Writing automated tests is an important part of development.
We write tests to:
Help improve quality by reducing number of regression defects
Help understand the product and its components
Reduce risk and fears when making bold changes into our product structure
Make sure the final product works when all pieces are put together and integrated with external systems (functional and system tools)
Make work easier and fun
All tests should have the following characteristics:
Be simple without being just an assertion check.
Be easy to write and maintain as product evolves
Be easy to read and and have a clear, easy to understand purpose
Be easy to run, easy to re-run, and have fully automated execution and reporting
Run very fast
Each test should be an independent test which can be run by itself
Writing tests is not only done to check that code works as expected, it is also done to make sure that code has a simple design, decoupled and easy to be reused.
Writing tests is also about making sure that you fully understand what the code should do and how it is used.
Writing tests is not a silver bullet for improving code quality - it’s easy to write bad tests.
Once used to having tests that show how things work (and that they do work), you will start using the key phrase: Do you have a test for that?.
You are going to test your code anyway, so spend the time to do it right from the beginning.
We write tests as a tool for regression checking, ie preventing bugs. Tests should be easy to run and should be fast. Tests will also help us find bugs and avoid long debugging sessions, but the primary goal is to keep the bugs out of our product.
We should write code and do clean-up / refactoring without any fear and with a confident and free mind. Don’t take decisions out of fear.
Imperfect tests run frequently, and are much better than perfect tests which are never written at all.
Writing tests does not mean writing twice as much code or coding slower.
Most of the time the associated test code is three times longer, or more in lines of code. Test code should be trivial, easy to write and easy to understand. Even if there are many lines of test code, they should be easy to write and should not add a big overhead to what you’re doing.
On the long run, it’s faster and more robust than coding without tests.
Another important part of writing tests is to help us have a clean code base. You can not refactor and do clean-up without an easy mechanism to check for regression. Clean-up tasks are just normal business and an integrated part of coding process. We will perform cleanups all the time, and there is no dedicated development window for cleanup.
The goal is to write the tests first and code later.
If you are starting and learning how to write tests, you will write tests after writing the code simply because you have so much to learn and find that writing tests first is impossible. This is a normal start. It’s just a stepping stone on your way to test-first development.
Passing software to another developer without tests is like saying ‘Good luck dude!’ - but instead we should say ‘I’ve got you covered!’.
2.3.1. Test Data
When using data for the test, make sure that all the data is part of the same versioning system as the test code.
In the best case, all test data should be generated by the test.
If for various reason you have pre-generated data, try to keep it in the same file and as close as possible to the test itself.
For example, keep the content of the file as string, and if the test really need to read that content from a file or data, write it before the test, run the test and the remove the file / database entry.
If keeping it in the same as the test code is not possible, as a third best keep it in a file which is versioned with the test code itself.
In some exceptional cases (chicken and egg problems) there is no option to generate the data at test time or to keep in the same versioning system as the test code.
For example when you want to run an end to end test to check that you can read and write a document from a database, you will have to start with testing the read operation against an existing data.
All testing data that is controlled outside of the tests for which is not directly generated by the test should be immutable. Once a branch using the external data is merged into master, the external data should never be changed.
This includes pre-generated access to the outside resource (username / password / server address / port). For this sort of persistent configuration make sure to use a prefix like ci-, making it explict that the resource is used for automated tests.
In the same way the version for the resource should be added as a suffix, like -v1. For example a username used by automated tests could be ci-testuser-v1.
In this way you can still run older versions of the code.
2.3.2. Types of tests
Tests can be placed in many categories according to various criteria.
Below is an attempt to identify various types of tests based on their interaction with other code / systems.
184.108.40.206. Unit testing
This is the least controversial type of tests, and people usually know what unit tests are.
Test for single method or functions. As the number of unit tests significantly grows throughout the lifecycle of the project one key requirement is that they run as fast as possible. Therefore the tests should use only data from memory for both input and output.
220.127.116.11. Integration testing
These are the tests which take what was tested using unit tests (functions, methods) and combine / integrate them in bigger tests for the whole module or even module interactions.
The time / speed restrictions still apply. They should also use only data from memory.
Try to test all corner cases regarding the component’s integration.
Don’t write corner cases for a specific component since they should be handled at unit testing level. If that is not the case please revise the unit tests accordingly.
The purpose of integration testing is to detect any inconsistencies between the software units / modules that are integrated together or between any modules and the hardware.
Since they should be fast, you can put them together with unit tests.
