在Python中重定向外部库的stdout和stderr

问题描述

我正在尝试收集Google OR工具库的搜索日志。具体来说，我使用python编写了一个使用该库来解决TSP实例的程序。我想做的是使库使用不同的方法解析相同的实例，并收集搜索的结果（库将其写入stderr）。问题在于该库是用C ++编写和编译的，而只编写了python的包装器。我尝试使用redirect_stderr和sys模块来将stderr设置为另一个文件，但是结果是相同的：输出仍然写在stderr上并显示在控制台上。因为我已经编写了多线程程序，所以shell重定向并不是真正的选择。有办法解决这个问题吗？

以下脚本取自Google OR-Tools文档，并进行了略微修改，以包括使用和重定向srderr文件的方法的规范：

"""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],[2451,1745,1524,831,1240,959,2596,403,1589,357,579],[713,355,920,803,1737,851,1858,262,940,1453,1260],[1018,700,862,1395,1123,1584,466,1056,1280,987],[1631,663,1021,1769,949,796,879,586,371],[1374,1681,1551,1765,547,225,887,999],[2408,2493,678,1724,1891,1114,701],[213,2699,1038,1605,2300,2099],[2571,1744,1645,653,600],[875,679,1272,1162],[1420,1017,1200],[2145,504],[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,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]

解决方法

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

有一个neat article about this，但可以归结为类似

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

，与stderr相同。（链接的文章显示了如何也保持原始的stdout / stderr来恢复以前的行为。）

python stderr