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mock：mock_tmp不是上下文管理器，而是返回上下文管理器。将您的设置行替换为：

mock_tmp.return_value.__enter__.return_value.name = mytmpname

你的测试会成功的。

Answer 2

为了扩展Nathaniel的答案，此代码阻止

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

有效地做了三件事

# 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

用Mock或MagicMock 的实例替换tempfile.NamedTemporaryFile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 fixture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

这里有一个pytest和mocker fixture的替代方案，这也是一种常见的做法：

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一个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)