18.104.22.168. System testing
In contrast with unit or functional tests, we have system test which have no restriction for interacting with external systems.
A test is a system test if:
It touches the file system.
It talks to the database.
It communicates across the network.
It uses some type of shared resource and can’t run at the same time as any other unit tests.
You have to do special things to prepare your environment.
Requires a special OS account / OS credentials to exists.
They interact with local filesystem, network and other services provided by the operating system.
While interacting with external system, system tests require additional steps in configuration and preparing the external system for running the tests, so their execution depend on these external systems. Example: configure an OS account, do special network configuration, configure a printer, create certain files or folder structures on the filesystem.
The external services are “black boxes”, as we don’t have access to their internal structure.
They test the integration with external services, and at some extend they are integration tests.
Due to interaction with external system, system tests are slower than unit or integration tests.
While configuring external systems, take special care to avoid side effects or leaving the system in an inconsistent tests that will not allow other tests to execute. At the end of the test, leave the system in the same state as the one from the start.
We put them in together with unit tests, since they should be fast, but we tag them since they require special handling due to dependency on external systems. Ex, when porting on a new platform, they are likely to fail.
22.214.171.124. Story / Functional / Customer tests
These are the tests for the final product. All modules are put together just like in the production (real) system.
In other places, they are also called acceptance tests.
This kind of tests help us detect what feature of our product is not working. In case we have good unit, integration and system test, we look there to find why that feature is not working.
Functional tests will not help with development and locating the source of an error. Use unit tests for this. In case unit test pass, and we have failing functional tests, then we have just discovered a hole in our unit testing and we should fix it first.
The functional tests are just recording the same steps required in manual testing. If a functional tests pass, you can look around by starting the real server and manually perform the same steps as in the functional test.
You can consider them black box testing as their role is to check that the system, as a whole, works.
While for system tests only external systems are handled as black boxes, for functional testing the system under test is also handled as a black box.
They will read actual input just like the real life application and will produce actual, real life, results.
Don’t do too much work here and don’t bother with corner cases. A simple success scenario and a failure scenario should be enough.
Sometimes reading and writing actual input can slow tests, so for performance reasons, some system tests also read and write data in memory, but the format used closely resembles the one used in real life, production environments.
2.3.3. Developing using tests
As there is no guarantee for the order in which the tests will execute there is one major requirement, regardless of the test type. The test should not have any side effect.
When writing application code, only write enough code to make a test work. It helps you to realise when to stop coding and move on to the next task. If you know there should be more code to handle other cases, you should write the tests for those particular cases. This technique prevents writing code that is never executed and ensures that you always have a test for the code you write.
When you find a bug, start by writing a test reproducing the bug then continue your work in fixing the bug. Ask the 5 whys in order to find the root cause of the bug and fix the problem there. The initial test written to reproduce the bug, might be a high level tests which is not at the same level to the code which was fixed. Removed the high level tests and write a specific unit test, directly associated with the fixed code.
Running the test suite should be fast, but sometimes some tests are just slow. Functional tests are always slow, and system tests tend to be slow. We mark these slow tests so that we can skip them using the test runner.
Principle of developing using tests:
Write test first - the test will save a lot of debugging time and setup time for each time you would have to run the manual test to check your code.
Design for testability - Now, if you don’t write your test first, you should at least let the test design your code and not design the test after your code. If you write test first you don’t need to worry about this.
Use Front Door First, this means that you should first try to write tests only using the public interface.
Verify one condition per test. Don’t test more than one thing in a test, as it will make the test hard to read. There is an exception for customer tests, which are story based and those tests will check a complete work-flow.
Comunicate Intent write short tests, which are clean and easy to read and serve as documentation for the code.
Keep testing login out of production code. Don’t add hooks or conditional statements in the production code to help with testing.
Keep tests independent each test should run on it’s own and should also run together with the other tests.
