Cleaner unit testing with the Arrange Act Assert pattern James Cooke Licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License. 

Outline ● Background ● Problem ● AAA pattern ● Smells and solutions ● Questions in the break 

Background ● Python developer ● TDD with AAA ● github.com/txels 

Givens ● PEP008 is beneficial ● PEP020 is our mantra ● We all TDD ● Every test is a function, uncoupled ● Simplicity trumps performance ● Eliminate duplication 

Problem ● Pythonic code in project, carnage in tests ● Tests of OSS projects are often hard to understand ● Tests are non-uniform 

Example Member Account Membership 1 1 m m 

The Pattern def test_x(*args, **kwargs): """__docstring__ """ # Arrange # Act # Assert AKA ‘AAA’ or ‘3A’ 

def test_x(*args, **kwargs): """__docstring__ """ # Arrange # Act # Assert 

def test_printable_name(): """__docstring__ """ # Arrange # Act # Assert 

Docstring ● Follow existing style of your project ● First line positive statement of tested action ● Extra info about test, conditions 

def test_printable_name(): """__docstring__ """ # Arrange # Act # Assert 

def test_printable_name(): """New members have printable names """ # Arrange # Act # Assert 

Arrange ● Single block of code ● No assertions ● Only prepare non-deterministic results not available after Act ● Should not require comments 

def test_printable_name(): """New members have printable names """ # Arrange # Act # Assert 

def test_printable_name(): """New members have printable names """ person = Member() person.first_name = 'René' person.last_name = ' 北京 ' # Act # Assert 

Act ● `result = ` format ● One line only ● Can be wrapped in `with raises` for expected exceptions 

def test_printable_name(): """New members have printable names """ person = Member() person.first_name = 'René' person.last_name = ' 北京 ' # Act # Assert 

def test_printable_name(): """New members have printable names """ person = Member() person.first_name = 'René' person.last_name = ' 北京 ' result = person.printable_name() # Assert 

Assert ● Single block of code ● No actions should happen ● Test result first, then side effects ● Use simple blocks of assertions 

def test_printable_name(): """New members have printable names """ person = Member() person.first_name = 'René' person.last_name = ' 北京 ' result = person.printable_name() # Assert 

def test_printable_name(): """New members have printable names """ person = Member() person.first_name = 'René' person.last_name = ' 北京 ' result = person.printable_name() assert result == ' 北京 , René' 

Smell: Assertions in Arrange 

def test_account_list(): """Member can list their accounts """ person = Member() account = Account() person.join(account) assert person in account.members result = person.account_list() assert len(result) == 1 assert account in result 

person = Member() account = Account() person.join(account) assert person in account.members 

person = Member() account = Account() person.join(account) result = account.members assert person in account.members 

person = Member() account = Account() person.join(account) result = account.members assert person in result 

def test_account_member(): """Member can join account """ person = Member() account = Account() person.join(account) result = account.members assert person in result 

def test_account_member(): """Member can join account """ person = Member() account = Account() person.join(account) result = account.members assert person in result def test_account_list(): """Member can list their accounts """ person = Member() account = Account() person.join(account) assert person in account.members result = person.account_list() assert len(result) == 1 assert account in result 

def test_account_member(): """Member can join account """ person = Member() account = Account() person.join(account) result = account.members assert person in result def test_account_list(): """Member can list their accounts """ person = Member() account = Account() person.join(account) result = person.account_list() assert len(result) == 1 assert account in result ● Eliminate duplication 

def test_account_member(): """Member can join account """ person = Member() account = Account() person.join(account) result = account.members assert person in result def test_account_list(): """Member can list their accounts """ person = Member() account = Account() person.join(account) result = person.account_list() assert len(result) == 1 assert account in result person = Member() account = Account() person.join(account) return person, account 

def test_account_member(): """Member can join account """ person = Member() account = Account() person.join(account) result = account.members assert person in result def test_account_list(): """Member can list their accounts """ person = Member() account = Account() person.join(account) result = person.account_list() assert len(result) == 1 assert account in result def account_member_factory(): """Create Member and Account pair """ person = Member() account = Account() person.join(account) return person, account 

