Improve Your Tests With the Python Mock Object Library
May 03, 2024https://realpython.com/courses/python-mock-object-library/
What is Mocking
- Creating a fake object that represents the real object
- Used in test environments
- More control over code behavior
- Deeper insight into your code
- when functions were called
- how many times they were called
- what arguments were passed
unittest.mock
- Built in to Python 3.3+
- Provides the
Mock()class - Provides the
patch()method
Debugging
print(dir(json))dump the methods of jsonprint(json.dumps.call_args)prints argsprint(json.dumps.call_count)countsprint(json.dumps.method_calls)all methods calledcal 05 2024in terminal to show the moth 5 of year 2024
Lazy Attributes and Methods
The mock object will create attributes and methods on demand. Arguments aren't validated by default.
from unittest.mock import Mock
import json
mock = Mock()
print(mock)
data = json.dumps({'a': 1})
json = mock
data = json.dumps({'a': 1})
print(data) # <Mock name='mock.dumps()' id='4392722704'>
from unittest.mock import Mock
import json
json = Mock()
json.dumps({'a': 1, })
print(json.dumps.assert_called()) # None, was called
print(json.dumps.assert_called_once()) # None
print(json.dumps.assert_called_with({'a': 1, })) # none
print(json.dumps.assert_called_with({'a': 2, })) # exception
Return Value
from datetime import datetime
from unittest.mock import Mock
tuesday = datetime(year=2024, month=5, day=7)
saturday = datetime(year=2024, month=5, day=11)
datetime = Mock()
def is_weekday():
# type: ignore
today = datetime.today()
day_of_the_week = today.weekday()
return (0 <= day_of_the_week < 5)
datetime.today.return_value = tuesday
assert is_weekday()
datetime.today.return_value = saturday
assert is_weekday() is False
Unit Tests
python -m unittest test_example.py run test
if __name__ == '__main__':
unittest.main()
Side Effects
def get_holidays():
r = requests.get('https://localhost/api/holidays');
if r.status_code == 200:
return r.json()
return None
class TestGetHolidays(unittest.TestCase):
def test_get_holidays_connection(self):
requests.get.side_effect = ConnectionError
with self.assertRaises(ConnectionError):
get_holidays()
def test_get_holidays_timeout(self):
requests.get.side_effect = Timeout
with self.assertRaises(Timeout):
get_holidays()
def log_request(self, url):
print(f"Making request to {url}")
response_mock = Mock()
response_mock.status_code = 200
response_mock.json.return_value = {
'25/12': 'Christmas',
'01/01': 'New Years'
}
return response_mock
def test_request_with_logging(self):
requests.get.side_effect = self.log_request
assert get_holidays()['25/12'] == 'Christmas'
if __name__ == '__main__':
unittest.main()
With iterable
def test_request_with_logging(self):
requests.get.call_count = 0
response_mock = Mock()
response_mock.status_code = 200
response_mock.json.return_value = {
'25/12': 'Christmas',
'01/01': 'New Years'
}
requests.get.side_effect = [Timeout, response_mock]
with self.assertRaises(Timeout):
get_holidays()
assert get_holidays()['25/12'] == 'Christmas'
assert requests.get.call_count == 2
Configure Mocks
Define during construction
mock = Mock(return_value=True, side_effect=AttributeError)
Configure existing
mock.configure_mock(side_effect=None)
Using objects
In [4]: holidays = { '25/12': 'Christmas', '01/01': 'New Years' }
In [5]: response_mock = Mock(**{'json.return_value': holidays})
In [6]: response_mock.json()
Out[6]: {'25/12': 'Christmas', '01/01': 'New Years'}
patch() as context manager
from unittest.mock import Mock, patch
from my_calendar import is_weekday, get_holidays
import unittest
from requests.exceptions import Timeout
class TestCalendar(unittest.TestCase):
def test_get_holidays_timeout(self):
with patch('my_calendar.requests') as mocked_requests:
mocked_requests.get.side_effect = Timeout
with self.assertRaises(Timeout):
get_holidays()
if __name__ == '__main__':
unittest.main()
patch.object()
from unittest.mock import Mock, patch
from my_calendar import is_weekday, get_holidays, requests
import unittest
from requests.exceptions import Timeout
class TestCalendar(unittest.TestCase):
def test_get_holidays_timeout(self):
with patch.object(requests, 'get', side_effect=Timeout) as _:
with self.assertRaises(Timeout):
get_holidays()
if __name__ == '__main__':
unittest.main()
# or
class TestCalendar(unittest.TestCase):
@patch.object(requests, 'get', side_effect=Timeout)
def test_get_holidays_timeout(self, mocked_get):
with self.assertRaises(Timeout):
get_holidays()
Common Problems
- Changes to object interfaces
- Changes to external dependencies
Spec List
In [2]: from unittest.mock import Mock
In [3]: calendar = Mock(spec=['is_weekday', 'get_holidays'])
In [4]: calendar
Out[4]: <Mock id='4436584656'>
In [5]: calendar.is_weekday()
Out[5]: <Mock name='mock.is_weekday()' id='4394641168'>
In [6]: calendar.is_weekdayy()
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
Cell In[6], line 1
----> 1 calendar.is_weekdayy()
File ~/.asdf/installs/python/3.11.7/lib/python3.11/unittest/mock.py:653, in NonCallableMock.__getattr__(self, name)
651 elif self._mock_methods is not None:
652 if name not in self._mock_methods or name in _all_magics:
--> 653 raise AttributeError("Mock object has no attribute %r" % name)
654 elif _is_magic(name):
655 raise AttributeError(name)
AttributeError: Mock object has no attribute 'is_weekdayy'
Spec Module
In [9]: import my_calendar
In [10]: calendar = Mock(spec=my_calendar)
In [12]: calendar.is_weekday()
Out[12]: <Mock name='mock.is_weekday()' id='4440262544'>
In [13]: calendar.is_weekdayy()
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
Cell In[13], line 1
----> 1 calendar.is_weekdayy()
File ~/.asdf/installs/python/3.11.7/lib/python3.11/unittest/mock.py:653, in NonCallableMock.__getattr__(self, name)
651 elif self._mock_methods is not None:
652 if name not in self._mock_methods or name in _all_magics:
--> 653 raise AttributeError("Mock object has no attribute %r" % name)
654 elif _is_magic(name):
655 raise AttributeError(name)
AttributeError: Mock object has no attribute 'is_weekdayy'
Autospec
In [14]: from unittest.mock import create_autospec
In [15]: calendar = create_autospec(my_calendar)
In [16]: calendar
Out[16]: <NonCallableMagicMock spec='module' id='4354278288'>
In [18]: calendar.is_weekdayy()
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
Cell In[18], line 1
----> 1 calendar.is_weekdayy()
File ~/.asdf/installs/python/3.11.7/lib/python3.11/unittest/mock.py:653, in NonCallableMock.__getattr__(self, name)
651 elif self._mock_methods is not None:
652 if name not in self._mock_methods or name in _all_magics:
--> 653 raise AttributeError("Mock object has no attribute %r" % name)
654 elif _is_magic(name):
655 raise AttributeError(name)
AttributeError: Mock object has no attribute 'is_weekdayy'
In [24]: from unittest.mock import patch
In [25]: with patch('__main__.my_calendar', autospec=True) as calendar:
...: calendar.is_weekday()
...: calendar.is_weekdayy()
...:
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
Cell In[25], line 3
1 with patch('__main__.my_calendar', autospec=True) as calendar:
2 calendar.is_weekday()
----> 3 calendar.is_weekdayy()
File ~/.asdf/installs/python/3.11.7/lib/python3.11/unittest/mock.py:653, in NonCallableMock.__getattr__(self, name)
651 elif self._mock_methods is not None:
652 if name not in self._mock_methods or name in _all_magics:
--> 653 raise AttributeError("Mock object has no attribute %r" % name)
654 elif _is_magic(name):
655 raise AttributeError(name)
AttributeError: Mock object has no attribute 'is_weekdayy'