Mocking two functions with patch for a unit test
Question:
I have a function I want to unit test contains calls two other functions. I am unsure how can I mock both functions at the same time properly using patch. I have provided an example of what I mean below. When I run nosetests, the tests pass but I feel that there must be a cleaner way to do this and I do not really Understand the piece regarding f.close()…
The directory structure looks like this:
program/
program/
data.py
tests/
data_test.py
data.py:
import cPickle
def write_out(file_path, data):
f = open(file_path, 'wb')
cPickle.dump(data, f)
f.close()
data_test.py:
from mock import MagicMock, patch
def test_write_out():
path = '~/collection'
mock_open = MagicMock()
mock_pickle = MagicMock()
f_mock = MagicMock()
with patch('__builtin__.open', mock_open):
f = mock_open.return_value
f.method.return_value = path
with patch('cPickle.dump', mock_pickle):
write_out(path, 'data')
mock_open.assert_called_once_with('~/collection', 'wb')
f.close.assert_any_call()
mock_pickle.assert_called_once_with('data', f)
Results:
$ nosetests
.
----------------------------------------------------------------------
Ran 1 test in 0.008s
OK
Answers:
Here’s a simple example on how to test raising ConflictError
in create_collection
function using mock:
import os
from unittest import TestCase
from mock import patch
from ..program.data import ConflictError, create_collection
class TestCreateCollection(TestCase):
def test_path_exists(self):
with patch.object(os.path, 'exists') as mock_method:
mock_method.return_value = True
self.assertRaises(ConflictError, create_collection, 'test')
Please, also see mock docs and Michael Foord’s awesome introduction to mock.
You can simplify your test by using the patch decorator and nesting them like so (they are MagicMock
objects by default):
from unittest.mock import patch
@patch('cPickle.dump')
@patch('__builtin__.open')
def test_write_out(mock_open, mock_pickle):
path = '~/collection'
f = mock_open.return_value
f.method.return_value = path
write_out(path, 'data')
mock_open.assert_called_once_with('~/collection', 'wb')
mock_pickle.assert_called_once_with('data', f)
f.close.assert_any_call()
Calls to a MagicMock
instance return a new MagicMock
instance, so you can check that the returned value was called just like any other mocked object. In this case f
is a MagicMock
named 'open()'
(try printing f
).
In addition to the response @Matti John you can also use patch
inside function test_write_out
:
from mock import MagicMock, patch
def test_write_out():
path = '~/collection'
with patch('__builtin__.open') as mock_open,
patch('cPickle.dump') as mock_pickle:
f = mock_open.return_value
...
As of Python 3.10 you can do use Parenthesized Context Managers like this
from unittest.mock import patch
def test_write_out():
with (
patch('cPickle.dump'),
patch('__builtin__.open') as open_mock, # example of using `as`
):
yield
I have a function I want to unit test contains calls two other functions. I am unsure how can I mock both functions at the same time properly using patch. I have provided an example of what I mean below. When I run nosetests, the tests pass but I feel that there must be a cleaner way to do this and I do not really Understand the piece regarding f.close()…
The directory structure looks like this:
program/
program/
data.py
tests/
data_test.py
data.py:
import cPickle
def write_out(file_path, data):
f = open(file_path, 'wb')
cPickle.dump(data, f)
f.close()
data_test.py:
from mock import MagicMock, patch
def test_write_out():
path = '~/collection'
mock_open = MagicMock()
mock_pickle = MagicMock()
f_mock = MagicMock()
with patch('__builtin__.open', mock_open):
f = mock_open.return_value
f.method.return_value = path
with patch('cPickle.dump', mock_pickle):
write_out(path, 'data')
mock_open.assert_called_once_with('~/collection', 'wb')
f.close.assert_any_call()
mock_pickle.assert_called_once_with('data', f)
Results:
$ nosetests
.
----------------------------------------------------------------------
Ran 1 test in 0.008s
OK
Here’s a simple example on how to test raising ConflictError
in create_collection
function using mock:
import os
from unittest import TestCase
from mock import patch
from ..program.data import ConflictError, create_collection
class TestCreateCollection(TestCase):
def test_path_exists(self):
with patch.object(os.path, 'exists') as mock_method:
mock_method.return_value = True
self.assertRaises(ConflictError, create_collection, 'test')
Please, also see mock docs and Michael Foord’s awesome introduction to mock.
You can simplify your test by using the patch decorator and nesting them like so (they are MagicMock
objects by default):
from unittest.mock import patch
@patch('cPickle.dump')
@patch('__builtin__.open')
def test_write_out(mock_open, mock_pickle):
path = '~/collection'
f = mock_open.return_value
f.method.return_value = path
write_out(path, 'data')
mock_open.assert_called_once_with('~/collection', 'wb')
mock_pickle.assert_called_once_with('data', f)
f.close.assert_any_call()
Calls to a MagicMock
instance return a new MagicMock
instance, so you can check that the returned value was called just like any other mocked object. In this case f
is a MagicMock
named 'open()'
(try printing f
).
In addition to the response @Matti John you can also use patch
inside function test_write_out
:
from mock import MagicMock, patch
def test_write_out():
path = '~/collection'
with patch('__builtin__.open') as mock_open,
patch('cPickle.dump') as mock_pickle:
f = mock_open.return_value
...
As of Python 3.10 you can do use Parenthesized Context Managers like this
from unittest.mock import patch
def test_write_out():
with (
patch('cPickle.dump'),
patch('__builtin__.open') as open_mock, # example of using `as`
):
yield