Edwin Jung - Mocking and Patching Pitfalls

Mocking and patching are powerful techniques for testing, but they can be easily abused, with negative effects on code quality, maintenance, and application architecture. These pain-points can be hard to verbalize, and consequently hard to address. If your unit tests are a PITA, but you cannot explain why, this talk may be for you.

Mocking as a technique has deep roots within OOD and TDD, going back 20+ years, but many Python developers know mocks and patches merely as a technique to isolate code under test. In the absence of knowledge around OOD and TDD, best practices around mocking are completely unknown, misunderstood, or ignored. Developers who use mocks and patches without doing TDD or OOD are susceptible to falling into many well-understood and documented traps.

This talk will draw a historical connection between the way mocks are taught today, and their origins in TDD, OOD, and Java. It will also demonstrate some pitfalls, and provide some guidance and alternatives to mocking and patching (e.g., dependency injection, test doubles, functional style).

https://us.pycon.org/2019/schedule/presentation/216/

1. Mocking and Patching Pitfalls PyCon 2019
2. Mocking and Patching Pitfalls Misadventures In Mocking A People’s History of Mocking Mocking: Forms and Perversions Mocking: A Polemic Mock Hell PyCon 2019
3. MOCK HELL PyCon 2019
4. who i am • Staff Software Engineer, Platform @ Quid • Data engineering and microservices • Python is ~4th language email: edwin.jung@outlook.com twitter: @ejjcatx github: xtaje
5. who you are • Have used mocking or patching before. • Use the mock and patch APIs regularly, but are not sure why it’s sometimes painful. • Comfortable with mocks, but want to know more about different options. Advanced Beginner
6. what you might learn • What are the styles of unit testing • Where did mocks come from? • What are alternatives to mocking? • Anti-patterns • Relationship between mocking and design Advanced Beginner
7. 1. complex patch targets 2. numerous mocks or patches 3. mocks with brittle assertions 4. mocks with complex setup 5. deep mocks/recursive mocks/ mocks returning mocks 6. tests that test nothing 7. using the debugger to reverse- engineer mocks 8. mocks that prevent refactoring
8. 1. Mock is a really cool library used to isolate code for testing. 2. Here’s how to use MagicMock, patch, spec, autospec, side_effect, etc. 3. Patching is tricky, use the right namespace target. the promise
9. # my_module.py from db import db_read def total_value(item_id): items = db_read(item_id) return sum(items) code under test
10. mock patch # test_my_module.py from unittest.mock import patch from my_module import total_value def test_total_value(): with patch('my_module.db_read') as db_read: db_read.return_value = [100, 200] assert 300 == total_value(1234) db_read.assert_called_with(1234)
11. total value db read
12. total value test db read test
13. total value test db read patch
14. total value test mock substitute a mock
15. total value test mock run
16. test mock assert on the result total value
17. test mock assert on the interaction total value
18. real example: Guardian feed
20. Given some topic of interest (e.g. “Brexit”), Find all relevant articles from a particular time period, and Feed those article urls to a scraper service.