def account_member_factory(): """Create Member and Account pair """ person = Member() account = Account() person.join(account) return person, account def test_account_member(): """Member can join account """ person = Member() account = Account() person.join(account) result = account.members assert person in result def test_account_list(): """Member can list their accounts """ person = Member() account = Account() person.join(account) result = person.account_list() assert len(result) == 1 assert account in result 

def account_member_factory(): """Create Member and Account pair """ person = Member() account = Account() person.join(account) return person, account def test_account_list(): """Member can list their accounts """ person = Member() account = Account() person.join(account) result = person.account_list() assert len(result) == 1 assert account in result def test_account_member(): """Member can join account """ person, account = account_member_factory() result = account.members assert person in result 

def account_member_factory(): """Create Member and Account pair """ person = Member() account = Account() person.join(account) return person, account def test_account_member(): """Member can join account """ person, account = account_member_factory() result = account.members assert person in result def test_account_list(): """Member can list their accounts """ person, account = account_member_factory() result = person.account_list() assert len(result) == 1 assert account in result 

def account_member_factory(): """Create Member and Account pair """ person = Member() account = Account() person.join(account) return person, account def test_account_member_factory(): """Member can join account """ person, account = account_member_factory() result = account.members assert person in result def test_account_list(): """Member can list their accounts """ person, account = account_member_factory() result = person.account_list() assert len(result) == 1 assert account in result 

def account_member_factory(): """Create Member and Account pair """ person = Member() account = Account() person.join(account) return person, account def test_account_member_factory(): """account_member_factory creates joined Member Account pair """ person, account = account_member_factory() result = account.members assert person in result def test_account_list(): """Member can list their accounts """ person, account = account_member_factory() result = person.account_list() assert len(result) == 1 assert account in result 

Smell: Duplication between tests 

Example Member A Account 1 Member B Member C Account 2 Account 3 

def test_a_transfer_b(): """Member A in Account 1 can transfer to B """ user_a, account_one = account_member_factory() user_b, account_two = account_member_factory() user_b.join(account_one) user_c, account_three = account_member_factory() # Act # Assert def test_a_not_transfer_c(): """Member A in Account 1 can not tranfer to C """ user_a, account_one = account_member_factory() user_b, account_two = account_member_factory() user_b.join(account_one) user_c, account_three = account_member_factory() # Act # Assert def test_c_not_transfer_a(): """Member C in Account 3 can not tranfer to A """ user_a, account_one = account_member_factory() user_b, account_two = account_member_factory() ● Eliminate duplication 

class TestTranfers(unittest.TestCase): def test_a_transfer_b(): """Member A in Account 1 can transfer to B """ user_a, account_one = account_member_factory() user_b, account_two = account_member_factory() user_b.join(account_one) user_c, account_three = account_member_factory() # Act # Assert def test_a_not_transfer_c(): """Member A in Account 1 can not tranfer to C """ user_a, account_one = account_member_factory() user_b, account_two = account_member_factory() user_b.join(account_one) user_c, account_three = account_member_factory() # Act # Assert def test_c_not_transfer_a(): """Member C in Account 3 can not tranfer to A """ user_a, account_one = account_member_factory() user_b, account_two = account_member_factory() 

class TestTranfers(unittest.TestCase): def test_a_transfer_b(self): """Member A in Account 1 can transfer to B """ user_a, account_one = account_member_factory() user_b, account_two = account_member_factory() user_b.join(account_one) user_c, account_three = account_member_factory() # Act # Assert def test_a_not_transfer_c(self): """Member A in Account 1 can not tranfer to C """ user_a, account_one = account_member_factory() user_b, account_two = account_member_factory() user_b.join(account_one) user_c, account_three = account_member_factory() # Act # Assert def test_c_not_transfer_a(self): """Member C in Account 3 can not tranfer to A """ user_a, account_one = account_member_factory() user_b, account_two = account_member_factory() 

class TestTranfers(unittest.TestCase): def setUp(self): """ """ def test_a_transfer_b(self): """Member A in Account 1 can transfer to B """ user_a, account_one = account_member_factory() user_b, account_two = account_member_factory() user_b.join(account_one) user_c, account_three = account_member_factory() # Act # Assert 

