Types of tests

There are a number of different kinds of tests, which will be discussed here.

Firstly there are positive tests and negative tests. Positive tests check that something works, for example testing that a function that multiplies some numbers together outputs the correct answer. Negative tests check that something generates an error when it should. For example nothing can go quicker than the speed of light, so a plasma physics simulation code may contain a test that an error is outputted if there are any particles faster than this, as it indicates there is a deeper problem in the code.

In addition to these two kinds of tests, there are also different levels of tests which test different aspects of a project. These levels are outlined below and both positive and negative tests can be present at any of these levels. A thorough test suite will contain tests at all of these levels (though some levels will need very few).

Table of contents

Overview of testing

Smoke testing: Very brief initial checks that ensures the basic requirements required to run the project hold. If these fail there is no point proceeding to additional levels of testing until they are fixed.

Unit testing: A level of the software testing process where individual units of a software are tested. The purpose is to validate that each unit of the software performs as designed.

Integration testing: A level of software testing where individual units are combined and tested as a group. The purpose of this level of testing is to expose faults in the interaction between integrated units.

System testing: A level of the software testing process where a complete, integrated system is tested. The purpose of this test is to evaluate whether the system as a whole gives the correct outputs for given inputs.

Acceptance testing: A level of the software testing process where a system is tested for acceptability. The purpose of this test is to evaluate the system’s compliance with the project requirements and assess whether it is acceptable for the purpose.

Here’s an analogy: during the process of manufacturing a ballpoint pen, the cap, the body, the tail, the ink cartridge and the ballpoint are produced separately and unit tested separately. When two or more units are ready, they are assembled and integration testing is performed, for example a test to check the cap fits on the body. When the complete pen is integrated, system testing is performed to check it can be used to write like any pen should. Acceptance testing could be a check to ensure the pen is the colour the customer ordered.

There is also another kind of testing called regression testing. Regression testing is a type of testing that can be performed at any of the four main levels and compares the results of tests before and after a change is made to the code, and gives an error if these are different.

These different types of tests will now be discussed in more detail.

Smoke testing

Smoke tests (also known as build verification tests) are a special kind of initial checks designed to ensure very basic functionality as well as some basic implementation and environmental assumptions. Smoke tests are generally run at the very start of each testing cycle as a sanity check before running a more complete test suite.

The idea behind this type of test is to help to catch big red flags in an implementation and to bring attention to problems that might indicate that further testing is either not possible or not worthwhile. Normally, the tester is asking whether any components are so obviously or badly broken that the build is not worth testing or some components are broken in obvious ways that suggest a corrupt build or some critical fixes that are the primary intent of the new build didn’t work. Smoke tests are not very extensive, but should be extremely quick. If a change to a project causes it to fail a smoke test, its an early signal that core assertions were broken and that you should not devote any more time to testing until the problem is resolved. For example if a function that reads in the data a project requires to run is broken there’s no point testing any further before that’s fixed. The typical result of a failed smoke test is rejection of the build (testing of the build stops) not just a new set of bug reports.

Unit tests

Unit tests are responsible for testing individual elements of code in an isolated and highly targeted way. The functionality of individual functions and classes are tested on their own. The purpose is to validate that each unit of the software performs as designed. A unit is the smallest testable part of any software. In procedural programming, a unit may be an individual program, function or procedure. In object-oriented programming the smallest unit is typically a method. It usually has one or a few inputs and usually a single output. Any external dependencies should be replaced with stub or mock implementations to focus the test completely on the code in question.

Unit tests are essential to test the correctness of individual code components for internal consistency and correctness before they are placed in more complex contexts. The limited extent of the tests and the removal of dependencies makes it easier to hunt down the cause of any defects. It also is the best time to test a variety of inputs and code branches that might be difficult to hit later on. For example system tests are often time consuming to run and it will likely be impractical to have system tests for every possible path through a code that has more than a few conditional statements. Unit tests are smaller, faster, and so it is more practical to cover all possible cases with them.

Often, after any smoke tests, unit tests are the first tests that are run when any changes are made.

Benefits of unit testing

If a researcher makes a change to a piece of code or how it is run then how can they be sure that doing so has not broken something? They may run a few tests, but without testing every small piece of code individually how can they be certain? Unit testing gives researchers that certainty, and allows them to be confident when changing and maintaining their code.

