Python：仅将共享库 (C++) 标准输出重定向到文件

Question

I am using a Python library (written in C++) that outputs a lot of messages in the console. Unfortunately the library is closed source, so there is no way to change its behavior. My goal is to write a Python context handler that redirects the library C++ output (both stdout ans stderr) into two files, while leaving the Python output unchanged (shown in the console). This is what I managed to accomplish so far:

import os
import sys
from pathlib import Path

log_dir = Path('./')

class logSaver():
    def __init__(self, logname):
        self.logname = logname
        sys.stdout.flush()
        sys.stderr.flush()

        if self.logname == None:
            self.logpath_out = os.devnull
            self.logpath_err = os.devnull
        else:
            self.logpath_out = log_dir / (logname + "_out.log")
            self.logpath_err = log_dir / (logname + "_err.log")

        self.logfile_out = os.open(self.logpath_out, os.O_WRONLY|os.O_TRUNC|os.O_CREAT)
        self.logfile_err = os.open(self.logpath_err, os.O_WRONLY|os.O_TRUNC|os.O_CREAT)
    
    def __enter__(self):
        self.orig_stdout = sys.stdout # save original stdout
        self.orig_stderr = sys.stderr # save original stderr

        self.new_stdout = os.dup(1)
        self.new_stderr = os.dup(2)

        os.dup2(self.logfile_out, 1)
        os.dup2(self.logfile_err, 2)

        sys.stdout = os.fdopen(self.new_stdout, 'w')
        sys.stderr = os.fdopen(self.new_stderr, 'w')
        
    def __exit__(self, exc_type, exc_val, exc_tb):
        sys.stdout.flush()
        sys.stderr.flush()

        sys.stdout = self.orig_stdout # restore original stdout
        sys.stderr = self.orig_stderr # restore original stderr
        
        os.close(self.logfile_out)
        os.close(self.logfile_err)

The context handler is then used as follows:

with logSaver("log_filename"):
    some_cpp_and_python_code()

The result is the creation of two files:

• log_filename_out.log
• log_filename_err.log

that contain all the output produced by the C++ library, while the output produced by Python is shown regularly into the console, and is not written into the log files.

The code works fine, and redirects only the C++ stdout while leaving unchanged the Python print() calls, however, for some reason, it works only one time: all subsequent uses of the context handler result in the broken behaviour of the Python print() calls, as they are not shown anywhere (console or log files). I suspect the issue might lie in the incorrect restoration of the original stdout and stderr.

How can the code be changed to fix this behaviour?

Thank you very much in advance

Answer 1

Thanks to @SergeyA comment, I managed to fix the problem. Here is the final working code for whoever might be interested:

class logSaver():
    def __init__(self, logname):
        self.logname = logname
        sys.stdout.flush()
        sys.stderr.flush()
        if self.logname == None:
            self.logpath_out = os.devnull
            self.logpath_err = os.devnull
        else:
            self.logpath_out = logname + "_out.log"
            self.logpath_err = logname + "_err.log"
        self.logfile_out = os.open(self.logpath_out, os.O_WRONLY|os.O_TRUNC|os.O_CREAT)
        self.logfile_err = os.open(self.logpath_err, os.O_WRONLY|os.O_TRUNC|os.O_CREAT)
    
    def __enter__(self):
        self.orig_stdout = os.dup(1)
        self.orig_stderr = os.dup(2)
        self.new_stdout = os.dup(1)
        self.new_stderr = os.dup(2)
        os.dup2(self.logfile_out, 1)
        os.dup2(self.logfile_err, 2)
        sys.stdout = os.fdopen(self.new_stdout, 'w')
        sys.stderr = os.fdopen(self.new_stderr, 'w')
        
    def __exit__(self, exc_type, exc_val, exc_tb):
        sys.stdout.flush()
        sys.stderr.flush()

        os.dup2(self.orig_stdout, 1)
        os.dup2(self.orig_stderr, 2)
        os.close(self.orig_stdout)
        os.close(self.orig_stderr)
        
        os.close(self.logfile_out)
        os.close(self.logfile_err)

The problem was that I was restoring the original streams with

sys.stdout = self.orig_stdout

while the correct way was to do:

os.dup2(self.orig_stdout, 1)