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to Phone Number in the Python Track

Published at Feb 16 2021 · 0 comments
Test suite

Clean up user-entered phone numbers so that they can be sent SMS messages.

The North American Numbering Plan (NANP) is a telephone numbering system used by many countries in North America like the United States, Canada or Bermuda. All NANP-countries share the same international country code: 1.

NANP numbers are ten-digit numbers consisting of a three-digit Numbering Plan Area code, commonly known as area code, followed by a seven-digit local number. The first three digits of the local number represent the exchange code, followed by the unique four-digit number which is the subscriber number.

The format is usually represented as


where N is any digit from 2 through 9 and X is any digit from 0 through 9.

Your task is to clean up differently formatted telephone numbers by removing punctuation and the country code (1) if present.

For example, the inputs

  • +1 (613)-995-0253
  • 613-995-0253
  • 1 613 995 0253
  • 613.995.0253

should all produce the output


Note: As this exercise only deals with telephone numbers used in NANP-countries, only 1 is considered a valid country code.

Exception messages

Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include a message.

To raise a message with an exception, just write it as an argument to the exception type. For example, instead of raise Exception, you should write:

raise Exception("Meaningful message indicating the source of the error")

Running the tests

To run the tests, run pytest phone_number_test.py

Alternatively, you can tell Python to run the pytest module: python -m pytest phone_number_test.py

Common pytest options

  • -v : enable verbose output
  • -x : stop running tests on first failure
  • --ff : run failures from previous test before running other test cases

For other options, see python -m pytest -h

Submitting Exercises

Note that, when trying to submit an exercise, make sure the solution is in the $EXERCISM_WORKSPACE/python/phone-number directory.

You can find your Exercism workspace by running exercism debug and looking for the line that starts with Workspace.

For more detailed information about running tests, code style and linting, please see Running the Tests.


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Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


import unittest

from phone_number import PhoneNumber

# Tests adapted from `problem-specifications//canonical-data.json`

class PhoneNumberTest(unittest.TestCase):
    def test_cleans_the_number(self):
        number = PhoneNumber("(223) 456-7890").number
        self.assertEqual(number, "2234567890")

    def test_cleans_numbers_with_dots(self):
        number = PhoneNumber("223.456.7890").number
        self.assertEqual(number, "2234567890")

    def test_cleans_numbers_with_multiple_spaces(self):
        number = PhoneNumber("223 456   7890   ").number
        self.assertEqual(number, "2234567890")

    def test_invalid_when_9_digits(self):
        with self.assertRaisesWithMessage(ValueError):

    def test_invalid_when_11_digits_does_not_start_with_a_1(self):
        with self.assertRaisesWithMessage(ValueError):

    def test_valid_when_11_digits_and_starting_with_1(self):
        number = PhoneNumber("12234567890").number
        self.assertEqual(number, "2234567890")

    def test_valid_when_11_digits_and_starting_with_1_even_with_punctuation(self):
        number = PhoneNumber("+1 (223) 456-7890").number
        self.assertEqual(number, "2234567890")

    def test_invalid_when_more_than_11_digits(self):
        with self.assertRaisesWithMessage(ValueError):

    def test_invalid_with_letters(self):
        with self.assertRaisesWithMessage(ValueError):

    def test_invalid_with_punctuations(self):
        with self.assertRaisesWithMessage(ValueError):

    def test_invalid_if_area_code_starts_with_0(self):
        with self.assertRaisesWithMessage(ValueError):
            PhoneNumber("(023) 456-7890")

    def test_invalid_if_area_code_starts_with_1(self):
        with self.assertRaisesWithMessage(ValueError):
            PhoneNumber("(123) 456-7890")

    def test_invalid_if_exchange_code_starts_with_0(self):
        with self.assertRaisesWithMessage(ValueError):
            PhoneNumber("(223) 056-7890")

    def test_invalid_if_exchange_code_starts_with_1(self):
        with self.assertRaisesWithMessage(ValueError):
            PhoneNumber("(223) 156-7890")

    def test_invalid_if_area_code_starts_with_0_on_valid_11_digit_number(self):
        with self.assertRaisesWithMessage(ValueError):
            PhoneNumber("1 (023) 456-7890")

    def test_invalid_if_area_code_starts_with_1_on_valid_11_digit_number(self):
        with self.assertRaisesWithMessage(ValueError):
            PhoneNumber("1 (123) 456-7890")

    def test_invalid_if_exchange_code_starts_with_0_on_valid_11_digit_number(self):
        with self.assertRaisesWithMessage(ValueError):
            PhoneNumber("1 (223) 056-7890")

    def test_invalid_if_exchange_code_starts_with_1_on_valid_11_digit_number(self):
        with self.assertRaisesWithMessage(ValueError):
            PhoneNumber("1 (223) 156-7890")

    # Additional tests for this track
    def test_area_code(self):
        number = PhoneNumber("2234567890")
        self.assertEqual(number.area_code, "223")

    def test_pretty_print(self):
        number = PhoneNumber("2234567890")
        self.assertEqual(number.pretty(), "(223)-456-7890")

    def test_pretty_print_with_full_us_phone_number(self):
        number = PhoneNumber("12234567890")
        self.assertEqual(number.pretty(), "(223)-456-7890")

    # Utility functions
    def assertRaisesWithMessage(self, exception):
        return self.assertRaisesRegex(exception, r".+")

if __name__ == "__main__":
import re

class PhoneNumber:
    def __init__(self, number):
            n = re.sub(r"\D", "", number)
            self.number = re.match(r"(1)?([2-9]\d{2}[2-9]\d{6})$", n).group(2)
            self.area_code = re.match(r"(1)?([2-9]\d\d)", n).group(2)
            raise ValueError("Invalid input")

    def pretty(self):
        return f"({self.area_code})-{self.number[-7:-4]}-{self.number[-4:]}"

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