Here’s a little example. Say a researcher has a small function that does one simple thing (here only a single line for brevity). In this example this will be raising a number to the 5th power:

def take_fifth_power(x):
  result = x * x * x * x * x
  return result

The unit test for this function could look like this:

def test_take_fifth_power():
  assert take_fifth_power(1.5) == 7.59375

So it checks that the correct result is outputted for a given input. If not the test will fail. The researcher carries on with their work. In the middle of it they decide to tidy up this function, multiplying the number five times like this is a bit crude. They change the result = x * x * x * x * x line to result = x * 5. Next time they run their unit tests, this test will fail, because they just made a mistake. Maybe they needed a coffee, maybe their finger slipped, maybe their coworker shot them in the ear with a nerf dart and distracted them, but when they were tidying up this function they should have written result = x ** 5 not result = x * 5. The failed test will flag up the mistake and it can quickly be corrected. If a mistake like this went unobserved it could lead to serious errors in the researcher’s work.

So unit testing leads to more reliable code, but there are other benefits too. Firstly it makes development faster by making bugs easier to find. Larger-scale tests which test large chunks of code (while still useful) have the disadvantage that if they fail it is difficult to pinpoint the source of the bug. Because unit tests by their very definition test small pieces of code they help developers find the cause of a bug much more quickly than higher-level tests or code with no tests at all. Unit tests also make fixing bugs faster and easier because they catch bugs early while they only impact small individual units. If bugs are not detected early via unit tests then it may be a long time before they are discovered, impacting later work that built on the faulty code. This means much more code is at risk and fixing the bug is more time consuming.

The other major benefit of unit testing is that it strongly incentivises researchers to write modular code because modular code is far easier to write unit tests for. Modular code is code that is broken up into manageable chunks which each accomplish simple tasks. This is typically achieved by dividing the code into functions and groups of functions. In contrast a script which is just one long continuous series of lines which produces a result is highly non-modular.

Modular code is much easier to reuse, for example if a researcher has a individual function that does some Useful Thing and in a future project they need to do that thing again it is trivial to copy or import the function. In contrast if the code that does this Useful Thing is entwined with a great deal of other code in a long script it is much harder to separate it out for re-use.

Unit testing tips

  • Many testing frameworks have tools specifically geared towards writing and running unit tests.

  • Isolate the development environment from the test environment.

  • Write test cases that are independent of each other. For example, if a unit A utilises the result of another unit B supplies you should test unit A with a test double, rather than actually calling the unit B. If you don’t do this your test failing may be due to a fault in either unit A or unit B, making the bug harder to trace.

  • Aim at covering all paths through a unit. Pay particular attention to loop conditions.

  • In addition to writing cases to verify the behaviour, write cases to ensure the performance of the code. For example, if a function that is supposed to add two numbers takes several minutes to run there is likely a problem.

  • If you find a defect in your code write a test that exposes it. Why? First, you will later be able to catch the defect if you do not fix it properly. Second, your test suite is now more comprehensive. Third, you will most probably be too lazy to write the test after you have already fixed the defect. Say a code has a simple function to classify people as either adults or children:

def adult_or_child(age):

  # If the age is greater or equal to 18 classify them as an adult
  if age >= 18:
    person_status = 'Adult'

  # If the person is not an adult classify them as a child
    person_status = 'Child'

  return person_status

And say this code has a unit test like this:

def test_adult_or_child():

  # Test that an adult is correctly classified as an adult
  assert adult_or_child(22) == 'Adult'

  # Test that an child is correctly classified as a child
  assert adult_or_child(5) == 'Child'


There’s a problem with this code that isn’t being tested: if a negative age is supplied it will happily classify the person as a child despite negative ages not being possible. The code should throw an error in this case. So once the bug is fixed:

def adult_or_child(age):

# Check age is valid
if age < 0:
  raise ValueError, 'Not possible to have a negative age'

# If the age is greater or equal to 18 classify them as an adult
if age >= 18:
  person_status = 'Adult'

# If the person is not an adult classify them as a child
  person_status = 'Child'

return person_status

go ahead and write a test to ensure that future changes in the code can’t cause it to happen again:

def test_adult_or_child():

#Test that an adult is correctly classified as an adult
assert adult_or_child(22) == 'Adult'

# Test that an child is correctly classified as a child
assert adult_or_child(5) == 'Child'

# Test that supplying an invalid age results in an error
with pytest.raises(ValueError):

Integration testing

Integration testing is a level of software testing where individual units are combined and tested as a group. While unit tests validate the functionality of code in isolation, integration tests ensure that components cooperate when interfacing with one another. The purpose of this level of testing is to expose faults in the interaction between integrated units.

For example, maybe a unit that reads in some data is working and passes its unit tests, and the following unit that cleans up the data once it’s been read in is also working and passes its tests. However say the first unit outputs the data as (time_data, temperature_data) but the function that cleans the data expects input of the form (temperature_data, time_data). This can obviously lead to bugs. While the units are correct there in an error in their integration.

An example of an integration test for this case could be to supply a test data file, use these functions to read it in and clean it, and check the resulting cleaned data against what would be expected. If a bug like this is present then the cleaned data outputted would be very unlikely to match the expected result, and an error would be raised.