126.96.36.199. Domain Specific Language
Group multiple / related calls into dedicated, helper methods. Give the method an easy to read name. Try to create a Domain Specific Language for your tests.
class TestSuperUser(TestCase): """ Tests for super user. """ def test_rename_ulgy(self): """ Users can be renamed just by calling rename() on the user object. Ugly initialization code. """ username = factory.makeUsername() new_username = factory.makeUsername() configuration = factory.makeSuperConfiguration() configuration.addUser(username) user = configuration.getUser(username) user.enabled = True user.rename(new_username) self.assertTrue(configuration.userExists(new_username)) self.assertFalse(configuration.userExists(username)) def makeUser(self, username, configuration=None): """ Return a new username created for `configuration`. If `configuration` is `None` it will use a new configuration. """ if configuration is None: configuration = factory.makeSuperConfiguration() configuration.addUser(username) user = configuration.getUser(username) user.enabled = True return user def test_rename_clean(self): """ Users can be renamed just by calling rename() on the user object. Clean version. """ username = factory.makeUsername() new_username = factory.makeUsername() user = self.makeUser(username) user.rename(new_username) self.assertTrue(configuration.userExists(new_username)) self.assertFalse(configuration.userExists(username))
188.8.131.52. Dependency injection of system components
For unit testing, we want to make them easy to write, run them fast and without touching the system (filesystem / network / os services).
In order to be useful, methods need to interact with the system.
As a first practice, methods interacting with the system should be grouped and isolated into component dedicated with input / output operations.
When writing tests for code which touches the system, the tests will also use the system. This can slow the tests or create unwanted side effects, since most of the time system resources are persistent.
In some cases, especially when testing code for failures, it is very hard to setup the external system to raise a certain failure condition. For example we have the code which handles a socket which can raise a timeout error. Timeout errors are complicated since they require a certain amount of time to pass before they appear and this can slow down the whole tests.
import socket class ClientWithoutDependecy(object): """ A network client. """ def connect(self, address): """ Try to connect to a server and return False if connection was not successful. """ result = False try: socket.connect(address) result = True except socket.TimeoutError: do_something_on_timeout() result = False return result def test_connect_with_slow_timeout(self): """ When a server does not exist at the address, the timeout is handled in some way. """ client = ClientWithoutDependecy() result = client.connect_to_server('bad.address') # Wait a lot for method to return. self.assertFalse(result) class ClientWithDependecy(object): """ A network client which has its dependencies as class members. """ socket = socket def connect(self, address): """ Try to connect to a server and return False if connection was not successful. """ result = False try: self.socket.connect(address) result = True except socket.TimeoutError: do_something_on_timeout() result = False return result def test_connect_with_fast_timeout(self): """ When a server does not exist at the address, the timeout is handled in some way. """ class TimingOutSocket(object): """ A socket which times out at connection. """ def connect(self, address): raise socket.TimeoutError() client = ClientWithDependecy() client.socket = TimingOutSocket() result = connect_to_server('bad.address') # Returns very fast. self.assertFalse(result)
184.108.40.206. Don’t mock
With great power, comes great responsibility! Don’t abuse the mock library.
Before using a Mock object consider using a minimal implementation or a Bunch object.
Is OK to use the Mock object as part of the patch process, but before using patching consider redesigning the code to support dependency injection.
When youu use the mock library for patching, make sure you don’t replace the patched code with a Mock object. Replace it with a minimal implementation instead.
2.3.4. Structure of a test
Use the Assert, Act, Arrange, pattern: each part must have it’s own paragraph.
Arrange is variable declaration and initialization code. Set up all conditions and environment for testing.
Act is invoking the code being tested. Call the method or trigger the necessary state.
Assert is using the assert methods or any other code to verify that expectations were met.
For integration tests, this can also be called Assemble, Activate, Assert.
The tests needs to be short and easy to read.
Some test might not require the arrange part, but this is usually a code smell and most of the time you should have something in arrange part.
Make sure to test only a single thing at once.
When asserting that acting on a code raised an exception, these two steps might get intertwined and look like the following code. This is OK.
def test_getAllProperties_no_accounts(self): """ An error is raised if no accounts are defined. """ with self.assertRaises(ConfigurationError): some.getAllProperties()
Especially on some integration test or system tests, you also have one last part for a test: the cleanup. This should be implemented using the addCleanup helper which should be called as soon as possible, including in the arrange part.
The arrange part can get very long. Try to move as much code in helper method which can create test fixtures.
