在Python中重定向外部库的stdout和stderr

Question

我正在尝试收集Google OR-tools库的搜索日志。具体来说，我正在使用 python 编写一个程序，该程序使用该库来解决 TSP 实例。我想做的是让库用不同的方法解决相同的实例并收集搜索的输出（库将其写入 stderr）。问题是该库是用 C++ 编写和编译的，并且只编写了 python 的包装器。我尝试使用

redirect_stderr

以及

sys

模块将

stderr

设置为另一个文件，但结果是相同的：输出仍然写入 stderr 并显示在控制台上。因为我编写了一个多线程程序，所以 shell 重定向实际上并不是一个选项。有办法解决这个问题吗？

下面的脚本取自 Google OR-Tools 文档，并稍加修改以包括使用方法的规范和我尝试重定向 srderr 文件：

"""Simple travelling salesman problem between cities."""

from __future__ import print_function
from ortools.constraint_solver import routing_enums_pb2
from ortools.constraint_solver import pywrapcp
from contextlib import redirect_stderr, redirect_stdout
import sys


def create_data_model():
    """Stores the data for the problem."""
    data = {}
    data['distance_matrix'] = [
        [0, 2451, 713, 1018, 1631, 1374, 2408, 213, 2571, 875, 1420, 2145, 1972],
        [2451, 0, 1745, 1524, 831, 1240, 959, 2596, 403, 1589, 1374, 357, 579],
        [713, 1745, 0, 355, 920, 803, 1737, 851, 1858, 262, 940, 1453, 1260],
        [1018, 1524, 355, 0, 700, 862, 1395, 1123, 1584, 466, 1056, 1280, 987],
        [1631, 831, 920, 700, 0, 663, 1021, 1769, 949, 796, 879, 586, 371],
        [1374, 1240, 803, 862, 663, 0, 1681, 1551, 1765, 547, 225, 887, 999],
        [2408, 959, 1737, 1395, 1021, 1681, 0, 2493, 678, 1724, 1891, 1114, 701],
        [213, 2596, 851, 1123, 1769, 1551, 2493, 0, 2699, 1038, 1605, 2300, 2099],
        [2571, 403, 1858, 1584, 949, 1765, 678, 2699, 0, 1744, 1645, 653, 600],
        [875, 1589, 262, 466, 796, 547, 1724, 1038, 1744, 0, 679, 1272, 1162],
        [1420, 1374, 940, 1056, 879, 225, 1891, 1605, 1645, 679, 0, 1017, 1200],
        [2145, 357, 1453, 1280, 586, 887, 1114, 2300, 653, 1272, 1017, 0, 504],
        [1972, 579, 1260, 987, 371, 999, 701, 2099, 600, 1162, 1200, 504, 0],
    ]  # yapf: disable
    data['num_vehicles'] = 1
    data['depot'] = 0
    return data


def print_solution(manager, routing, solution):
    """Prints solution on console."""
    print('Objective: {} miles'.format(solution.ObjectiveValue()))
    index = routing.Start(0)
    plan_output = 'Route for vehicle 0:\n'
    route_distance = 0
    while not routing.IsEnd(index):
        plan_output += ' {} ->'.format(manager.IndexToNode(index))
        previous_index = index
        index = solution.Value(routing.NextVar(index))
        route_distance += routing.GetArcCostForVehicle(previous_index, index, 0)
    plan_output += ' {}\n'.format(manager.IndexToNode(index))
    plan_output += 'Route distance: {}miles\n'.format(route_distance)
    print(plan_output)


def main():
    """Entry point of the program."""
    # Instantiate the data problem.
    data = create_data_model()

    # Create the routing index manager.
    manager = pywrapcp.RoutingIndexManager(len(data['distance_matrix']),
                                           data['num_vehicles'], data['depot'])

    # Create Routing Model.
    routing = pywrapcp.RoutingModel(manager)


    def distance_callback(from_index, to_index):
        """Returns the distance between the two nodes."""
        # Convert from routing variable Index to distance matrix NodeIndex.
        from_node = manager.IndexToNode(from_index)
        to_node = manager.IndexToNode(to_index)
        return data['distance_matrix'][from_node][to_node]

    transit_callback_index = routing.RegisterTransitCallback(distance_callback)

    # Define cost of each arc.
    routing.SetArcCostEvaluatorOfAllVehicles(transit_callback_index)

    # Setting first solution heuristic.
    search_parameters = pywrapcp.DefaultRoutingSearchParameters()
    search_parameters.first_solution_strategy = (
        routing_enums_pb2.FirstSolutionStrategy.PATH_CHEAPEST_ARC)
    search_parameters.local_search_metaheuristic = (routing_enums_pb2.LocalSearchMetaheuristic.TABU_SEARCH)
    search_parameters.time_limit.seconds = 1
    search_parameters.log_search = True

    # This is the first approach and it does not work
    # (Yes, I'm sure that it's routing.SolveWithParameters() that logs the search
    # parameters to stderr.
    with open("output.txt", "wt") as f:
        with redirect_stderr(f):
            solution = routing.SolveWithParameters(search_parameters)

    # Print solution on console.
    if solution:
        print_solution(manager, routing, solution)


if __name__ == '__main__':
    main()

第二种方法是：

#[code omitted: see above]
    search_parameters.time_limit.seconds = 1
    search_parameters.log_search = True

    # This is the second approach and it does not work
    # (Yes, I'm sure that it's routing.SolveWithParameters() that logs the search
    # parameters to stderr.
    sys.stderr = open("output.txt", "wt")
    solution = routing.SolveWithParameters(search_parameters)
#[code omitted: see above]

Answer 1

您可能需要重定向 stdout/stderr 文件描述符，而不是像 contextmanager 位那样重定向流对象。

有一篇关于此的文章，但归结为类似

redirect_file = tempfile.TemporaryFile(mode='w+b')
stdout_fd = sys.stdout.fileno()
sys.stdout.close()
os.dup2(redirect_file.fileno(), stdout_fd)

和

stderr

相同。 （链接的文章展示了如何保留原来的

stdout

/

stderr

来恢复以前的行为。）