Integration testing is particularly important in collaborative projects where different people work on different parts of the code. If two different people complete separate units and then need to integrate then integration issues are more likely as neither may understand the other’s code. A famous example of this is a multi-million dollar satellite which crashed because one piece of code outputted distance data in feet, while another assumed data in meters. This is another example of an integration issue.

A sub type of integration testing is system integration testing. This tests the integration of systems, packages and any interfaces to external organizations (such as Electronic Data Interchange, Internet). Depending on the nature of a project system integration testing may or may not be applicable.


There are several different approaches to integration testing. Big Bang is an approach to integration testing where all or most of the units are combined together and tested at one go. This approach is taken when the testing team receives the entire software in a bundle. So what is the difference between Big Bang integration testing and system testing? Well, the former tests only the interactions between the units while the latter tests the entire system.

Top Down is an approach to integration testing where top-level sections of the code (that themselves contain many smaller units) are tested first and lower level units are tested step by step after that. So is a code can be split into the main steps A, B, and C, and each of those contain steps to complete them, and these steps may have substeps like:

  • A

  • A.1

    • A.1.1

    • A.1.2

  • A.2

  • B

  • B.1

  • B.2

    • B.2.1

    • B.2.2

    • B.2.3

  • B.3

  • C

  • C.1

    • C.1.1

    • C.1.2

  • C.2

    • C.2.1

    • C.2.2

So in the top down approach the integration between sections at the top level (A, B and C) are tested, then integration between sections at the next level (for example, A.1 -> A.2) and so on. Testing upper level units by running all the code they contain including running lower level ones can lead to upper level tests breaking due to bugs in low level units. This is undesirable, so to prevent this the lower level sections should not be run, but test stubs should be used to simulate the outputs from them.

Bottom Up is an approach to integration testing where integration between bottom level sections are tested first and upper-level sections step by step after that. Again test stubs should be used, in this case to simulate inputs from higher level sections.

Sandwich/Hybrid is an approach to integration testing which is a combination of Top Down and Bottom Up approaches.

Which approach you should use will depend on which best suits the nature/structure of your project.

Integration testing tips

  • Ensure that you have a proper Detail Design document where interactions between each unit are clearly defined. It is difficult or impossible to perform integration testing without this information.

  • Make sure that each unit is unit tested and fix any bugs before you start integration testing. If there is a bug in the individual units then the integration tests will almost certainly fail even if there is no error in how they are integrated.

  • Use mocking/stubs where appropriate.

System tests

Once integration tests are performed, another level of testing called system testing can begin. System testing is a level of software testing where a complete and integrated software is tested. The tester supplies the program with input and verifies if the program’s output is correct. If it is not then there is a problem somewhere in the system. Note that this does not have to be done manually, it can be automated. The purpose of these tests is to evaluate the system’s compliance with the specified requirements. In many ways, system testing acts as an extension to integration testing. The focus of system tests are to make sure that groups of components function correctly as a cohesive whole. However, instead of focusing on the interfaces between components, system tests typically evaluate the outward functionality of a full piece of software. This set of tests ignores the constituent parts in order to gauge the composed software as a unified entity. Because of this distinction, system tests usually focus on user- or externally-accessible outputs.

System testing can also test features of the system other than correctness. Examples include:

  • Performance testing: does the program performance meet the minimum requirements? A performance test may measure how long the system takes to run in a given case.

  • Migration testing: does the program work when transferred to another computational environment?

  • Stress/scale/load testing: testing how the program behaves when under stress, for example, when required to process very large volumes of data.

  • Usability testing: how user-friendly is the program (more common in commercial software, tests typically conducted by humans rather than automated).

  • Recovery testing: Can the program continue if errors occur (again, more common in commercial software).

System testing tips

System tests, also called end-to-end tests, run the program, well, from end to end. As such these are the most time consuming tests to run. Therefore you should only run these if all the lower-level tests (smoke, unit, integration) have already passed. If they haven’t fix the issues they have detected first before wasting time running system tests.

Because of their time-consuming nature it will also often be impractical to have enough system tests to trace every possible route through a program, especially is there are a significant number of conditional statements. Therefore you should consider the system test cases you run carefully and prioritise:

  • The most common routes through a program.

  • The most important routes for a program. For example, the LIGO detector aims to find gravitational wave events, which are extremely rare. If there’s a bug in that path through the program which monitors the detector then it’s a huge problem.

  • Cases that are prone to breakage due to structural problems within the program. Though ideally it’s better to just fix those problems, but cases exist where this may not be feasible.

Because system tests can be time consuming it may be impractical to run them very regularly (such as multiple times a day after small changes in the code). Therefore it can be a good idea to run them each night (and to automate this process) so that if errors are introduced that only system testing can detect the programmer will be made of them relatively quickly.