When the code is used by multiple tests tests, put it in a dedicated, reusable, method.
def test_section_navigation_long_arrange(self): """ A test which is self contained. """ account = factory.makeTestAccount() browser = factory.makeTestBrowser() browser.open(self.BASE_URL + '/login') browser.setField('username', account.name) browser.setField('password', account.password) browser.clickButton('Submit') browser.open(self.BASE_URL + '/some_section') self.assertEqual('section_title', browser.title) def getAccountAndBrowser(self): """ Object used by almost all tests. """ account = factory.makeTestAccount() browser = factory.makeTestBrowser() self.addCleanup(self.logout, account) return (account, browser) def login(self, browser, account): """ Go to login page and submit username and password. """ browser.open(self.BASE_URL + '/login') browser.setField('username', account.name) browser.setField('password', account.password) browser.clickButton('Submit') def test_section_navigation(self): """ A test which used helper method to get its fixtures. """ account, sut = self.getAccountAndBrowser() self.login(sut, account) sut.open(self.BASE_URL + '/some_section') self.assertEqual('section_title', sut.title)
The arrange part might create multiple fixtures and initialize multiple objects. To make the code easier to read and to make it easier to identify which object is targeted by the test, name the system under test object as sut.
def test_buildProcotol_no_client_response(self): """ The client connection is closed when client connects without sending a message. """ client_factory = self.getClient(port=1234) sut = self.getServerFactory(port=1234) client_protocol = client_factory.connect() sut.buildProtocol(client_protocol) self.assertTrue(client_protocol.closed)
Don’t abuse the debugger. The tests should have a good coverage so that any code can be debugged just by using the debugger on a failed tests. In case you need more than 1 breakpoint in the code, this is a sign that you are missing a test.
Hard work in finding / debugging an error is often an indication of failure in writing good code or good tests.
In case using the automatic breakpoint provided test runner is enough to detect the problem, that the code might be good :)
If an end to end test fails but no integration test fails, than we have at least one missing integration test.
The assert part of the tests can be replaced with plain assert calls. The assertion should check for real behaviour and state changes, and not just fixed values or type checks.
2.3.6. Naming conventions
All test cases should have names prefixed with Test.
Include the tested class name in the name of the test case.
When multiple test cases exist for the same class, suffix the test case with some hints about the special cases in each test case.
class TestSuperButtonInSpace(TestCase): """ Test for super button behavior in space. """ class TestSuperButtonOnEarth(TestCase): """ Test for super button behavior on earth. """
All methods that perform tests should be prefixed with test_.
If testing a specific method, include the exact name of the method, in the test name, at the beginning, just after the test_ marker.
Only use private members when they are used by a single test. Any code which is used by multiple tests should be public and with comprehensive documentation.
When there are multiple tests for the same method, suffix the test with a short underline (“_”) delimited summary. No need to add all details in the name. Just make sure it is unique in the test case. Avoid long descriptions, you can add everything in the docstring.
def test_getAllProperties_no_accounts(self): """ An empty dictionary is returned if no accounts are defined. """
2.3.7. Test fixtures, addCleanup, setUp and tearDown
Reusing base test cases and refactoring fixtures into dedicated methods are great ways of reducing code duplications.
Avoid using setUp and tearDown. Instead call helper methods to get the test fixtures at the beginning of each tests. Use addCleanup to have code called at the end of a test.
In this way, when tests are refactored is much easier to detect which helper code is no longer needed as it will be reported by the coverage tools. With code in setUp, the helper code will always be executed and is much harder to detect and we are left with objects created in setUp but not used in any other test.
As a code smell, is not of to use setUp and tearDown for classed which have test methods. Is OK to use setUp/tearDown for high level test cases which don’t have any self test method and are designed to be reused by more than 2 other final test cases.
In the same time, objects created in setUp will create a stronger dependency between the tests using that object. Changing the way an object is created in setUp in order to please a test might have the unwanted consequence of seeing many other tests failing.
We write test cases based on the class under test. For example, when we have an object called Account which can have two important states: Application and OS, we will write two test case AccountOSTestCase and AccountApplicationTestCase.
All tests from a specific test case will need to instantiate the same object, and to reduce duplication you should create a helper method which will get a new instance of the system under test.
