模拟Popen和Popen.communicate的python单元测试

Question

我正在尝试编写一个执行子进程调用的python，所以我想模拟这个调用。

我已经讨论过这些问题了(但没有结果)：

• Mocking a subprocess call in Python
• mocking subprocess.Popen
• mocking subprocess.Popen dependant on import style
• Mocking two functions with patch for a unit test

benchmark.py

from subprocess import Popen, PIPE, STDOUT

def some_func():

  with Popen(some_list, stdout=PIPE, stderr=STDOUT) as process:

    stdout, stderr = process.communicate(timeout=timeout)

test.py

import mock

@mock.patch('benchmark.Popen.communicate')
@mock.patch('benchmark.Popen')
def test_some_func(self, mock_popen, mock_comm):

  mock_popen.return_value = 0
  mock_comm.return_value = ('output', 'error')

  foo = benchmark.some_func()

当运行unittest时，我得到：

    stdout, stderr  = process.communicate(timeout=timeout)
ValueError: not enough values to unpack (expected 2, got 0)

看起来我没有正确地模拟communicate的返回值；我做错了什么？

解决方案

我接受了这些评论，并提出了解决以下问题的答案：

test.py

import mock

@mock.patch('benchmark.Popen')
def test_some_func(self, mock_popen):

  process = mock_popen.return_value.__enter__.return_value
  process.returncode = 0
  process.communicate.return_value = (b'some output', b'some error')

  foo = benchmark.some_func()

Answer 1

正如jonrsharpe所提到的，with Popen(...) as process使用Popen实例作为上下文管理器，它调用__enter__方法并将其值分配给process。

jonsharpe的解决方案使用了return_value魔术的mock，它工作得很好。但是，您也可以实现上下文管理器，并在其中包装模拟逻辑：

import mock
import subprocess


class MockedPopen:

    def __init__(self, args, **kwargs):
        self.args = args
        self.returncode = 0

    def __enter__(self):
        return self

    def __exit__(self, exc_type, value, traceback):
        pass

    def communicate(self, input=None, timeout=None):
        if self.args[0] == 'ls':
            stdout = '\n'.join(['hello.txt', 'world.txt'])
            stderr = ''
            self.returncode = 0
        else:
            stdout = ''
            stderr = 'unknown command'
            self.returncode = 1

        return stdout, stderr


@mock.patch('subprocess.Popen', MockedPopen)
def foo():
    with subprocess.Popen(['ls']) as proc:
        stdout, stderr = proc.communicate()
        print(stdout, stderr)


foo()

输出：

hello.txt
world.txt