
{"name": "Ada", "score": 9},
{"name": "Grace", "score": 8},
]

print(students[0]["name"])

Run the cell once without editing it. If the result is different from your prediction, leave the prediction visible and write one sentence about the difference. That sentence is more useful than a perfect first guess.

Explain Example 1.1

• students is a list, so students[0] means the first record.
• The first record is a dictionary, so ["name"] looks up a labeled value inside that record.
• The final value is the string Ada, which is what gets printed.

Now explain the example out loud or in a Markdown cell. Use short sentences: “this line creates…”, “this name stores…”, “this output appears because…”. If you cannot explain a line yet, run only the lines above it and inspect the values that exist at that moment.

Challenge 1.1

NoteChallenge

Change the outer index to 1 and predict the name. Then change the key to "score". Only one level changed each time, so the cause and effect stay visible.

Show a safe way to approach the challenge

1. Copy Example 1.1 into a new Colab cell.
2. Change exactly one value, name, condition, or line.
3. Write the expected output before running the cell.
4. Run the cell and compare the actual result with your prediction.
5. If the result surprises you, undo the change and try a smaller one.

Suggested first move: Change the outer index to 1 and predict the name.

Debugging checkpoint 1.1

WarningDebugging checkpoint

The most common nested-data bug is mixing up list indexes and dictionary keys. If you see TypeError or KeyError, pause and ask: what type is the value at this level? Print type(students[0]) before going deeper.

Do not debug by rewriting the whole example. Read the error type or surprising output, inspect the closest value with print(...) or type(...), then change one thing. This is the same routine you will use in larger projects.

Apply it

Add a third student record, loop through the list, and print name: score for each learner. Then compute the average score by accumulating one field from each dictionary.

Finish by adding a Markdown cell that answers: What did this example teach me that I can reuse in a project?

Key points

• Learn the concept by running a complete, small example first.
• Predict before execution so your thinking becomes visible.
• Change one thing at a time so cause and effect stay clear.
• Treat errors as clues about the exact line or value Python could not handle.

Why this matters

Represent structured information with containers inside containers.

Beginners often try to learn Python as a list of commands. This lesson teaches one idea at a time: what problem it solves, how to recognize it in code, how to practice it, and how to debug a common mistake. Treat the code examples as small experiments, not as text to memorize.

NoteGuiding questions

By the end of this lesson, you should be able to answer:

• What problem does Nested Data help me solve?
• What words or symbols should I recognize in a small example?
• What should I check first when the example does not behave as expected?

NoteLearning objectives

You will practice how to:

• explain the main idea in everyday language;
• read a short example line by line;
• predict the result before running the code;
• make one safe variation of the example;
• debug one common beginner mistake.

TipAnalogy

Nested data is like folders inside folders: each level narrows your path to the value you want.

How nested data is arranged

Nested data is a path through containers. Move one level at a time instead of trying to understand the whole structure at once.

flowchart TD
  students["students<br/>list"] --> row0["index 0<br/>dict"]
  students --> row1["index 1<br/>dict"]
  row0 --> name0["key 'name'<br/>'Ada'"]
  row0 --> scores0["key 'scores'<br/>list"]
  scores0 --> score00["index 0<br/>90"]
  scores0 --> score01["index 1<br/>95"]
  path["students[0]['scores'][1]"] --> row0
  path --> scores0
  path --> score01

Read nested data paths from left to right: list index, dictionary key, list index, and so on.

Vocabulary

• nested — a key term for this lesson; after the example, write a one-sentence definition in your own words.
• record — a key term for this lesson; after the example, write a one-sentence definition in your own words.
• path — a key term for this lesson; after the example, write a one-sentence definition in your own words.
• level — a key term for this lesson; after the example, write a one-sentence definition in your own words.
• structure — a key term for this lesson; after the example, write a one-sentence definition in your own words.

What this means

Nested data uses lists, dictionaries, tuples, or sets inside each other to model real records.

A useful explanation has three parts:

1. Name the thing. Say what concept you are using.
2. Name the input. Identify the values, files, objects, or settings involved.
3. Name the result. Explain what changes, what is returned, or what is printed.

Example 1

Predict what will happen before you run the code.

students = [
    {"name": "Ada", "scores": [90, 95]},
    {"name": "Grace", "scores": [100, 98]},
]
print(students[0]["scores"][1])

Step-by-step explanation

1. students = [ — pause here and say what this line reads, creates, changes, or displays.
2. {"name": "Ada", "scores": [90, 95]}, — pause here and say what this line reads, creates, changes, or displays.
3. {"name": "Grace", "scores": [100, 98]}, — pause here and say what this line reads, creates, changes, or displays.
4. ] — pause here and say what this line reads, creates, changes, or displays.
5. print(students[0]["scores"][1]) — pause here and say what this line reads, creates, changes, or displays.

After running the example, compare the actual output with your prediction. If they differ, do not erase your prediction. The difference is the part that can teach you the most.

Worked example: read, predict, modify

Use this routine with every example in the course:

Step	What to do	Why it helps
Read	Point to each name, value, and operator.	Slows the code down enough to understand it.
Predict	Write what you think will happen.	Creates a testable prediction.
Run	Execute the smallest complete example.	Lets Python give evidence.
Explain	Say what happened in plain language.	Converts recognition into understanding.
Modify	Change one small thing and run again.	Shows which part caused which result.

Challenge

NotePractice

Change one input value, predict the new output, run the code, and explain the difference in one sentence.

