Phone Number2026-04-21

My Solution: Exercism Phone Number solution

Instructions

Clean, validate, and format North American phone numbers from a raw input string.

Rules:

• Reject letters with letters not permitted.
• Reject unsupported punctuation with punctuations not permitted.
• The cleaned number must be 10 digits, or 11 digits starting with 1.
• Area code and exchange code cannot start with 0 or 1.
• Expose the area code and a pretty string format.

Solution

class PhoneNumber:
    def __init__(self, number):
        self.number = self._clean_number(number)
        
    def _clean_number(self, raw_number):
        clean_number = ''
        for ch in raw_number:
            if ch.isalpha():
                raise ValueError("letters not permitted")
            if not ch.isdigit() and ch not in " +-.()":
                raise ValueError("punctuations not permitted")
            if ch.isdigit():
                clean_number += ch
                
        if len(clean_number) < 10:
            raise ValueError("must not be fewer than 10 digits")
        if len(clean_number) > 11:
            raise ValueError("must not be greater than 11 digits")

        if len(clean_number) == 11:
            if clean_number[0] != '1':
                raise ValueError("11 digits must start with 1")
            clean_number = clean_number[1:]

        if clean_number[0] == '0':
            raise ValueError("area code cannot start with zero")
        if clean_number[0] == '1':
            raise ValueError("area code cannot start with one")
        if clean_number[3] == '0':
            raise ValueError("exchange code cannot start with zero")
        if clean_number[3] == '1':
            raise ValueError("exchange code cannot start with one")

        return clean_number

    @property
    def area_code(self):
        return self.number[:3]

    def pretty(self):
        return f"({self.number[:3]})-{self.number[3:6]}-{self.number[6:]}"

Syntax Notes

• A leading underscore in _clean_number signals an internal/helper method by convention.
• The @property decorator lets area_code be accessed like an attribute (obj.area_code) instead of a method call.
• __init__ is the constructor that runs on object creation and stores the cleaned value in self.number.