Python：模拟上下文管理器

Question

我不明白为什么我不能在这个例子中模拟 NamedTemporaryFile.name：

from mock import Mock, patch
import unittest
import tempfile

def myfunc():
    with tempfile.NamedTemporaryFile() as mytmp:
        return mytmp.name

class TestMock(unittest.TestCase):
    @patch('tempfile.NamedTemporaryFile')
    def test_cm(self, mock_tmp):
        mytmpname = 'abcde'
        mock_tmp.__enter__.return_value.name = mytmpname
        self.assertEqual(myfunc(), mytmpname)

测试结果：

AssertionError: <MagicMock name='NamedTemporaryFile().__enter__().name' id='140275675011280'> != 'abcde'

Answer 1

您设置了错误的模拟：mock_tmp 不是上下文管理器，而是返回一个上下文管理器。将您的设置行替换为：

mock_tmp.return_value.__enter__.return_value.name = mytmpname

你的测试会成功。

Answer 2

为了扩展纳撒尼尔的答案，这个代码块

with tempfile.NamedTemporaryFile() as mytmp:
    return mytmp.name

有效地做三件事

# Firstly, it calls NamedTemporaryFile, to create a new instance of the class.
context_manager = tempfile.NamedTemporaryFile()  

# Secondly, it calls __enter__ on the context manager instance.
mytmp = context_manager.__enter__()  

# Thirdly, we are now "inside" the context and can do some work. 
return mytmp.name

当您将tempfile.NamedTemporaryFile 替换为Mock 或MagicMock 的实例 时

context_manager = mock_tmp()
# This first line, above, will call mock_tmp().
# Therefore we need to set the return_value with
# mock_tmp.return_value

mytmp = context_manager.__enter__()
# This will call mock_tmp.return_value.__enter__() so we need to set 
# mock_tmp.return_value.__enter__.return_value

return mytmp.name
# This will access mock_tmp.return_value.__enter__.return_value.name

Answer 3

使用 pytest 和 mocker 夹具扩展 Peter K 的答案。

def myfunc():
    with tempfile.NamedTemporaryFile(prefix='fileprefix') as fh:
        return fh.name


def test_myfunc(mocker):
    mocker.patch('tempfile.NamedTemporaryFile').return_value.__enter__.return_value.name = 'tempfilename'
    assert myfunc() == 'tempfilename'

Answer 4

这是pytest 和mocker fixture 的替代方案，这也是一种常见做法：

def test_myfunc(mocker):
    mock_tempfile = mocker.MagicMock(name='tempfile')
    mocker.patch(__name__ + '.tempfile', new=mock_tempfile)
    mytmpname = 'abcde'
    mock_tempfile.NamedTemporaryFile.return_value.__enter__.return_value.name = mytmpname
    assert myfunc() == mytmpname

Answer 5

我将 hmobrienv 的答案扩展到一个小型工作程序

import tempfile
import pytest


def myfunc():
    with tempfile.NamedTemporaryFile(prefix="fileprefix") as fh:
        return fh.name


def test_myfunc(mocker):
    mocker.patch("tempfile.NamedTemporaryFile").return_value.__enter__.return_value.name = "tempfilename"
    assert myfunc() == "tempfilename"


if __name__ == "__main__":
    pytest.main(args=[__file__])

Answer 6

另一种可能性是使用工厂创建实现上下文管理器接口的对象：

import unittest
import unittest.mock
import tempfile


def myfunc():
    with tempfile.NamedTemporaryFile() as mytmp:
        return mytmp.name


def mock_named_temporary_file(tmpname):
    class MockNamedTemporaryFile(object):
        def __init__(self, *args, **kwargs):
            self.name = tmpname

        def __enter__(self):
            return self

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

    return MockNamedTemporaryFile()


class TestMock(unittest.TestCase):
    @unittest.mock.patch("tempfile.NamedTemporaryFile")
    def test_cm(self, mock_tmp):
        mytmpname = "abcde"
        mock_tmp.return_value = mock_named_temporary_file(mytmpname)
        self.assertEqual(myfunc(), mytmpname)