from enum import IntEnum
from typing import Optional

class Priority(IntEnum):
    LOW = 0
    MEDIUM = 1
    HIGH = 2

values = [
    Priority.LOW,
    Priority.MEDIUM,
    Priority.HIGH]

def format(priority: Priority) -> str:
    if priority == Priority.LOW:
        return 'Low'
    elif priority == Priority.MEDIUM:
        return 'Medium'
    elif priority == Priority.HIGH:
        return 'High'

def parse(string: str) -> Optional[Priority]:
    if string == 'Low':
        return Priority.LOW
    elif string == 'Medium':
        return Priority.MEDIUM
    elif string == 'High':
        return Priority.HIGH
    else:
        return None