Python: Mocking a context manager

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__])

Python: Mocking a context manager

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