21. Guardian REST API GET https://content.guardianapis.com/search? from-date=2019-03-01&to- date=2019-03-31&page=1&q=brexit&api- key=<apikey>
22. { "response": { "status": "ok", “total": 1113, "startIndex": 1, "pageSize": 10, "currentPage": 1, "pages": 112, “results": [ { "id": “world/2019/mar/21/thursday-briefing-mays-brexi...”, "type": "article", "webPublicationDate": "2019-03-21T06:30:38Z", "webTitle": "Thursday briefing: May's Brexit broadside", "webUrl": “https://www.theguardian.com/world/2019/m...”, }, . . . and 9 more results ] } }
23. if __name__ == "__main__": logging.basicConfig() logger.setLevel(logging.DEBUG) Feed("brexit").run() main.py
24. import redis import requests class Feed(object): + ---- < def __init__ > --------------------------------- + ---- < def format_request > --------------------------- + ---- < def run > -------------------------------------- feed.py
25. feed.py import redis import requests class Feed(object): def __init__(self, query, redis_host=None, page_size=None, date_range=None): self._query, self.page_size = query, page_size self.date_range = date_range or (None, None) self._redis_con = redis.Redis(redis_host or "localhost") self._session = requests.Session() + ---- < def format_request > --------------------------------- + ---- < def run > --------------------------------------------
26. feed.py import redis import requests class Feed(object): + ---- < def __init__ > --------------------------------- + ---- < def format_request > --------------------------- + ---- < def run > --------------------------------------
27. feed.py import redis import requests class Feed(object): + ---- < def __init__ > -------------------------------------- def format_request(self, page): def param(key, val): return {key: val} if val is not None else {} params={"page": page, "query": self._query, "api-key": _APIKEY } params.update(param("page_size", self.page_size)) # repeat for from-date, to-date return Request('GET', _BASE_URL, params=params).prepare() + ---- < def run > -------------------------------------------
28. import redis import requests class Feed(object): + ---- < def __init__ > --------------------------------- + ---- < def format_request > --------------------------- + ---- < def run > -------------------------------------- feed.py
29. import redis import requests class Feed(object): + ---- < def __init__ > -------------------------------------- + ---- < def format_request > -------------------------------- def run(self): page, page_count = (1, 1) while page <= page_count: rsp = self._session.send(self.format_request(page)) rsp.raise_for_status() response = rsp.json()['response'] page_count = response['pages'] results = response[‘results'] def push(web_url): self._redis_con.rpush("scrape_urls", web_url) list(map(push, (r['webUrl'] for r in results))) page += 1 feed.py
30. Feed test guardian Session Request redispy redis server
31. Feed test guardian Session Request redispy redis server
32. Feed test guardian Session Request redispy redis server
33. Feed test guardian Session Request redispy redis server
34. Feed test guardian Session Request redispy redis server
35. @patch("feed.redis.Redis", autospec=True) @patch("feed.requests.Session", autospec=True) @patch("feed.Request", autospec=True) def test_run(request_mock, session_mock, mock_Redis): Feed("brexit").run()
36. @patch("feed.redis.Redis", autospec=True) @patch("feed.requests.Session", autospec=True) @patch("feed.Request", autospec=True) def test_run(request_mock, session_mock, mock_Redis): mock_response = MagicMock(spec=requests.Response) mock_response.json.return_value = { "response": { "pages": 1, "results": [ {"webUrl": "http://article1"}, {"webUrl": "http://article2"} ] } } session_mock.return_value.send.return_value = mock_response Feed("brexit").run()
37. @patch("feed.redis.Redis", autospec=True) @patch("feed.requests.Session", autospec=True) @patch("feed.Request", autospec=True) def test_run(request_mock, session_mock, mock_Redis): mock_response = MagicMock(spec=requests.Response) mock_response.json.return_value = { "response": { "pages": 1, "results": [ {"webUrl": "http://article1"}, {"webUrl": "http://article2"} ] } } session_mock.return_value.send.return_value = mock_response Feed("brexit").run() request_mock.return_value.prepare.assert_called() params = {"page": 1, "api-key": ‘<apikey>', 'query': 'brexit'} expected_call = mock.call("GET", _GUARDIAN_URL, params=params) assert expected_call == request_mock.call_args
