good test bad test ! ! ! ! ! dan crosta http://late.am/ @lazlofruvous magnetic

virtues of good tests

fast selective repeatable reliable helpful

fast selective repeatable reliable helpful

fast selective repeatable reliable helpful

fast selective repeatable reliable helpful

fast selective repeatable reliable helpful

http://www.free-desktop-backgrounds.net/Miscellaneous-pictures/Signs-wallpaper-images/Caution-sign.html opinions ahead

myth 1 100% coverage

if every line is covered, it must be correct. ! right?

https://www.sparkholder.com/wp-content/uploads/2015/01/sparkholder-api.png (modified)

https://s-media-cache-ak0.pinimg.com/originals/ca/4c/56/ca4c56fcdc04212f1bd11eca4a2fca36.jpg "mutually assured destruction"

! some code should not
 be tested

what would cause
 my test to fail? !

keep your “plumbing” dumb. !

myth 2 assert about everything

more assertions means
 higher confidence ! right?

def test_it_parses_log_lines(self): line = '2015-03-11T20:09:25|GET /foo?bar=baz|...' ! parsed = parse_log_line(line) ! ! ! ! ! ! ! ! ! ! ! !

def test_it_parses_log_lines(self): line = '2015-03-11T20:09:25|GET /foo?bar=baz|...' ! parsed = parse_log_line(line) ! self.assertEqual({ "date": datetime(2015, 3, 11, 20, 9, 25), "method": "GET", "path": "/foo", "query": "bar=baz", }, parsed) ! ! ! ! !

def test_it_parses_log_lines(self): line = '2015-03-11T20:09:25|GET /foo?bar=baz|...' ! parsed = parse_log_line(line) ! self.assertEqual( datetime(2015, 3, 11, 20, 9, 25), parsed["date"], ) self.assertEqual("GET", parsed["method"]) self.assertEqual("/foo", parsed["path"]) self.assertEqual("bar=baz", parsed["query"]) ! ! ! !

def test_it_parses_get_request_log_lines(self): # ... self.assertEqual("GET", parsed["method"]) self.assertEqual("/foo", parsed["path"]) self.assertEqual("bar=baz", parsed["query"]) ! def test_it_parses_post_request_log_lines(self): # ... self.assertEqual("POST", parsed["method"]) self.assertEqual("/foo", parsed["path"]) self.assertEqual("bar=baz", parsed["query"]) ! def test_it_parses_log_lines_with_oof_requests(self): # ... self.assertEqual("GET", parsed["method"]) self.assertEqual("/oof", parsed["path"]) self.assertEqual("bar=baz", parsed["query"])

what would I do when a test fails? !

have your tests tell you what is wrong. !

! only make one assertion per test

def test_it_parses_request_method(self): # ... self.assertEqual("GET", parsed["method"]) ! def test_it_parses_request_path(self): # ... self.assertEqual("/foo", parsed["path"]) ! def test_it_parses_query_string(self): # ... self.assertEqual("bar=baz", parsed["query"]) ! def test_it_gives_none_when_no_query_string(self): # ... self.assertIsNone(parsed["query"]) !

! myth 3 mock makes tests better

mock helps you isolate tests to just one layer ! right?

def authenticate(username): """Return a True if the user is authenticated """ # ... return True ! ! def login(username): if not authenticate(self.params["username"]): return Response(status=401) ! # ... do more stuff ... ! return Response(status=200) !

def test_login_returns_200_on_success(self): with mock.patch(app, "authenticate") as auth: auth.return_value = True ! response = app.login("dcrosta") ! self.assertEqual(200, response.status) ! ! def test_login_returns_401_on_failure(self): with mock.patch(app, "authenticate") as auth: auth.return_value = False ! response = app.login("dcrosta") ! self.assertEqual(401, response.status)

def authenticate(username): """Return a tuple of (authenticated, user_roles) """ # ... return (True, ["admin"]) ! ! def login(username): if not authenticate(self.params["username"]): return Response(status=401) ! # ... do more stuff ... ! return Response(status=200) !

why might my test pass when it really should fail? !

use verified doubles

class StubDB(object): ! def __init__(self): self.users = {} ! def register_user(self, username, roles): self.users[username] = roles ! def authenticate(self, username): if username not in self.users: return (False, []) return (True, self.users[username]) ! ! !

def test_login_returns_200_on_success(self): db = StubDB() db.register_user("dcrosta", ["admin"]) ! app = MyApp(db) response = app.login("dcrosta") ! self.assertEqual(200, response.status) ! def test_login_returns_401_on_failure(self): db = StubDB() ! app = MyApp(db) response = app.login("dcrosta") ! self.assertEqual(401, response.status)

http://gingersnapworks.com/wp-content/uploads/2012/12/facebook_like-hang-tag.jpg library authors: ship test doubles
 with your packages

tests can smell, too !

design your tests !

good tests are your best friends

good test bad test ! ! ! ! ! dan crosta http://late.am/ @lazlofruvous magnetic