En Python, classieux

#! /usr/bin/python3
# Advent of Code 2022, day 2
from typing import Iterable, Optional, TypeVar
# '@' is not a valid item type, but it does no harm, and having it makes the
# value of each item type equal to its index in this string. :-)
letters = "@abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
priorities = {letter: value for (value, letter) in enumerate(letters)}
class Rucksack:
 def __init__(self, left: str, right: str):
 self.left = left
 self.right = right
 def anomaly(self) -> str:
 for letter in self.left:
 if letter in self.right:
 return letter
 raise ValueError("no anomaly in this rucksack")
 def __contains__(self, letter: str) -> bool:
 return letter in self.left or letter in self.right
def import_sack(line):
 n = len(line) - 1 # do not count final '\n'
 left = line[0:n//2]
 right = line[n//2:n]
 return Rucksack(left, right)
T = TypeVar('T')
def group(iterable: Iterable[T], n: int) -> Iterable[list[T]]:
 result = [] # type: list[T]
 for item in iterable:
 result.append(item)
 if len(result) >= n:
 yield result
 result = []
 yield result
def part1(lines: Iterable[str]) -> int:
 total = 0
 for line in lines:
 sack = import_sack(line)
 total += priorities[sack.anomaly()]
 return total
def part2(lines: Iterable[str]) -> int:
 total = 0
 for line_group in group(lines, 3):
 sacks = [import_sack(line) for line in line_group]
 for letter in letters:
 if all(letter in sack for sack in sacks):
 total += priorities[letter]
 break
 return total