38. @patch("feed.redis.Redis", autospec=True) @patch("feed.requests.Session", autospec=True) @patch("feed.Request", autospec=True) def test_run(request_mock, session_mock, mock_Redis): mock_response = MagicMock(spec=requests.Response) mock_response.json.return_value = { "response": { "pages": 1, "results": [ {"webUrl": "http://article1"}, {"webUrl": "http://article2"} ] } } session_mock.return_value.send.return_value = mock_response Feed("brexit").run() request_mock.return_value.prepare.assert_called() params = {"page": 1, "api-key": ‘<apikey>', 'query': 'brexit'} expected_call = mock.call("GET", _GUARDIAN_URL, params=params) assert expected_call == request_mock.call_args mock_Redis.assert_called_with("localhost") expected_calls = [call('scrape_urls', 'http://article1'), ... ] assert expected_calls == mock_Redis.return_value.rpush.call_args_list
40. @patch("feed.redis.Redis", autospec=True) @patch("feed.requests.Session", autospec=True) @patch("feed.Request", autospec=True) def test_run(request_mock, session_mock, mock_Redis): mock_response = MagicMock(spec=requests.Response) mock_response.json.return_value = { "response": { "pages": 1, "results": [ {"webUrl": "http://article1"}, {"webUrl": "http://article2"} ] } } session_mock.return_value.send.return_value = mock_response Feed("brexit").run() request_mock.return_value.prepare.assert_called() params = {"page": 1, "api-key": ‘<apikey>', 'query': 'brexit'} expected_call = mock.call("GET", _GUARDIAN_URL, params=params) assert expected_call == request_mock.call_args mock_Redis.assert_called_with("localhost") expected_calls = [call('scrape_urls', 'http://article1'), ... ] assert expected_calls == mock_Redis.return_value.rpush.call_args_list How many mocks? A. 3 B. 4 C. __
41. @patch("feed.redis.Redis", autospec=True) @patch("feed.requests.Session", autospec=True) @patch("feed.Request", autospec=True) def test_run(request_mock, session_mock, mock_Redis): mock_response = MagicMock(spec=requests.Response) mock_response.json.return_value = { "response": { "pages": 1, "results": [ {"webUrl": "http://article1"}, {"webUrl": "http://article2"} ] } } session_mock.return_value.send.return_value = mock_response Feed("brexit").run() request_mock.return_value.prepare.assert_called() params = {"page": 1, "api-key": ‘<apikey>', 'query': 'brexit'} expected_call = mock.call("GET", _GUARDIAN_URL, params=params) assert expected_call == request_mock.call_args mock_Redis.assert_called_with("localhost") expected_calls = [call('scrape_urls', 'http://article1'), ... ] assert expected_calls == mock_Redis.return_value.rpush.call_args_list How many mocks? A. 3 B. 4 C. 11
42. @patch("feed.redis.Redis", autospec=True) @patch("feed.requests.Session", autospec=True) @patch("feed.Request", autospec=True) def test_run(request_mock, session_mock, mock_Redis): mock_response = MagicMock(spec=requests.Response) mock_response.json.return_value = { "response": { "pages": 1, "results": [ {"webUrl": "http://article1"}, {"webUrl": "http://article2"} ] } } session_mock.return_value.send.return_value = mock_response Feed("brexit").run() request_mock.return_value.prepare.assert_called() params = {"page": 1, "api-key": ‘<apikey>', 'query': 'brexit'} expected_call = mock.call("GET", _GUARDIAN_URL, params=params) assert expected_call == request_mock.call_args mock_Redis.assert_called_with("localhost") expected_calls = [call('scrape_urls', 'http://article1'), ... ] assert expected_calls == mock_Redis.return_value.rpush.call_args_list
43. is this a violation of information hiding? encapsulation? @patch("feed.redis.Redis", autospec=True) @patch("feed.requests.Session", autospec=True) @patch("feed.Request", autospec=True)
44. @patch("feed.redis.Redis", autospec=True) @patch("feed.requests.Session", autospec=True) @patch("feed.Request", autospec=True) def test_run(request_mock, session_mock, mock_Redis): mock_response = MagicMock(spec=requests.Response) mock_response.json.return_value = { "response": { "pages": 1, "results": [ {"webUrl": "http://article1"}, {"webUrl": "http://article2"} ] } } session_mock.return_value.send.return_value = mock_response Feed("brexit").run() request_mock.return_value.prepare.assert_called() params = {"page": 1, "api-key": ‘<apikey>', 'query': 'brexit'} expected_call = mock.call("GET", _GUARDIAN_URL, params=params) assert expected_call == request_mock.call_args mock_Redis.assert_called_with("localhost") expected_calls = [call('scrape_urls', 'http://article1'), ... ] assert expected_calls == mock_Redis.return_value.rpush.call_args_list
45. def test_run(request_mock, session_mock, mock_Redis): mock_response = MagicMock(spec=requests.Response) mock_response.json.return_value = { session_mock.return_value.send.return_value = mock_response request_mock.return_value.prepare.assert_called() assert expected_calls == mock_Redis.return_value.rpush.call_args_list law of demeter violation?