Show one possible solution path

1. Copy Example 1 into Colab, Jupyter, or a .py file.
2. Mark the line you plan to change.
3. Write a one-sentence prediction.
4. Run the changed code.
5. If the result surprises you, restore the original and change a smaller part.

The goal is not to find the only correct answer. The goal is to create a small experiment where you can explain cause and effect.

Common mistakes

WarningCommon mistake

Deep nesting is easy to misread. Format data over multiple lines and trace one level at a time.

When you get stuck, use this debugging routine:

1. Read the last line of the error message or inspect the unexpected output.
2. Find the smallest line of code that could be responsible.
3. Print or inspect the value and type at that point.
4. Change one thing.
5. Run again and record what changed.

Checkpoint quiz

Answer these questions before moving on. The quiz runs with Quarto OJS in the browser, so it does not need a Python kernel during website rendering.

{
  const scripts = Array.from(
    document.querySelectorAll("script.fcpython-ojs-quiz-config")
  );
  const script = scripts.find(
    (node) => node.dataset.fcpythonRendered !== "true"
  );

  if (!script) {
    return html`<div class="fcpython-quiz fcpython-quiz-warning">
      Quiz configuration was not found.
    </div>`;
  }

  script.dataset.fcpythonRendered = "true";
  const quiz = JSON.parse(script.textContent);
  const container = html`<div class="fcpython-quiz"></div>`;
  const title = document.createElement("h3");
  title.textContent = quiz.title;
  container.appendChild(title);

  const instructions = document.createElement("p");
  instructions.textContent = quiz.instructions;
  container.appendChild(instructions);

  const feedbackNodes = [];

  quiz.questions.forEach((question, questionIndex) => {
    const fieldset = document.createElement("fieldset");
    fieldset.className = "fcpython-quiz-question";

    const legend = document.createElement("legend");
    legend.textContent = `${questionIndex + 1}. ${question.prompt}`;
    fieldset.appendChild(legend);

    question.options.forEach((option, optionIndex) => {
      const label = document.createElement("label");
      label.className = "fcpython-quiz-option";

      const input = document.createElement("input");
      input.type = "radio";
      input.name = `${quiz.id}-${question.id}`;
      input.value = String(optionIndex);

      const text = document.createElement("span");
      text.textContent = option;

      label.appendChild(input);
      label.appendChild(text);
      fieldset.appendChild(label);
    });

    const feedback = document.createElement("p");
    feedback.className = "fcpython-quiz-feedback";
    feedback.setAttribute("aria-live", "polite");
    feedbackNodes.push(feedback);
    fieldset.appendChild(feedback);
    container.appendChild(fieldset);
  });

  const actions = document.createElement("div");
  actions.className = "fcpython-quiz-actions";

  const check = document.createElement("button");
  check.type = "button";
  check.textContent = "Check answers";

  const reset = document.createElement("button");
  reset.type = "button";
  reset.textContent = "Reset";

  const score = document.createElement("p");
  score.className = "fcpython-quiz-score";
  score.setAttribute("aria-live", "polite");

  check.addEventListener("click", () => {
    let correctCount = 0;

    quiz.questions.forEach((question, questionIndex) => {
      const selected = container.querySelector(
        `input[name="${quiz.id}-${question.id}"]:checked`
      );
      const feedback = feedbackNodes[questionIndex];

      if (!selected) {
        feedback.textContent = "Choose an answer before checking.";
        feedback.className = "fcpython-quiz-feedback";
        return;
      }

      const selectedIndex = Number(selected.value);
      if (selectedIndex === question.answer_index) {
        correctCount += 1;
        feedback.textContent = `✅ Correct. ${question.explanation}`;
        feedback.className = "fcpython-quiz-feedback is-correct";
      } else {
        const answer = question.options[question.answer_index];
        feedback.textContent = `❌ Not yet. Correct answer: ${answer}. ${question.explanation}`;
        feedback.className = "fcpython-quiz-feedback is-incorrect";
      }
    });

    score.textContent = `Score: ${correctCount}/${quiz.questions.length}`;
  });

  reset.addEventListener("click", () => {
    container.querySelectorAll("input[type='radio']").forEach((input) => {
      input.checked = false;
    });
    feedbackNodes.forEach((feedback) => {
      feedback.textContent = "";
      feedback.className = "fcpython-quiz-feedback";
    });
    score.textContent = "";
  });

  actions.appendChild(check);
  actions.appendChild(reset);
  container.appendChild(actions);
  container.appendChild(score);
  return container;
}

Notebook and Colab practice

Open a blank notebook at https://colab.new, copy Example 1, and run three small variations. You can also use a local Jupyter notebook or VS Code. Keep one cell for the original example, one cell for your prediction, and one cell for your modified version.

Instructor note

Teaching notes

• Ask learners to predict before execution; do not skip this step.
• Invite one learner to explain the analogy and another to explain the code.
• When an error appears, model calm traceback reading instead of immediately fixing it.
• If time is short, keep Example 1 and quiz; move the challenge to homework.

Key points

TipKey points

• Nested data uses lists, dictionaries, tuples, or sets inside each other to model real records.
• Small examples are more useful than large copied programs when a concept is new.
• Prediction, execution, explanation, and one small modification form the core practice loop.
• Debugging starts by reading clues and changing one thing at a time.

References

• Python data structures tutorial: https://docs.python.org/3/tutorial/datastructures.html
• Built-in types: https://docs.python.org/3/library/stdtypes.html
• Python Tutorial: https://docs.python.org/3/tutorial/
• Quarto OJS documentation: https://quarto.org/docs/interactive/ojs/
• ipywidgets documentation: https://ipywidgets.readthedocs.io/en/stable/