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Level 1 / Project 01 - Input Validator Lab

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Before You Start

Recall these prerequisites before diving in: - Can you open a file and read its contents into a list of lines? - Can you use if/elif/else to check multiple conditions? - Can you check if a character is in a string? ("@" in "user@example.com")

Estimated time: 20 minutes

Learn Your Way

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Concept This project Walkthrough Quiz Flashcards — Browser

Focus

  • validate required fields and safe defaults

Why this project exists

Validate common input formats -- emails, phone numbers, and zip codes -- using string methods and basic regex. You will learn how to dispatch different validators based on input type and return structured pass/fail results.

Run (copy/paste)

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

cd <repo-root>/projects/level-1/01-input-validator-lab
python project.py --input data/sample_input.txt
pytest -q

Expected terminal output

=== Validation Results ===

  PASS  [email] user@example.com
  FAIL  [email] bad-email-no-at -- must contain exactly one @
  FAIL  [email] missing@domain -- domain must contain a dot

  1/3 passed validation
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 validates a date string in YYYY-MM-DD format.

Step 1: Think about what makes a date string valid. It has three parts separated by dashes. The year is 4 digits, the month is 1-12, the day is 1-31.

def validate_date(text):
    parts = text.strip().split("-")
    if len(parts) != 3:
        return {"valid": False, "reason": "must have three parts separated by -"}
    year, month, day = parts
    if not (year.isdigit() and month.isdigit() and day.isdigit()):
        return {"valid": False, "reason": "year, month, day must be numbers"}
    if not (1 <= int(month) <= 12):
        return {"valid": False, "reason": f"month {month} is out of range 1-12"}
    if not (1 <= int(day) <= 31):
        return {"valid": False, "reason": f"day {day} is out of range 1-31"}
    return {"valid": True, "reason": ""}

Step 2: Test it. validate_date("2024-01-15") returns valid. validate_date("2024-13-01") returns invalid (month 13). validate_date("not-a-date") returns invalid (not numbers).

The thought process: Validate one thing at a time: format first, then types, then ranges. Return structured results (dict with valid + reason) so the caller can display useful messages. This is the same pattern the input validator project uses.

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

Alter it (required) — Extension

  1. Add a new validation type: "url" that checks for http:// or https:// prefix and a dot in the domain.
  2. Add a --strict flag that rejects emails without a TLD of at least 2 characters.
  3. Re-run script and tests.

Break it (required) — Core

  1. Add a line with an unknown type like ssn: 123-45-6789 -- does validate_input() handle it or crash?
  2. Add a line with no colon separator like just some text -- does parsing fail gracefully?
  3. Add an email like user@ -- does validate_email() accept it when it should not?

Fix it (required) — Core

  1. Ensure validate_input() returns an "unknown type" result for unrecognised types instead of crashing.
  2. Handle lines without the type: value format by skipping them with a warning.
  3. Add a test for the unknown-type case.

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

Explain it (teach-back)

  1. Why does validate_email() use basic string checks ("@" in value) rather than a full regex?
  2. What does re.fullmatch() do differently from re.search()?
  3. Why return a dict with {"valid": True/False, "reason": "..."} instead of just True/False?
  4. Where would input validation appear in real software (form handlers, API endpoints, data import)?

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 Input Validator Lab. 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 exactly one @ symbol. I have tried count() but my logic is not quite right. Can you give me a hint?"
  • "Can you explain how to return structured results from a validation function, using a different example like checking phone numbers?"
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