46. what if you change imports? import redis import requests class Feed(object): + ---- < def __init__ > --------------------------------- + ---- < def format_request > --------------------------- + ---- < def run > --------------------------------------
47. from redis import Redis import requests class Feed(object): + ---- < def __init__ > --------------------------------- + ---- < def format_request > --------------------------- + ---- < def run > -------------------------------------- what if you change imports?
48. broken @patch("feed.redis.Redis", autospec=True) @patch("feed.requests.Session", autospec=True) @patch("feed.Request", autospec=True) def test_run(request_mock, session_mock, mock_Redis): mock_response = MagicMock(spec=requests.Response) ...
49. kwargs to args? class Feed(object): + ---- < def __init__ > --------------------------------- def format_request(self, page): def param(key, val): return {key: val} if val is not None else {} params={"page": page, "query": self._query, "api-key": _APIKEY } params.update(param("page_size", self.page_size)) # repeat for from-date, to-date return Request('GET', _BASE_URL, params=params).prepare() + ---- < def run > --------------------------------------
50. kwargs to args? class Feed(object): + ---- < def __init__ > --------------------------------- def format_request(self, page): def param(key, val): return {key: val} if val is not None else {} params={"page": page, "query": self._query, "api-key": _APIKEY } params.update(param("page_size", self.page_size)) # repeat for from-date, to-date return Request('GET', _BASE_URL, params).prepare() + ---- < def run > --------------------------------------
51. broken @patch("feed.redis.Redis", autospec=True) @patch("feed.requests.Session", autospec=True) @patch("feed.Request", autospec=True) def test_run(request_mock, session_mock, mock_Redis): # more code above ... Feed("brexit").run() request_mock.return_value.prepare.assert_called() params = {"page": 1, "api-key": ‘<apikey>', 'query': 'brexit'} expected_call = mock.call("GET", _GUARDIAN_URL, params=params) assert expected_call == request_mock.call_args # more code below...
52. 1. Mock is a really cool library used to isolate code for testing. 2. Here’s how to use Mock, MagicMock, patch, spec, autospec, side_effect, etc. 3. Patching is tricky, use the right namespace target. 4. (And by the way, be careful about over-mocking.) 5. (Mocks aren’t stubs) fine print
53. • PyCon Cleveland 2018 • EuroPython 2018 • PyCon UK 2018 • PyCon Australia 2017 • PyCon CZ 2017 • PyCon Israel 2017 • EuroPython 2016 • PyCon Portland 2016 • PyCon UK 2015 • PyCon SG 2015 mock talk at PyCons
54. PyCon Cleveland 2018: Demystifying the Patch Function mock talk at PyCons
55. A: … Patches can definitely get out of hand … … you maybe need to look into why there is so much heavy mocking. “Q&A: Demystifying the Patch Function” -PyCon 2018
56. Q: Hello… that was a really good talk… “Q&A: Demystifying the Patch Function” -PyCon 2018
57. Q: I’m Michael Foord. I was the original author of mock… “Q&A: Demystifying the Patch Function” -PyCon 2018
58. Q: “One thing I would say is that…” “Q&A: Demystifying the Patch Function” -PyCon 2018
59. Q: “Patches are a sign of failure.” “Q&A: Demystifying the Patch Function” -PyCon 2018
60. Q: “The more you have to patch, the worse your code is.” “Q&A: Demystifying the Patch Function” -PyCon 2018
61. ? !
62. tests code reflect determines symbiosis
63. where did mocks come from?
65. Mock Objects is an extension to Test Driven Development that supports good Object- Oriented design …
66. It turns out to be less interesting as a technique for isolating tests … than is widely thought.
67. 1. TDD 2. Object Oriented Design 3. mocks are not a tool for isolation mocks in context
68. 1. rapid cycles of small refactoring 2. Object Oriented Design 3. mocks are not a tool for isolation mocks in context
69. 1. rapid, cycles of small refactoring 2. object collaborations 3. mocks are not a tool for isolation mocks in context
70. 1. rapid, cycles of small refactoring 2. object collaborations 3. mocks are a tool for exploratory design and discovery mocks in context
71. 1. rapid, cycles of small refactoring 2. object collaborations 3. mocks are a tool for exploratory design and discovery mocks in context
72. contrary opinions
73. Always choose interaction testing [mocking]… as the last option. This is very important… Not everyone agrees with this point of view ...
