Question :
Python: Mocking a context manager
I don’t understand why I can’t mock NamedTemporaryFile.name in this example:
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)
Test results in:
AssertionError: <MagicMock name='NamedTemporaryFile().__enter__().name' id='140275675011280'> != 'abcde'
Answer #1:
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.
Answer #2:
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'
Answer #3:
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
Answer #4:
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
# This first line, below, will call mock_tmp().
# Therefore we need to set the return_value with
# mock_tmp.return_value
context_manager = mock_tmp()
# This will call mock_tmp.return_value.__enter__() so we need to set
# mock_tmp.return_value.__enter__.return_value
mytmp = context_manager.__enter__()
# This will access mock_tmp.return_value.__enter__.return_value.name
return mytmp.name
Answer #5:
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__])
