Try to Break the Phone Number Normalization Fix

Problem/Feature Description

A developer on the customer data platform team claims to have fixed a bug in the phone number normalization function. Previously, phone numbers with country codes were being stored inconsistently — sometimes with a leading +, sometimes without, and sometimes with spaces or dashes. The fix is supposed to normalize all incoming phone numbers to E.164 format (+[country code][number], digits only after the +).

This function runs on every customer record import, which processes millions of records from various international sources. The data team wants you to try to break it — find any inputs that are not handled correctly or produce a wrong output — before it is deployed.

Output Specification

Write your findings to verification_report.md. If you find a counterexample, document the exact input and the actual vs expected output.

Input Files

The following files are provided as inputs. Extract them before beginning.

=============== FILE: src/phone_normalizer.py =============== import re

def normalize_phone(raw: str) -> str: """ Normalize a phone number to E.164 format: +[digits only] Claimed to handle: - Numbers with country code prefix (e.g. +1, +44, +49) - Numbers with spaces, dashes, dots, parentheses - US numbers without country code (assumed +1) Returns normalized E.164 string or raises ValueError if unparseable. """ if not raw or not isinstance(raw, str): raise ValueError(f"Invalid input: {raw!r}")

# Strip whitespace
cleaned = raw.strip()

# Remove all formatting characters: spaces, dashes, dots, parens
digits_and_plus = re.sub(r'[\s\-\.\(\)]', '', cleaned)

# Validate remaining characters
if not re.match(r'^\+?[0-9]+$', digits_and_plus):
    raise ValueError(f"Cannot parse phone number: {raw!r}")

# If starts with +, treat as already having country code
if digits_and_plus.startswith('+'):
    return digits_and_plus

# If 10 digits, assume US number, prepend +1
if len(digits_and_plus) == 10:
    return '+1' + digits_and_plus

# If 11 digits starting with 1, assume US with country code
if len(digits_and_plus) == 11 and digits_and_plus.startswith('1'):
    return '+' + digits_and_plus

raise ValueError(f"Cannot determine country code for: {raw!r}")

=============== FILE: tests/test_phone_normalizer.py =============== import sys sys.path.insert(0, 'src') from phone_normalizer import normalize_phone

passed = 0 failed = 0

def check(name, fn): global passed, failed try: fn() print(f"PASS: {name}") passed += 1 except AssertionError as e: print(f"FAIL: {name} — {e}") failed += 1 except Exception as e: print(f"ERROR: {name} — {e}") failed += 1

def test_us_with_plus(): assert normalize_phone("+12125551234") == "+12125551234"

def test_us_10_digit(): assert normalize_phone("2125551234") == "+12125551234"

def test_us_formatted(): assert normalize_phone("(212) 555-1234") == "+12125551234"

def test_uk_number(): assert normalize_phone("+44 20 7946 0958") == "+442079460958"

def test_rejects_letters(): try: normalize_phone("abc") assert False except ValueError: pass

check("us_with_plus", test_us_with_plus) check("us_10_digit", test_us_10_digit) check("us_formatted", test_us_formatted) check("uk_number", test_uk_number) check("rejects_letters", test_rejects_letters) print(f"\n{passed} passed, {failed} failed")