AoC 2021 day 12: path finding

day12.py

#!/usr/bin/env python3
#
# https://adventofcode.com/2021/day/12

import sys, os,math
from collections import defaultdict

def walk(routes, path, double=None):
    for dst in routes[path[-1]]:
        if dst == 'end':
            yield path + [dst]
        elif dst.isupper() or (dst not in path):
            yield from walk(routes, path + [dst], double)
        elif dst == 'start':
            continue
        elif double is None and path.count(dst) == 1:
            yield from walk(routes, path + [dst], dst)

def build_routes(lines):
    routes = defaultdict(set)
    for line in lines:
        src,dst = line.rstrip().split('-')
        routes[src].add(dst)
        if src != 'start': #start is one-way
            routes[dst].add(src) # all others are two-way
    return routes

def part1(routes):
    return len(list(walk(routes,['start'],'start')))

def part2(lines):
    return len(list(walk(routes,['start'])))

if __name__ == '__main__':
    if len(sys.argv)<2:
        print("Usage",sys.argv[0],"filename")
        sys.exit(1)
    with open(sys.argv[1]) as f:
        lines = f.readlines()

    routes = build_routes(lines)
    print("part1:", part1(routes))
    print("part1:", part2(routes))

optimized a bit -- now using the part 2 function for both parts. for part 1, to stop it from going down any small caves twice, you just have to tell it that it's already seen "start" twice. I wonder if I could have named any variables or functions any better.