Python: Mocking a context manager

Question

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

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