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Level 1 / Project 02 - Password Strength Checker

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Before You Start

Recall these prerequisites before diving in: - Can you use any() with a generator expression? (any(c.isupper() for c in text)) - Can you read lines from a file into a list?

Estimated time: 20 minutes

Learn Your Way

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Concept This project — Quiz Flashcards — Browser

Focus

  • rule-based scoring and condition checks

Why this project exists

Score passwords on length, character variety, and common-password checks. You will learn rule-based scoring, the any() built-in with generator expressions, and how to build a multi-criteria evaluation system.

Run (copy/paste)

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

cd <repo-root>/projects/level-1/02-password-strength-checker
python project.py --input data/sample_input.txt
pytest -q

Expected terminal output

=== Password Strength Report ===

  "password"              => Score: 1/5 (weak)
  "abc123"                => Score: 2/5 (weak)
  "MyStr0ng!Pass#2024"    => Score: 5/5 (strong)

3 passwords checked. Output written to data/output.json
5 passed

Expected artifacts

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

Worked Example

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

Problem: Write a function that scores a username's strength based on rules: length, no spaces, no special characters at the start.

Step 1: Define the rules and assign points.

def score_username(username):
    score = 0
    reasons = []

    if len(username) >= 3:
        score += 1
    else:
        reasons.append("too short (need 3+ characters)")

    if " " not in username:
        score += 1
    else:
        reasons.append("contains spaces")

    if username[0].isalpha():
        score += 1
    else:
        reasons.append("must start with a letter")

    return {"score": score, "max": 3, "issues": reasons}

Step 2: Test it. score_username("alice") gives 3/3. score_username("a") gives 2/3 (too short). score_username("1bob") gives 2/3 (starts with number).

Step 3: Handle edge cases. What if username is empty? username[0] would crash with IndexError. Add a guard.

if not username:
    return {"score": 0, "max": 3, "issues": ["username is empty"]}

The thought process: Check one rule at a time, accumulate a score, collect reasons for failures. This is the same pattern the password strength checker uses with its multi-criteria scoring.

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

Alter it (required) — Extension

  1. Add a "sequential characters" penalty (e.g. "abc", "123" lose a point).
  2. Add a --min-score flag that only shows passwords scoring at or above the threshold.
  3. Re-run script and tests.

Break it (required) — Core

  1. Test an empty password (blank line) -- does score_password() crash or return 0?
  2. Test a password that is the string "password" -- does the common-password check catch it?
  3. Test a 1000-character password -- does any check break with very long input?

Fix it (required) — Core

  1. Handle empty passwords by returning a score of 0 with label "empty".
  2. Ensure the common-password list comparison is case-insensitive.
  3. Add a test for the empty-password edge case.

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

Explain it (teach-back)

  1. Why does check_character_variety() check for uppercase, lowercase, digits, and special characters separately?
  2. What does any(c.isupper() for c in password) do and why use any() instead of a loop?
  3. Why is "password" in a common-passwords list instead of checking for specific patterns?
  4. Where would password strength checking appear in real software (registration forms, password managers)?

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.

Stuck? Ask AI

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

  • "I am working on Password Strength Checker. I got this error: [paste error]. Can you explain what this error means without giving me the fix?"
  • "I am trying to check if a string contains at least one uppercase letter. I have tried a for loop but it feels clunky. Can you give me a hint about a more Pythonic way?"
  • "Can you explain how any() works with generator expressions, using an example about checking a list of numbers?"
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