The document discusses unit testing and mocking in Python, emphasizing the importance of automated tests and code coverage for ensuring software correctness. It covers various aspects of writing unit tests, integrating testing within CI pipelines, and the use of the unittest.mock library to simulate behavior of real objects. Additionally, it highlights common scenarios for utilizing mock objects to avoid side effects during testing and provides code examples demonstrating these practices.

1. How to fake properly
by
rainer schuettengruber

4. who am i
• database engineer
• currently keeps Oracle Exadata systems up and running
• uses Python to automate routine tasks

5. unit tests
• a piece of software that tests parts of your code base
• isolate a unit and validate its correctness
• form the basic pillar of Test-Driven Development
• should be written before actually implementing the intended
functionality
• commonly automated as part of your CI pipeline
• frameworks for all common languages, e.g. JUnit, NUnit, …
• Python comes with pytest

6. unit tests
test_matlib.py:
from matlib import div
def test_div():
assert div(4,2) == 2
$ pytest -v
collected 0 items / 1 errors
ImportError while importing test module
'/home/debadmin/mock_talk/unit_test/test_matlib.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
test_matlib.py:1: in <module>
from matlib import div
E ModuleNotFoundError: No module named 'matlib'
============================== 1 error in 0.36 seconds ================

7. unit testsmatlib.py:
def div(dividend, divisor):
if divisor != 0:
result = dividend / divisor
else:
print('not defined')
result = float('NaN')
return result
$ pytest -v
collected 1 item
test_div.py::test_div PASSED [100%]
=== 1 passed in 0.01 seconds =====================================

8. code coverage
• determines the percentage of code lines executed by a unit test
• should be close to 100%
• reports should be part of your CI pipeline
• provided by the pytest-cov plugin
• nicely integrates with Jenkins’ Cobertura plugin

9. code coverage
$ pytest --cov -v
collected 1 item
test_matlib.py::test_div PASSED [100%]
----------- coverage: platform linux, python 3.7.2-final-0 -----------
Name Stmts Miss Cover
------------------------------------
matlib.py 6 2 67%
test_matlib.py 4 0 100%
------------------------------------
TOTAL 10 2 80%
========== 1 passed in 0.03 seconds =====

10. code coverage
def test_div():
assert div(4,2) == 2
def div(dividend, divisor):
1 if divisor != 0:
2 result = dividend / divisor
3 else:
4 print('not defined')
5 result = float('NaN')
6 return result
this code path is not
considered, therefore code
coverage of 67% (4/6)

11. code coverage
testmatlib.py:
import math
from matlib import div
def test_div():
assert div(4,2) == 2
def test_div_by_zero():
assert math.isnan(div(4,0))
$ pytest --cov -v
collected 2 items
test_matlib.py::test_div PASSED [ 50%]
test_matlib.py::test_div_by_zero PASSED [100%]
----------- coverage: platform linux, python 3.6.6-final-0 -----------
Name Stmts Miss Cover
------------------------------------
matlib.py 6 0 100%
test_matlib.py 6 0 100%
------------------------------------
TOTAL 12 0 100%

12. branch coverage
def my_partial_fn(x):
if x:
y = 10
return y
def test_my_partial_fn():
assert my_partial_fn(1) == 10
----------- coverage: platform linux, python 3.6.6-final-0 -----------
Name Stmts Miss Cover
-----------------------------------
mylib.py 4 0 100%
test_mylib.py 3 0 100%
-----------------------------------
TOTAL 7 0 100%
code coverage 100%, however, code is bound to fail …
>>> my_partial_fn(0)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 4, in my_partial_fn
UnboundLocalError: local variable 'y' referenced before assignment

13. code coverage
there is room for
improvement

15. fake tests
def unit_under_test:
.
SendEmail.send_message(msg)
.
class SendEmail:
.
.
def send_message(self, message):
mail_client = smtplib.SMTP(‘localhost’)
mail_client.send_message(message)
mail_client.close()
.
.
• running this in a test suite would spark off unintended mails
• however, seen from a code coverage perspective it would suffice to know
whether SendEmail.send_message has been called

