由于某些其他原因，我使用的 c++ 共享库将一些文本输出到标准输出。在 python 中，我想捕获输出并保存到变量。关于重定向标准输出有很多类似的问题，但在我的代码中不起作用。

示例:Suppressing output of module calling outside library

1 import sys
2 import cStringIO
3 save_stdout = sys.stdout
4 sys.stdout = cStringIO.StringIO()
5 func()
6 sys.stdout = save_stdout

在第 5 行，func() 将调用共享库，共享库生成的文本仍然输出到控制台!如果把 func() 改成 print "hello" 就可以了!

我的问题是:

1. 如何将 c++ 共享库的标准输出捕获到变量？
2. 为什么使用 StringIO，无法捕获共享库的输出？

最佳答案

感谢 nice answer由 Adam ，我能够得到这个工作。他的解决方案不太适合我的情况，因为我需要多次捕获文本、恢复和再次捕获文本，所以我必须进行一些相当大的更改。另外，我想让它也适用于 sys.stderr(可能适用于其他流)。

所以，这是我最终使用的解决方案(带或不带线程):

代码

import os
import sys
import threading
import time


class OutputGrabber(object):
    """
    Class used to grab standard output or another stream.
    """
    escape_char = "\b"

    def __init__(self, stream=None, threaded=False):
        self.origstream = stream
        self.threaded = threaded
        if self.origstream is None:
            self.origstream = sys.stdout
        self.origstreamfd = self.origstream.fileno()
        self.capturedtext = ""
        # Create a pipe so the stream can be captured:
        self.pipe_out, self.pipe_in = os.pipe()

    def __enter__(self):
        self.start()
        return self

    def __exit__(self, type, value, traceback):
        self.stop()

    def start(self):
        """
        Start capturing the stream data.
        """
        self.capturedtext = ""
        # Save a copy of the stream:
        self.streamfd = os.dup(self.origstreamfd)
        # Replace the original stream with our write pipe:
        os.dup2(self.pipe_in, self.origstreamfd)
        if self.threaded:
            # Start thread that will read the stream:
            self.workerThread = threading.Thread(target=self.readOutput)
            self.workerThread.start()
            # Make sure that the thread is running and os.read() has executed:
            time.sleep(0.01)

    def stop(self):
        """
        Stop capturing the stream data and save the text in `capturedtext`.
        """
        # Print the escape character to make the readOutput method stop:
        self.origstream.write(self.escape_char)
        # Flush the stream to make sure all our data goes in before
        # the escape character:
        self.origstream.flush()
        if self.threaded:
            # wait until the thread finishes so we are sure that
            # we have until the last character:
            self.workerThread.join()
        else:
            self.readOutput()
        # Close the pipe:
        os.close(self.pipe_in)
        os.close(self.pipe_out)
        # Restore the original stream:
        os.dup2(self.streamfd, self.origstreamfd)
        # Close the duplicate stream:
        os.close(self.streamfd)

    def readOutput(self):
        """
        Read the stream data (one byte at a time)
        and save the text in `capturedtext`.
        """
        while True:
            char = os.read(self.pipe_out, 1)
            if not char or self.escape_char in char:
                break
            self.capturedtext += char

用法

使用 sys.stdout，默认:

out = OutputGrabber()
out.start()
library.method(*args) # Call your code here
out.stop()
# Compare the output to the expected value:
# comparisonMethod(out.capturedtext, expectedtext)

使用 sys.stderr:

out = OutputGrabber(sys.stderr)
out.start()
library.method(*args) # Call your code here
out.stop()
# Compare the output to the expected value:
# comparisonMethod(out.capturedtext, expectedtext)

在 with block 中:

out = OutputGrabber()
with out:
    library.method(*args) # Call your code here
# Compare the output to the expected value:
# comparisonMethod(out.capturedtext, expectedtext)

在带有 Python 2.7.6 的 Windows 7 和带有 Python 2.7.6 的 Ubuntu 12.04 上测试。

要在 Python 3 中工作，请更改 char = os.read(self.pipe_out,1)
到 char = os.read(self.pipe_out,1).decode(self.origstream.encoding).

关于python - 在 python 中，如何将标准输出从 c++ 共享库捕获到变量，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/24277488/