Python 子进程 Popen.communicate() 等价于 Popen.stdout.read()?

2022-01-18 00:00:00 python subprocess popen wait communicate

问题描述

非常具体的问题(我希望):以下三个代码有什么区别?

Very specific question (I hope): What are the differences between the following three codes?

(我希望它只是第一个不等待子进程完成，而第二个和第三个会这样做.但我需要确定这是 only 的区别...)

(I expect it to be only that the first does not wait for the child process to be finished, while the second and third ones do. But I need to be sure this is the only difference...)

我也欢迎其他评论/建议(尽管我已经很清楚 shell=True 的危险和跨平台限制)

I also welcome other remarks/suggestions (though I'm already well aware of the shell=True dangers and cross-platform limitations)

请注意，我已经阅读了 Python 子进程交互，为什么我的进程可以使用 Popen.communicate，而不是 Popen.stdout.read()? 而且我不想/不需要之后与程序交互.

Note that I already read Python subprocess interaction, why does my process work with Popen.communicate, but not Popen.stdout.read()? and that I do not want/need to interact with the program after.

另外请注意，我已经阅读了 Alternatives to Python Popen.communicate() memory限制?但我并没有真正明白......

Also note that I already read Alternatives to Python Popen.communicate() memory limitations? but that I didn't really get it...

最后，请注意，我知道当一个缓冲区使用一种方法填充一个输出时，某处存在死锁的风险，但我在互联网上寻找明确的解释时迷路了......

Finally, note that I am aware that somewhere there is a risk of deadlock when one buffer is filled with one output using one method, but I got lost while looking for clear explanations on the Internet...

第一个代码:

from subprocess import Popen, PIPE

def exe_f(command='ls -l', shell=True):
    """Function to execute a command and return stuff"""

    process = Popen(command, shell=shell, stdout=PIPE, stderr=PIPE)

    stdout = process.stdout.read()
    stderr = process.stderr.read()

    return process, stderr, stdout

第二个代码:

from subprocess import Popen, PIPE
from subprocess import communicate

def exe_f(command='ls -l', shell=True):
    """Function to execute a command and return stuff"""

    process = Popen(command, shell=shell, stdout=PIPE, stderr=PIPE)

    (stdout, stderr) = process.communicate()

    return process, stderr, stdout

第三个代码:

from subprocess import Popen, PIPE
from subprocess import wait

def exe_f(command='ls -l', shell=True):
    """Function to execute a command and return stuff"""

    process = Popen(command, shell=shell, stdout=PIPE, stderr=PIPE)

    code   = process.wait()
    stdout = process.stdout.read()
    stderr = process.stderr.read()

    return process, stderr, stdout

谢谢.

解决方案

如果您查看 subprocess.communicate() 的源代码，它显示了一个完美的差异示例:

If you look at the source for subprocess.communicate(), it shows a perfect example of the difference:

def communicate(self, input=None):
    ...
    # Optimization: If we are only using one pipe, or no pipe at
    # all, using select() or threads is unnecessary.
    if [self.stdin, self.stdout, self.stderr].count(None) >= 2:
        stdout = None
        stderr = None
        if self.stdin:
            if input:
                self.stdin.write(input)
            self.stdin.close()
        elif self.stdout:
            stdout = self.stdout.read()
            self.stdout.close()
        elif self.stderr:
            stderr = self.stderr.read()
            self.stderr.close()
        self.wait()
        return (stdout, stderr)

    return self._communicate(input)

您可以看到 communicate 确实使用了对 stdout 和 stderr 的读取调用，并且还调用了 wait().这只是操作顺序的问题.在您的情况下，因为您对 stdout 和 stderr 都使用 PIPE，所以它进入 _communicate():

You can see that communicate does make use of the read calls to stdout and stderr, and also calls wait(). It is just a matter of order of operations. In your case because you are using PIPE for both stdout and stderr, it goes into _communicate():

def _communicate(self, input):
    stdout = None # Return
    stderr = None # Return

    if self.stdout:
        stdout = []
        stdout_thread = threading.Thread(target=self._readerthread,
                                         args=(self.stdout, stdout))
        stdout_thread.setDaemon(True)
        stdout_thread.start()
    if self.stderr:
        stderr = []
        stderr_thread = threading.Thread(target=self._readerthread,
                                         args=(self.stderr, stderr))
        stderr_thread.setDaemon(True)
        stderr_thread.start()

    if self.stdin:
        if input is not None:
            self.stdin.write(input)
        self.stdin.close()

    if self.stdout:
        stdout_thread.join()
    if self.stderr:
        stderr_thread.join()

    # All data exchanged.  Translate lists into strings.
    if stdout is not None:
        stdout = stdout[0]
    if stderr is not None:
        stderr = stderr[0]

    # Translate newlines, if requested.  We cannot let the file
    # object do the translation: It is based on stdio, which is
    # impossible to combine with select (unless forcing no
    # buffering).
    if self.universal_newlines and hasattr(file, 'newlines'):
        if stdout:
            stdout = self._translate_newlines(stdout)
        if stderr:
            stderr = self._translate_newlines(stderr)

    self.wait()
    return (stdout, stderr)

这使用线程一次从多个流中读取.然后它在最后调用 wait().

This uses threads to read from multiple streams at once. Then it calls wait() at the end.

总结一下:

  1. 此示例一次从一个流中读取，并且不等待它完成该过程.
  2. 此示例通过内部线程同时从两个流中读取，并等待它完成该过程.
  3. 此示例等待进程完成，然后一次读取一个流.正如您所提到的，如果流中写入的内容过多，则可能会出现死锁.

另外，您在第二个和第三个示例中不需要这两个 import 语句:

Also, you don't need these two import statements in your 2nd and 3rd examples:

from subprocess import communicate
from subprocess import wait

它们都是 Popen 对象的方法.

They are both methods of the Popen object.

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