16. mock
You use mock to describe something which is not real or genuine, but
which is intended to be very similar to the real thing. (Collins
Dictionary)
In object-oriented programming, mock objects are simulated objects
that mimic the behaviour of real objects in controlled ways.
(Wikipedia)

17. use cases
• avoid undesired effects of API calls
• deal with non deterministic results, e.g. current time
• decrease runtime of your test suite
• deal with states that are difficult to reproduce, e.g. network error
• break dependency chain
• reduce complexity of your unit test setup/teardown

18. unittest.mock
unittest.mock is a library for testing in Python. It allows you to replace
parts of your system under test with mock objects and make
assertions about how they have been used.

19. unittest.mock.patch
def get_current_working_path():
path = os.getcwd()
return path
needs to be
replaced within
the scope of the
unit test
@mock.patch('os.getcwd')

20. unittest.mock.patch
def get_current_working_path():
path = os.getcwd()
return path
@mock.patch('os.getcwd')
def test_call_only(mocked_getcwd):
pytest takes the patch
decorator into consideration
and passes an instance of
MagicMock() to the test
function

21. unittest.mock.patch
def get_current_working_path():
path = os.getcwd()
return path
@mock.patch('os.getcwd')
def test_call_only(mocked_getcwd):
mocked_getcwd.return_value =
'/home/debadmin/mock_call'
before calling get_current_working_path()
the attribute return_value is set, which is
acutally the fake

22. unittest.mock.patch
def get_current_working_path():
path = os.getcwd()
return path
@mock.patch('os.getcwd')
def test_call_only(mocked_getcwd):
mocked_getcwd.return_value =
'/home/debadmin/mock_call'
path = get_current_working_path()
since everything is arranged
time to call the unit under test

23. unittest.mock.patch
def get_current_working_path():
path = MagicMock(
return_value =
'/home/deba..
)
return path
@mock.patch('os.getcwd')
def test_call_only(mocked_getcwd):
mocked_getcwd.return_value =
'/home/debadmin/mock_call'
path = get_current_working_path()
os.getcwd() is replaced by a
MagicMock object

24. unittest.mock.patch
def get_current_working_path():
path = MagicMock(
return_value =
'/home/deba..
)
return path
@mock.patch('os.getcwd')
def test_call_only(mocked_getcwd):
mocked_getcwd.return_value =
'/home/debadmin/mock_call'
path = get_current_working_path()
the faked path will be
returned to the caller

25. unittest.mock.patch
def get_current_working_path():
path = MagicMock(
return_value =
'/home/deba..
)
return path
@mock.patch('os.getcwd')
def test_call_only(mocked_getcwd):
mocked_getcwd.return_value =
'/home/debadmin/mock_call'
path = get_current_working_path()
assert path == '/home/debadmin/mock_call'
test condition evaluates to
true

26. unittest.mock
import os
def get_current_working_path():
path = os.getcwd()
print ('ncurrent directory is : ' + str(path))
return path
if __name__ == '__main__':
current_working_path = get_current_working_path()
debadmin@jenkins1:/tmp/mock_talk>python sample1.py
current directory is : /tmp/mock_talk
debadmin@jenkins1:/tmp/mock_talk>

27. unittest.mock.patch
>>> from unittest import mock
>>> with mock.patch('os.getcwd') as mocked_getcwd:
... dir(mocked_getcwd)
...
mocking out os.getcwd by
means of an context
manager
possible assertions about
mock object usage
['assert_any_call', 'assert_called', 'assert_called_once',
'assert_called_once_with', 'assert_called_with', 'assert_has_calls',
'assert_not_called', 'attach_mock', 'call_args', 'call_args_list',
'call_count', 'called', 'configure_mock', 'method_calls',
'mock_add_spec', 'mock_calls', 'reset_mock', 'return_value',
'side_effect']
>>>