class BadStorageTestCase(TestCase): """ A test that is hard to read. """ def setUp(self): stream = SomeStream() self.storage = StreamStorage(stream) self.object_1 = NewStoredObject(name='one') self.storage.add(self.object_1) self.object_2 = NewStoredObject(name='two') self.storage.add(self.object_2) super(BadHandlerTestCase, self).setUp() def test_get_all_objects(self): """ Without arguments, returns all objects for the storage. """ # This test smells, since it had no arrange part. result = self.storage.get() self.assertEqual(2, len(result)) self.assertContains(self.object_1, result) self.assertContains(self.object_2, result) def test_get_filtered_objects(self): """ A name can be specified to filter results. """ # This test smells, since it had no arrange part. result = self.storage.get(name='one') self.assertEqual(1, len(result)) self.assertContains(self.object_1, result) self.assertNotContains(self.object_2, result) class StorageMuchBetterTestCase(TestCase): """ A test that is easier to read. """ def getStorage(self): """ Return a new storage which is checked at the end of the test to make sure it is left clean. """ stream = SomeStream(self) storage = StreamStorage(stream) def assert_cleanup(storage): """ Called at the end of each test to make sure storage is in a sane state. """ self.assertIsEmpty(storage.getAll()) self.addCleanup(assert_cleanup, storage) return storage def test_get_all_objects(self): """ Without arguments, returns all objects for the storage. """ sut = self.getStorage() object_1 = NewStoredObject() sut.add(object_1) object_2 = NewStoredObject() sut.add(object_2) result = sut.get() self.assertEqual(2, len(result)) self.assertContains(object_1, result) self.assertContains(object_2, result) def test_get_all_objects(self): """ A name can be specified to filter results. """ sut = self.getStorage() object_1 = NewStoredObject(name='one') sut.add(object_1) object_2 = NewStoredObject(name='two') sut.add(object_2) result = sut.get(name='one') self.assertEqual(1, len(result)) self.assertContains(object_1, result) self.assertNotContains(object_2, result)
Try to put as much cleanup code in the addCleanup method and not after the assert block. If a test fails, the rest of the assert block is not executed, and putting everything in a try/finally increase indentation. You can register object for cleanup by using dedicated creation method.
class BadTestCase(TestCase): def test_someMethod_with_test_bad_cleanup(self): """ When an assertion fails, cleanup is not called. """ file = open('some_file') operator = SomeFancy(file) operator.read() self.assertEqual('something', operator.getAllContent()) # When assert fails, the remaining code is not executed. file.close() os.rm(file.path) class GoodTestCase, self(.tearDown()TestCase): def close_file(self, file): """ Do the best to close and remove the file. """ try: file.close() os.rm(file.path) except: # Pass or record the files which were not closed # and fail with more details. pass def getOpenedFile(self, path): """ Creation method which also registers the object for cleanup. """ file = open('some_file') self.addCleanup(close_file, file) return file def test_someMethod_with_test_bad_cleanup(self): """ When an assertion fails, cleanup is still called via cleanup. """ file = self.getOpenedFile('some_file') operator = SomeFancy(file) operator.read() self.assertEqual('something', operator.getAllContent())
You can still reuse object, in case creating a new instance takes a long time, but this should be an exception and not the rule.
2.3.8. Tests description - docstrings
Good tests can help document and define what something is supposed to do, so dedicate effort to write good docstrings.
Each test should have a description (docstring) with information about the purpose of the test or any other additional info that could help another person to understand the test.
Writing docstring for tests is not easy, but doing so will reduce the WTF/minute.
Describe in simple plain English what you are testing and what is the expected behaviour.
Think of the test’s docstring as an extension of documentation for the method under tests.
For integration tests add notes about pre-conditions or other requirements.
Add a brief description and don’t describe how the test is done. Detailed information about the test should be available by reading the code implementing the tests. When code fails to speak for itself use inline comments.
Avoid including the name of the method under tests. The test method name should already include it.
When testing for raised exceptions avoid adding the exception name in the test description. Just inform that an exception is raised. By reading the test code, it should be easy to get more details about the exception.
Here are some tips from Jonathan Lange as a handy five-step guide:
1. Write the first docstring that comes to mind. It will almost certainly be:
"""Test that input is parsed correctly."""
Get rid of “Test that” or “Check that”. We know it’s a test.:
"""Input should be parsed correctly."""
Seriously?! Why’d you have to go and add “should”? It’s a test, it’s all about “should”.
"""Input is parsed correctly."""
“Correctly”, “properly”, and “as we expect” are all redundant. Axe them too.
"""Input is parsed."""
Look at what’s left. Is it saying anything at all? If so, great. If not, consider adding something specific about the test behaviour and perhaps even why it’s desirable behaviour to have.
""" Input is parsed into an immutable dict according to the config schema, so we get config info without worrying about input validation all the time. """
2.3.9. Dealing with flaky tests
Once the test suite grows to more than a few hundred tests and you run the test on more than a couple of test environment you will experience flaky test.
The functional / integration tests are prone to result in flaky results, especially if they are executed on system with high load, or slow or exotic environment.
We have about 5000 tests, executed on 20 environments so you end up with 100.000 tests executed on each run. A single test failure will make the whole commit red and will block the merge.
To mitigate this our automated testing infrastructure allow re-running all the tests on a single environment.
Here are the pages I used to create this page.