Advent of code 2022 day 21

def day21(s, *, part2=False):  # Topological ordering for single-pass assignments.
  dependencies, ops, assigned = {}, {}, {}
  for line in s.splitlines():
    dst, rest = line.split(': ')
    if rest.isdigit():
      assigned[dst] = int(rest)
    else:
      field0, op, field1 = rest.split()
      dependencies[dst] = [field0, field1]
      ops[dst] = op

  if part2:
    del assigned['humn']
    ops['root'] = '='

  # Assign nodes whose children are assigned.
  import graphlib  # pylint: disable=import-error # Python 3.9.
  for dst in graphlib.TopologicalSorter(dependencies).static_order():
    if dep := dependencies.get(dst, None):
      if dep[0] in assigned and dep[1] in assigned:
        a, b, op = assigned[dep[0]], assigned[dep[1]], ops[dst]
        assigned[dst] = (a + b if op == '+' else a - b if op == '-' else
                         a * b if op == '*' else a // b)  # if op == '/'

  if not part2:
    return assigned['root']

  # Trace a path from the root down, deriving the value of each unassigned node.
  dst = 'root'
  while dst != 'humn':
    dep, op = dependencies[dst], ops[dst]
    if dep[0] in assigned:
      a = assigned[dep[0]]
      value: int
      value = (value - a if op == '+' else a - value if op == '-' else
               value // a if op == '*' else a // value if op == '/' else a)
      dst = dep[1]
    else:
      b = assigned[dep[1]]
      value = (value - b if op == '+' else value + b if op == '-' else
               value // b if op == '*' else value * b if op == '/' else b)
      dst = dep[0]

  return value