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Level 2 / Project 01 - Dictionary Lookup Service

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Before You Start

Recall these prerequisites before diving in: - Can you use dict.get(key, default) to safely look up a key that might not exist? - Can you write a dictionary comprehension? ({k: v for k, v in items})

Estimated time: 30 minutes

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Focus

  • nested lookup safety and defaults

Why this project exists

This project gives you level-appropriate practice in a realistic operations context. Goal: run the baseline, alter behavior, break one assumption, recover safely, and explain the fix.

Run (copy/paste)

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

cd <repo-root>/projects/level-2/01-dictionary-lookup-service
python project.py --dict data/sample_input.txt --lookup python java haskell
python project.py --dict data/sample_input.txt --stats
pytest -q

Expected terminal output

[{"found": true, "term": "python", "definition": "A high-level ...", ...}, ...]
9 passed

Expected artifacts

  • JSON lookup results on stdout
  • Passing tests
  • Updated notes.md

Worked Example

Here is a similar (but different) problem, solved step by step.

Problem: Write a function that looks up country codes (like "US" -> "United States") with fuzzy matching for typos.

Step 1: Build the lookup dictionary.

from difflib import get_close_matches

COUNTRIES = {"us": "United States", "uk": "United Kingdom", "ca": "Canada", "de": "Germany"}

def lookup_country(code):
    code_lower = code.strip().lower()
    if code_lower in COUNTRIES:
        return {"found": True, "code": code_lower, "name": COUNTRIES[code_lower]}
    # Try fuzzy matching
    suggestions = get_close_matches(code_lower, COUNTRIES.keys(), n=3, cutoff=0.6)
    return {"found": False, "code": code_lower, "suggestions": suggestions}

Step 2: Test it. lookup_country("US") returns found. lookup_country("ux") returns not found, but suggests "us" and "uk".

Step 3: Handle edge cases. What if code is empty? get_close_matches will not crash, but we should return a clear result.

if not code or not code.strip():
    return {"found": False, "code": "", "suggestions": []}

The thought process: Normalize input first (lowercase, strip). Try exact match. If that fails, try fuzzy match. Always return a structured result. This is the same pattern the dictionary lookup project uses.

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

Alter it (required) — Extension

  1. Add a --case-sensitive flag that disables lowercase normalisation.
  2. Change get_close_matches cutoff from 0.6 to 0.5 — observe more suggestions.
  3. Add a --add option that appends a new key=value line to the dictionary file.

Break it (required) — Core

  1. Create a dictionary file with duplicate keys — which value wins?
  2. Put a line with multiple = signs — does the definition split correctly?
  3. Search for an empty string "" — what happens?

Fix it (required) — Core

  1. Add a guard for empty search terms in lookup().
  2. Add a test that verifies duplicate-key behaviour is intentional.
  3. Ensure = inside definitions is preserved (split on first = only).

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

Explain it (teach-back)

  1. Why does the code normalise keys to lowercase?
  2. How does difflib.get_close_matches decide what is "close"?
  3. What is the difference between str.split("=") and str.split("=", 1)?
  4. When would you use a dictionary lookup service in a real codebase?

Mastery check

You can move on when you can: - explain how dict comprehensions differ from regular for-loop dict building, - describe what try/except KeyError does vs. dict.get(), - add a new feature (e.g. reverse lookup) without breaking existing tests, - explain fuzzy matching in your own words.

Stuck? Ask AI

If you are stuck after trying for 20 minutes, use one of these prompts:

  • "I am working on Dictionary Lookup Service. I got this error: [paste error]. Can you explain what this error means without giving me the fix?"
  • "I am trying to use difflib.get_close_matches. Can you explain how the cutoff parameter works with a simple example?"
  • "Can you explain the difference between str.split('=') and str.split('=', 1) with examples?"
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