74. Freeman and Pryce advocate what many call “the London school of TDD,” …
75. I’m not fully disagreeing … but for maintainability … tests using mocks creates more trouble than it’s worth.
76. Mocking frameworks are potent tools that let you wield incredible power. However, power nearly always corrupts. …
77. how to test without mocks?
78. mocks aren’t stubs 1. mocks are not stubs 2. mocks != stubs 3. stubs are not mocks
79. test doubles name behavior mock stub fake dummy spy
80. test doubles name behavior mock records calls to the object stub returns canned data, no logic fake implements fake version of production logic dummy does nothing spy records and delegates to the real thing
81. # test_my_module.py from unittest.mock import patch from my_module import total_value def test_total_value(): with patch('my_module.db_read') as db_read: db_read.return_value = [100, 200] assert 300 == total_value(1234) db_read.assert_called_with(1234) remember mock patch? mock stub
82. alternative: fake patch # test_my_module.py from unittest.mock import patch from my_module import total_value def test_total_value(): with patch('my_module.db_read'): assert 300 == total_value(1234)
83. alternative: fake patch # test_my_module.py from unittest.mock import patch from my_module import total_value def test_total_value(): def fake_db_read(item_id): with open(f'./fixtures/{item_id}.json') as fobj: return json.load(fobj) with patch('my_module.db_read'): assert 300 == total_value(1234)
84. alternative: fake patch # test_my_module.py from unittest.mock import patch from my_module import total_value def test_total_value(): def fake_db_read(item_id): with open(f'./fixtures/{item_id}.json') as fobj: return json.load(fobj) with patch('my_module.db_read', new=fake_db_read): assert 300 == total_value(1234)
85. patch total value test db read
86. define a fake total value test db read fake db_read
87. substitute the fake total value test db_rea d fake db_read
88. run total value test db_rea d fake db_read
89. total value test db_rea d assert on result fake db_read
90. alternative: fake injection # my_module.py from db import db_read def total_value(item_id): items = db_read(item_id) return sum(items) # my_module.py def total_value(item_id, db_read): items = db_read(item_id) return sum(items)
91. alternative: fake injection def test_total_value2(): def fake_db_read(item_id): with open(f'./fixtures/{item_id}.json') as fobj: return json.load(fobj) assert 300 == total_value2(1234, fake_db_read)
92. total_value2 expects to be given a collaborator to fill a role total value
93. test => define a fake fake total value test
94. test => inject the fake fake total value test
95. run fake total value test
96. fake total value test assert on result
97. app => create a real DB adapter db read total value app
98. app => inject the adapter db read total value app
99. db read total value app => run app
100. tactical questions 1.Which Test Doubles (mock or other)? 2.Mockist or Classical? (aka London or Detroit) 3.Patch or Inject?
101. plan: Guardian feed
102. Mock Roles, not Objects Tactics 3 Dependency Injection Tactics 4 Inject the Collaborator Tactics 5 Go Functional Tactics 1 Find a Seam Patch a Fake
103. what are the roles in this collaboration?
104. Master Student
105. Master Student
106. Villain Hero
107. Hero Villain
108. Father Son
109. Feed test Session Request redispy redis server guardian what are the roles in this collaboration?
110. Feed test Session Request redispy redis server guardian Parser Sink Source
111. Mock Roles, not Objects Tactics 2 Dependency Injection Tactics 3 Inject the Collaborator Tactics 4 Go Functional Tactics 1 Find a Seam Patch a Fake
112. Feed test guardian Session Request redispy redis server
113. Feed test Session Request redispy redis server guardian
114. seam “A seam is a place where you can alter behavior in your program without editing in that place.”
