aoc-2018-day7.py

from collections import defaultdict

tasks = defaultdict(list)
for line in open('day7.txt'):
    components = line.split(' ')
    dep, task = components[1], components[7]
    tasks[task].append(dep)
    if dep not in tasks: tasks[dep] = []

result = ''
while tasks:
    task = next(task for task in sorted(tasks) if not tasks[task])
    result += task
    del tasks[task]
    for other in tasks:
        if task in tasks[other]:
            tasks[other].remove(task)

print(result)