Python 在伪子进程中模拟标准输出.打开

我想测试一个函数，它调用subprocess.Popen并捕获stdout.特别是，我需要测试stdout个物理捕获在磁盘上的文件中的内容，而无需调用实际的进程.

示例函数:

import subprocess

def func():
    with open('stdout.txt', 'wt') as out:
        subprocess.Popen(_cmdline(), stdout=out)

def _cmdline() -> list[str]:
    # Or whatever process we want to run
    return ['ls']

测试功能:

from unittest import mock

def test_captures_stdout():
    with mock.patch('subprocess.Popen') as mock_popen:
        func()

    # Try to intercept stdout and write to it
    [(_, kwargs)] = mock_popen.call_args_list
    with kwargs['stdout']:
        buf.write('some standard output')

    with open('stdout.txt') as buf:
        assert buf.read() == 'some standard output'

在这里，我模拟subprocess.Popen，然后拦截传递给它的构造函数的stdout，并try 写入缓冲区.然后，我打算对stdout.txt文件的内容运行断言.

显然，当我try 写入stdout缓冲区时，它已经关闭，并且我收到IO错误.

================================== FAILURES ===================================
____________________________ test_captures_stdout _____________________________

    def test_captures_stdout():
        with mock.patch('subprocess.Popen') as mock_popen:
            func()

        # Try to intercept stdout and write to it
        [(_, kwargs)] = mock_popen.call_args_list
>       with kwargs['stdout']:
E       ValueError: I/O operation on closed file.

test_subprocess.py:22: ValueError
=========================== short test summary info ===========================

我想知道是否有一种方便的方法来模拟Popen并以某种方式模拟写入到文件中的stdout.