28. unittest.mock.patch
debadmin@jenkins1:~/mock_talk$ pytest -v -s -k test_call_only
=========== test session starts =======================================
platform linux -- Python 3.7.2, pytest-4.3.0, py-1.8.0, pluggy-0.9.0 --
/opt/python/bin/python3.7
cachedir: .pytest_cache
rootdir: /home/debadmin/mock_talk, inifile:
collected 1 item
test/test_sample1.py::test_call_only
current directory is : <MagicMock name='getcwd()' id='139723199533968'>
PASSED
========== 1 passed in 0.01 seconds =====================================
os.getcwd has been
replaced by a MagicMock
object
@mock.patch('os.getcwd')
def test_call_only(mocked_getcwd):
path = get_current_working_path()
mocked_getcwd.assert_called_once()

29. unittest.mock.patch
debadmin@jenkins1:~/mock_talk$ pytest -v -s -k test_return_value
=========== test session starts =======================================
platform linux -- Python 3.7.2, pytest-4.3.0, py-1.8.0, pluggy-0.9.0 --
/opt/python/bin/python3.7
cachedir: .pytest_cache
rootdir: /home/debadmin/mock_talk, inifile:
collected 1 item
test/test_sample1.py::test_return_value
current directory is : /home/debadmin/mock_call
PASSED
========== 1 passed in 0.01 seconds =====================================
@mock.patch('os.getcwd')
def test_call_only(mocked_getcwd):
mocked_getcwd.return_value = '/home/debadmin/mock_call'
path = get_current_working_path()
assert path == '/home/debadmin/mock_call'
faked return value
print considers faked
return value

30. unittest.mock.patch
@mock.patch('subprocess.check_call')
def test_stop_database(self, mock_check_call):
fb = Flashback(
restore_point_name='TEST1',
oracle_home='/opt/oracle/product/12.1.0.2.180417/db_test',
oracle_sid='TTEST1'
)
fb._stop_database()
mock_check_call.assert_called_with(
['/opt/oracle/product/12.1.0.2.180417/db_test/bin/srvctl',
'stop',
'database',
'-d',
'TTEST_XD0104',
]
def _stop_database(self):
LOG.info('stopping database %s ...', self.db_unique_name)
subprocess.check_call(
[self.srvctl_command, 'stop', 'database', '-d', self.db_unique_name]
)
srvctl deals with stopping
a cluster database, which
is not available for the
test system
testing the srvctl command is
out of scope, however,
determining that is has been
called with appropriate
parameters is of interest

31. unittest.mock.patch
@mock.patch('rlb.common.oradb.DatabaseLib.sql_plus_task')
def test_run_flashback_restore(self, mock_sql_plus_task):
fb = Flashback(
restore_point_name='TEST1',
oracle_home='/opt/oracle/product/12.1.0.2.180417/db_test',
oracle_sid='TTEST1'
)
fb._run_flashback_restore()
mock_sql_plus_task.assert_any_call(
'nflashback database to restore point TEST1;n'
)
mock_sql_plus_task.assert_any_call(
'alter database open resetlogs;'
)
def _run_flashback_restore(self):
LOG.info('restoring database ... ')
self.dl.sql_plus_task(< restore command >)
self.dl.sql_plus_task('alter database open resetlogs;')
2 calls of sql_plus_task
with different parameters
the restore command
the open database
command

32. references
• Kent B. (1999). Extreme Programming. Addison-Wesley.
• unittest.mock – mock object library. [online] Available at:
https://docs.python.org/3/library/unittest.mock.html
• What the mock? – A cheatsheet for mocking in Pyhton. [online] Available at:
https://medium.com/@yeraydiazdiaz/what-the-mock-cheatsheet-mocking-in-
python-6a71db997832

33. questions and answers

34. speaker unit test
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