Python: 嘲笑上下文管理器

You are setting the wrong mock: mock_tmp is not the context manager, but instead returns a context manager. Replace your setup line with:

mock_tmp.return_value.__enter__.return_value.name = mytmpname

and your test will work.

Here is an alternative with pytest and mocker fixture, which is a common practice as well:

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

Extending Peter K's answer using pytest and the mocker fixture.

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'

To expand on Nathaniel's answer, this code block

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

effectively does three things

# 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

When you replace tempfile.NamedTemporaryFile with an instance of Mock or MagicMock

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

I extended hmobrienv's answer to a small working program

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__])

Another possibility is to use a factory to create an object that implements the context manager interface:

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)