Addendum: TestCase.setUp ● A pure Arrange block ● Docstring if required ● Assert valid with a special test called test_set_up 

class TestTranfers(unittest.TestCase): def setUp(self): """Create three Accounts and three members --------+--------------------------- Member | Accounts --------+--------------------------- user_a | account_one user_b | account_one, account_two user_c | account_three --------+--------------------------- """ super().setUp() self.user_a, self.account_one = account_member_factory() self.user_b, self.account_two = account_member_factory() self.user_b.join(self.account_one) self.user_c, self.account_three = account_member_factory() def test_a_transfer_b(self): """Member A in Account 1 can transfer to B """ user_a, account_one = account_member_factory() user_b, account_two = account_member_factory() user_b.join(account_one) user_c, account_three = account_member_factory() # Act # Assert 

class TestTranfers(unittest.TestCase): def setUp(self): """Create three Accounts and three members --------+--------------------------- Member | Accounts --------+--------------------------- user_a | account_one user_b | account_one, account_two user_c | account_three --------+--------------------------- """ super().setUp() self.user_a, self.account_one = account_member_factory() self.user_b, self.account_two = account_member_factory() self.user_b.join(self.account_one) self.user_c, self.account_three = account_member_factory() def test_a_transfer_b(self): """Member A in Account 1 can transfer to B """ # Act # Assert def test_a_not_transfer_c(self): """Member A in Account 1 can not tranfer to C """ # Act 

class TestTranfers(unittest.TestCase): def setUp(self): """Create three Accounts and three members --------+--------------------------- Member | Accounts --------+--------------------------- user_a | account_one user_b | account_one, account_two user_c | account_three --------+--------------------------- """ super().setUp() self.user_a, self.account_one = account_member_factory() self.user_b, self.account_two = account_member_factory() self.user_b.join(self.account_one) self.user_c, self.account_three = account_member_factory() def test_set_up(self): def test_a_transfer_b(self): """Member A in Account 1 can transfer to B """ # Act # Assert 

Addendum: test_set_up ● A pure Assert block ● Docstring if required 

class TestTranfers(unittest.TestCase): def setUp(self): """Create three Accounts and three members --------+--------------------------- Member | Accounts --------+--------------------------- user_a | account_one user_b | account_one, account_two user_c | account_three --------+--------------------------- """ super().setUp() self.user_a, self.account_one = account_member_factory() self.user_b, self.account_two = account_member_factory() self.user_b.join(self.account_one) self.user_c, self.account_three = account_member_factory() def test_set_up(self): assert self.user_a.account_list() == [self.account_one] assert self.user_b.account_list() == [self.account_one, self.account_two assert self.user_c.account_list() == [self.account_three] def test_a_transfer_b(self): """Member A in Account 1 can transfer to B """ # Act # Assert 

class TestTranfers(unittest.TestCase): def setUp(self): """Create three Accounts and three members --------+--------------------------- Member | Accounts --------+--------------------------- user_a | account_one user_b | account_one, account_two user_c | account_three --------+--------------------------- """ super().setUp() self.user_a, self.account_one = account_member_factory() self.user_b, self.account_two = account_member_factory() self.user_b.join(self.account_one) self.user_c, self.account_three = account_member_factory() def test_set_up(self): assert self.user_a.account_list() == [self.account_one] assert self.user_b.account_list() == [self.account_one, self.account_two] assert self.user_c.account_list() == [self.account_three] def test_a_transfer_b(self): """Member A in Account 1 can transfer to B """ # Act # Assert def test_a_not_transfer_c(self): """Member A in Account 1 can not tranfer to C """ # Act # Assert def test_c_not_transfer_a(self): """Member C in Account 3 can not tranfer to A """ # Act # Assert 

Summary ● Demonstrated AAA ● Small and simple tests ● Opens the door to DRY and clarity 

Thanks! ● Resources: http://jamescooke.info/cleaner... ● @jamesfublo ● Come ask questions ● Let’s talk more about testing ● Thanks for listening! Licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License.