115. Feed test Session Request redispy redis server guardian
116. Feed test Session Request redispy redis server find a different “seam” urllib3 requests internals TCP guardian
117. Feed test Session Request redispy redis server patch a fake into a transport layer “seam” HTTMock guardian fakeguard ian
118. @httmock.urlmatch(netloc=r’content.guardianapis.com') def fake_guardian(url, request): page, page_size = parse_query_params(url.query) article_ids = range(page_size*(page-1)+1, page_size*page+1) results = [{"webUrl": "http://article%d" % (i)} for i in article_ids] fake_response = { "response": { "currentPage": page, "pages": 2, "results": results } } return json.dumps(fake_response)
119. from urllib.parse import parse_qs from httmock import urlmatch, HTTMock @mock.patch("feed.redis.Redis", autospec=True) def test_run(mock_Redis): with httmock.HTTMock(fake_guardian): Feed("brexit").run() mock_Redis.assert_called_with("localhost") expected_calls = [mock.call('scrape_urls', f'http://a{i}') for i in range(1, 21)] assert expected_calls == mock_Redis.return_value.rpush.call_args_list)
120. Mock Roles, not Objects Tactics 2 Dependency Injection Tactics 3 Inject the Collaborator Tactics 4 Go Functional Tactics 1 Find a Seam Patch a Fake
121. Feed test Session Request mock redispy redis server fake guardian HTTMock
122. Feed test Session Request redis server fake guardian HTTMock
123. Feed test Session Request redis server fake redis fake guardian HTTMock
124. Feed test Session Request fake redis redis server fake guardian HTTMock
125. inject the redis connection class Feed(object): def __init__(self, query, redis_host=None, page_size=None, date_range=None): self._query = query self.page_size = page_size self.date_range = date_range or (None, None) self._redis_con = redis.Redis(redis_host or "localhost") self._session = requests.Session() + ---- < def format_request > --------------------------- + ---- < def run > --------------------------------------
126. inject the redis connection class Feed(object): def __init__(self, query, redis_con, page_size=None, date_range=None): self._query = query self.page_size = page_size self.date_range = date_range or (None, None) self._redis_con = redis_con self._session = requests.Session() + ---- < def format_request > --------------------------- + ---- < def run > --------------------------------------
127. from httmock import urlmatch, HTTMock from feed import Feed import fakeredis def test_run(): fake_con = fakeredis.FakeStrictRedis() with HTTMock(fake_guardian): Feed("brexit", fake_con).run() expected = [f"http://article{i}" for i in range(1, 21)] received = [fake_con.lpop("scrape_urls").decode("utf-8") for _ in range(1,21)] self.assertEqual(expected, received)
128. Mock Roles, not Objects Tactics 3 Dependency Injection Tactics 4 Inject the Collaborator Tactics 5 Go Functional Tactics 1 Find a Seam Patch a Fake
129. inject the collaborator class Feed(object): def __init__(self, query, redis_con, page_size=None, date_range=None): self._query = query self.page_size = page_size self.date_range = date_range or (None, None) self._redis_con = redis_con self._session = requests.Session() + ---- < def format_request > --------------------------- + ---- < def run > --------------------------------------
130. inject the collaborator class Feed(object): def __init__(self, query, sink, page_size=None, date_range=None): self._query = query self.page_size = page_size self.date_range = date_range or (None, None) self._sink = sink self._session = requests.Session() + ---- < def format_request > --------------------------- + ---- < def run > --------------------------------------
131. class Feed(object): + ---- < def __init__ > --------------------------------- + ---- < def format_request > --------------------------- def run(self): page, page_count = (1, 1) while page <= page_count: rsp = self._session.send(self.format_request(page)) rsp.raise_for_status() response = rsp.json()['response'] logger.debug(response) page_count = response['pages'] results = response['results'] list(map(self._sink, (r['webUrl'] for r in results))) page += 1
132. from httmock import urlmatch, all_requests, HTTMock from feed import Feed def test_run(self): received = [] with HTTMock(fake_guardian): Feed("brexit", received.append).run() expected = [f"http://article{i}" for i in range(1, 21)] self.assertEqual(expected, received)