Acceptance testing

Acceptance tests are one of the last tests types that are performed on software prior to delivery. Acceptance testing is used to determine whether a piece of software satisfies all of the requirements from the business or user’s perspective. Does this piece of software do what it needs to do? These tests are sometimes built against the original specification.

Because research software is typically written by the researcher that will use it (or at least with significant input from them) acceptance tests may not be necessary.

Regression testing

Regression testing is a style of testing that focuses on retesting after changes are made. The results of tests after the changes are compared to the results before, and errors are raised if these are different. Regression testing is intended to ensure that changes (enhancements or defect fixes) to the software have not adversely affected it. The likelihood of any code change impacting functionalities that are not directly associated with the code is always there and it is essential that regression testing is conducted to make sure that fixing one thing has not broken another. Regression testing can be performed during any level of testing (unit, integration, system, or acceptance) but it is mostly relevant during system testing. Any test can be reused, and so any test can become a regression test.

Regression testing is obviously especially important in team working, but it is surprisingly easy to break your own code without noticing it, even if you are working on your own. And because regression testing is next to impossible to do satisfactorily by hand (it’s simply too tedious), it’s an obvious case for automation.

Regression tests are written by first running the (or part of the) code for given inputs and recording the outputs. This could be done by writing input files and saving the corresponding output files. These outputs serve as the expected outputs from the program given the corresponding inputs. Regression tests are then written. Each regression test runs the code for the set of inputs. It then compares the output from the code to the expected outputs, and raises an error if these do not match.

Regression testing approaches differ in their focus. Common examples include:

  • Bug regression: We retest a specific bug that has been allegedly fixed.

  • Old fix regression testing: We retest several old bugs that were fixed, to see if they are back. (This is the classical notion of regression: the program has regressed to a bad state.)

  • General functional regression: We retest the project broadly, including areas that worked before, to see whether more recent changes have destabilized working code.

  • Conversion or port testing: The program is ported to a new platform and a regression test suite is run to determine whether the port was successful.

  • Configuration testing: The program is run with a new device or on a new version of the operating system or in conjunction with a new application. This is like port testing except that the underlying code hasn’t been changed–only the external components that the software under test must interact with.


Regression tests are not guaranteed to test all parts of the code. Most importantly, regression tests do not test if the result outputted by a piece of code is correct, only that is has not changed. This the remit of other kinds of tests, though regression tests can serve as the starting point for introducing tests for correctness, by both the use of analytical solutions, and test functions which read output files and check the data for correctness, as defined by a researcher.

Runtime testing

Runtime tests are tests that run as part of the program itself. They may take the form of checks within the code, as shown below:

population = population + people_born - people_died

// test that the population is positive
if (population < 0):
error( 'The number of people can never be negative' )

Another example of a use of runtime tests is internal checks within functions that verify that their inputs and outputs are valid, as shown below:

function add_arrays( array1, array2 ):

// test that the arrays have the same size
if (array1.size() != array2.size()):
  error( 'The arrays have different sizes!' )

output = array1 + array2

if (output.size() != array1.size()):
  error( 'The output array has the wrong size!'' )

return output

Advantages of runtime testing:

  • Run within the program, so can catch problems caused by logic errors or edge cases.

  • Makes it easier to find the cause of the bug by catching problems early.

  • Catching problems early also helps prevent them escalating into catastrophic failures. It minimises the blast radius.

Disadvantages of runtime testing:

  • Tests can slow down the program.

  • What is the right thing to do if an error is detected? How should this error be reported? Exceptions are a recommended route to go with this.

Test driven development

One way of ensuring tests are not neglected in a project is to adopt test-driven development. This is an approach in which unit tests are written before the code. The tests thus describe a “contract” that the code is expected to comply with. This ensures that the code will be correct (as far as can be enforced by the testing contract) as written, and it provides a useful framework for thinking about how the code should be designed, what interfaces it should provide, and how its algorithms might work. This can be a very satisfying mental aid in developing tricky algorithms.

Once the tests are written, the code is developed so that it passes all the associated tests. Testing the code from the outset ensures that your code is always in a releasable state (as long as it passes the tests!). Test driven development forces you to break up your code into small discrete units, to make them easier to test; the code must be modular. The benefits of this were discussed in the section on unit testing.

An alternative development approach is behaviour driven development. Simply put test driven development tests “has the thing been done correctly?”, behaviour driven development tests “has the correct thing been done?”. It is more often used in commercial software development to focus development on making the software as simple and effective as possible for users. User experience is very rarely at the heart of code written for the purposes of research, but there are cases where such software is written with a large user-base in mind. In such cases behaviour-driven development is a path worth considering.