Command-Line Programs

projects-and-environments

Run scripts and accept command-line arguments with argparse.

Level: Beginner to intermediate
Estimated time: 35–50 minutes
You will learn: Run scripts and accept command-line arguments with argparse.
Practice in: VS Code and the terminal

Open this lesson in Google Colab

Questions

What problem does Command-Line Programs 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 command-line programs 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: Command-Line Programs

A command-line program reads inputs typed after the command, validates them, and produces output that can be repeated or automated.

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 what the script prints when the user supplies --name Ada --times 2.

import argparse

parser = argparse.ArgumentParser()
parser.add_argument("--name")
parser.add_argument("--times", type=int, default=1)
args = parser.parse_args()

for _ in range(args.times):
print(f"Hello, {args.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

ArgumentParser creates a small interface for terminal inputs.
--name is read as text unless you say otherwise.
--times uses type=int, so argparse converts the text argument into an integer.
The loop repeats the greeting according to the parsed value.

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

Challenge

Remove type=int and run with --times 2. The loop will not accept a string where it needs an integer, so the type conversion belongs at the boundary of the program.

Show a safe way to approach the challenge

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

Suggested first move: Remove type=int and run with --times 2.

Debugging checkpoint 1.1

Debugging checkpoint

Do not hard-code file paths or names that should be user choices. Also write helpful --help text; command-line programs should explain themselves when a user forgets how to run them.

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

Write a greeting script with --name and --times, run it with three different arguments, and then run it with --help to inspect the user-facing instructions.

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

Run scripts and accept command-line arguments with argparse.

This lesson combines related subtopics that belong together in one learning conversation. You will still pause for a quiz after each section, but you do not need to jump between separate pages while building one clear explanation.

Guiding questions

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

How do the sections in Command-Line Programs fit together?
Which small example demonstrates each section?
Which debugging clue should I check first for each section?

Learning objectives

You will practice how to:

explain the shared concept for this lesson;
use each section as one step in a larger workflow;
complete 2 short section quizzes before moving on;
connect examples, mistakes, and debugging routines.

Lesson map

1. Command-Line Scripts — Run Python files as repeatable command-line programs.
2. argparse — Accept command-line options and arguments clearly.

1. Command-Line Scripts

Run Python files as repeatable command-line programs.

Analogy

A script is a small appliance: give it input, press run, and receive output.

What this means

A script is a Python file designed to be executed from the terminal.

Example 1

Predict what will happen before you run the code.

python scripts/hello.py
python -m my_project

Step-by-step explanation

python scripts/hello.py — pause here and say what this line reads, creates, changes, or displays.
python -m my_project — 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.

Challenge

Practice

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

Show one possible solution path

Copy Example 1 into Colab, Jupyter, or a .py file.
Mark the line you plan to change.
Write a one-sentence prediction.
Run the changed code.
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

Common mistake

If a script depends on a relative file path, running from a different folder can break it.

When you get stuck, use this debugging routine:

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

Check your understanding

This quiz checks the ideas in this section before you move on.

{
  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;
}

2. argparse

Accept command-line options and arguments clearly.

Analogy

argparse is a receptionist for your script: it asks for the right information and explains the rules.

What this means

argparse parses terminal text into named Python values and generates help messages.

Example 2

Predict what will happen before you run the code.

import argparse

parser = argparse.ArgumentParser()
parser.add_argument("name")
args = parser.parse_args()
print(f"Hello, {args.name}")

Step-by-step explanation

import argparse — pause here and say what this line reads, creates, changes, or displays.
parser = argparse.ArgumentParser() — pause here and say what this line reads, creates, changes, or displays.
parser.add_argument("name") — pause here and say what this line reads, creates, changes, or displays.
args = parser.parse_args() — pause here and say what this line reads, creates, changes, or displays.
print(f"Hello, {args.name}") — 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.

Challenge

Practice

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

Show one possible solution path

Copy Example 1 into Colab, Jupyter, or a .py file.
Mark the line you plan to change.
Write a one-sentence prediction.
Run the changed code.
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

Common mistake

Command-line arguments are strings by default. Use type=int when you need numbers.

When you get stuck, use this debugging routine:

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

Check your understanding

This quiz checks the ideas in this section before you move on.

{
  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. Use one section at a time: copy the Example 1, predict the result, run it, answer the section quiz, and then move to the next section. This is better than copying the entire page at once.

Instructor note

Teaching notes

Treat each section as a short teaching episode.
Pause for the section quiz before introducing the next section.
Ask learners to compare sections: what stayed the same, and what changed?
If time is short, teach the first two sections live and assign the rest as practice.

Key points

Command-Line Scripts: A script is a Python file designed to be executed from the terminal.
argparse: argparse parses terminal text into named Python values and generates help messages.
Use the section quizzes as gates: review before moving on if a quiz feels uncertain.

References

Python modules tutorial: https://docs.python.org/3/tutorial/modules.html
venv documentation: https://docs.python.org/3/library/venv.html
argparse documentation: https://docs.python.org/3/library/argparse.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/