133. if __name__ == "__main__": logging.basicConfig() logger.setLevel(logging.DEBUG) redis_con = redis.Redis("localhost") def sink(msg): redis_con.rpush("scrape_urls", msg) Feed("brexit", sink).run()
134. Mock Roles, not Objects Tactics 3 Dependency Injection Tactics 4 Inject the Collaborator Tactics 5 Go Functional Tactics 1 Find a Seam Patch a Fake
135. glue code def feed(query): s = requests.Session() first_page = s.send(initial_request(query)) all_pages = map(s.send, page_requests(first_page.json(), query=query)) web_urls = munge(all_pages) redis.Redis("localhost").rpush("scrape_urls", *web_urls) if __name__ == "__main__": feed("brexit")
136. pure functions def flatmap(f, items): return itertools.chain.from_iterable(map(f, items)) def param(key, val): return {key: val} if val is not None else {} def format_request(query, page, page_size = None, date_range=None): date_range = date_range or (None, None) params= { "page": page, "query": query, "api-key": _APIKEY } params.update(param("page_size", page_size)) params.update(param("start_date", date_range[0])) params.update(param("end_date", date_range[1])) return Request('GET', _BASE_URL, params=params).prepare() def initial_request(query, page_size=None, date_range=None): return format_request(query, page=1, page_size=page_size, date_range=date_range) def page_requests(body, query): response = body['response'] page_count = min(3, int(response['pages'])) # IRL needs flow control for page in range(1, page_count+1): params = {"page": page, "query": query, "api-key": _APIKEY} yield Request('GET', _BASE_URL, params=params).prepare() def munge(all_pages): bodies = map(lambda r: r.json(), all_pages) results = flatmap(lambda body: body['response']['results'], bodies) return map(lambda result: result['webUrl'], results)
137. anti-patterns
138. bootleg TDD
139. bootleg toys
140. bootleg toys
141. • Aim for 100% test isolation, 100% coverage • Test Whenever, No Refactoring • Procedural Decomposition or Top-Down Design (no OOP) bootleg TDD
142. • Aim for 100% test isolation, 100% coverage • Test Whenever, No Refactoring • Procedural Decomposition or Top-Down Design (no OOP) bootleg TDD
143. procedural decomposition “In each step, one or several instructions of a program are decomposed into more detailed instructions”
144. P
145. P B A
146. P B A C D E
147. P B A C D E Done. Time to test.
148. P B A test
149. B A C D E test test
150. C D E test test test
151. C D E test test test “How do patch a database?”
152. C D E test test test “. . . mock the db connection?” “. . . mock the query language?”
153. inversion-lock
154. Abstract Detailed 1.Top down design => no dependency inversion 2.Patching + mocking “locks in” design decisions Patching prevents understanding of dependency inversion Issues?
155. dependency inversion total value client
156. fake total value test test => inject a fake
157. db read total value app => inject a real DB app
158. total value test db read app fake clean
159. total value test db read app fake ports-and-adapters
160. total value test hexagonal db read app fake
161. connections to “good” design? • Functional-Core/Imperative Shell • Ports-And-Adapters, Hexagonal • Clean, Onion
162. the blob
163. P
164. P How do I test this without the database? test
165. P Extreme Tactical Patching test
166. issues • What kind of test is this? • Is this • pragmatic or • just enabling bad habits?
167. patches as crosscuts
168. propositions 1.A test should test a single behavior. 2.A patch is a violation of some encapsulation boundary. 3.A test with many patches is violating multiple encapsulation boundaries. A test case with a lot of patches may represent a problem with cohesion or coupling. A test case with a lot of patches may represent a crosscutting concern.
169. opinions • always be refactoring • consider other test doubles • patching • should be rare, and the last tool you use • mocks (if you use them) • should target roles and not objects • are not for just for test isolation
170. thanks to
171. Debates and Counter-opinions • Dependency Injection is Not a Virtue (DHH) • TDD is Dead (Martin Fowler, Kent Beck, DHH) • Why Most Unit Testing is a Waste (Coplien) • The TDD Apostate (Seeman) Resources • Mocks aren’t Stubs (Fowler) • Clean Code (Uncle Bob) • Working Effectively with Legacy Code (Feathers) • Growing Object Oriented Software Guided by Tests • The Art of Unit Testing (Osherove) email: edwin.jung@outlook.com twitter: @ejjcatx