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' 6 Answers
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.
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 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' 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 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) 
