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Level 1 / Project 11 - Command Dispatcher

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Concept This project — Quiz Flashcards — Browser

Estimated time: 35 minutes

Focus

  • map commands to handler functions

Why this project exists

Map text commands (upper, lower, reverse, etc.) to handler functions using a dictionary dispatcher. You will learn the function-as-value pattern, where commands are looked up in a dict and called dynamically.

Run (copy/paste)

Use <repo-root> as the folder containing this repository's README.md.

cd <repo-root>/projects/level-1/11-command-dispatcher
python project.py --input data/sample_input.txt
pytest -q

Expected terminal output

=== Command Dispatcher ===

  upper "hello world"       => HELLO WORLD
  lower "THIS SHOULD BE..." => this should be lowercase
  reverse "Python is fun"   => nuf si nohtyP

3 commands processed. Output written to data/output.json
5 passed

Expected artifacts

  • data/output.json
  • Passing tests
  • Updated notes.md

Checkpoint: Baseline code runs and all tests pass. Commit your work before continuing.

Alter it (required) — Extension

  1. Add a new command: replace old new text that replaces all occurrences of old with new in the text.
  2. Add a --list flag that prints all available commands and exits.
  3. Re-run script and tests.

Break it (required) — Core

  1. Send a command with no arguments like just upper (no text) -- does dispatch() handle it?
  2. Send an unknown command like fly to the moon -- does the error dict include the command name?
  3. Send an empty line -- does the dispatcher skip it or crash?

Fix it (required) — Core

  1. Ensure dispatch() returns an error dict when no arguments are provided.
  2. Include the attempted command name in the "Unknown command" error message.
  3. Add a test for the no-arguments case.

Checkpoint: All modifications done, tests still pass. Good time to review your changes.

Explain it (teach-back)

  1. Why does COMMANDS map command names to functions instead of using if/elif?
  2. What does dispatch() returning a dict (not raising an exception) mean for error handling?
  3. Why does list_commands() return sorted keys from the COMMANDS dict?
  4. Where would command dispatchers appear in real software (chatbots, CLI frameworks, REST API routers)?

Mastery check

You can move on when you can: - run baseline without docs, - explain one core function line-by-line, - break and recover in one session, - keep tests passing after your change.

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