Building tests - part 1

There is a well-known and widespread unit/integration/function/end-to-end taxonomy of tests that describe what is tested - single program component, single service or an entire solution. There is also a less known taxonomy of how testing is performed - from not having tests at all to the current golden standard of “single method - single test case” to a more advanced techniques - sometimes I call them “levels” of testing, as they build upon each other - like floors in a building. Interestingly, “buildings” of all heights deserve to exist as each “level” has its pros and cons - taller “buildings” are generally harder to build and maintain, but pack more inner space for the same land area - so choosing the right “height” is important for long-term success.

This post consists of two parts:

  • Part 1 - sets up general background and covers the “simpler” approaches to testing (YOU ARE HERE).
  • Part 2 - continues on to talk about more sophisticated techniques necessary to build higher “buildings”.

“Levels” and Test Pyramid

The “levels” I’m talking about in this post are not related to the Test Pyramid. The different bands of the pyramid describe what is tested - a single class, a system, an ecosystem of microservices, or an entire application/solution. What I call “levels” are different approaches and techniques to testing - these describe how it is tested - specifically, how tests are created, executed and reported on.

Levels are orthogonal to the test pyramid bands - in fact, the higher the band, the harder it is to use “high” level; but one can certainly use different levels at different bands. For example, property-based unit tests supported by “standard” integration tests.

NB: “integration testing” term is a bit overloaded - especially in the space of microservices architectures. There’s an interservice integration tests, that verify how different services interact with each other; and intraservice tests that usually verify how the service behaves as a whole. To disambiguate, I’ll call the former “functional” tests, and the latter - “integration” tests.

What do we test, exactly?

To be more specific, let’s write some code to be tested. One quite common shortsight of many tutorials, how-tos and manuals is to pick a very simple use case - to avoid unnecessary complexity and focus attention on the topic itself. However, it is than quite challenging to translate it to a more complex real life situations. So, I’ll use not one, but three examples:

  1. A function without any side-effects (aka pure function)
  2. A method on a class that takes a parameter
  3. A class with a dependency.

For brevity though, I’ll use only small subset of them in the post text, but full listings can be found at GitHub.

Here’s the code under test:

from dataclasses import dataclass
from datetime import date
from dateutil.relativedelta import relativedelta

class User:
    id: int
    name: str
    date_of_birth: date

    def is_older(self, other) -> bool:
        assert(isinstance(other, User))
        return self.date_of_birth < other.date_of_birth

def age_at(user: User, date: date) -> int:
    return max(relativedelta(date, user.date_of_birth).years, 0)

class UserRepository:
    def get(self, id: int) -> User: pass
    def save(self, user: User) -> None: pass

class UserService:
    def __init__(self, user_repo: UserRepository):
        self._repo = user_repo

    def read_user(self, id: int) -> User:
        return self._repo.get(id)

    def update_user_name(self, id: int, new_name: str):
        old_record = self._repo.get(id) = new_name

This is not a TDD session, so we’re not going to evolve this code - it’s already doing what it is supposed to and doing it right (I’ve tested that!)

No tests - no building at all

Image by christoph-mueller; Pixabay License

Sometimes you really don’t want to bother building a house - a temporary accomodation will work just as good. Think of a camping site - even though you need to have some roof over your head, you won’t build a house - more likely just put a tent or something similar.

In software world, the analogy of a camping site is an one-time, an infrequently used or a very small software. In such cases, investing into test infrastructure is quite often not justified - the task can be completed faster and with acceptable quality1 without it.

Other notable use case in this category is testing Infrastructure as Code. A good test should execute “code under test”, and for IaC executing essentially means creating all that infrastructure and running certain assertions against it. This inevitably poses multiple challenges:

  • there needs to be some isolated environment where that infrastructure would be provisioned
  • it almost inevitably incurs costs, sometimes significant costs
  • it’s hard to come up with non-tautological assertions. Example of tautology: check that aws_instance creates an AWS EC2 instance.
  • … and so on

Because of that for smaller organizations it quite often makes sense to keep the IaC code at “no tests” level, especially when the infrastructure is still small and can be comprehended easily.

Building to this level: a conscious decision to not “waste” time on testing.
Pros: fastest to achieve - there’s literally nothing to do.
Cons: everything else.
Should I get here: proooobably not, unless you know what you’re doing, and why.

One method per test case - single-floor landed house (aka bungalow, aka cottage)

When we speak about testing, by default we mean this - tests are written and executed using some 3rd party testing framework, such as JUnit (and friends/clones/forks), unittest, specs, etc.

Long story short, our test suite will look like this (full listing):

class Users:
    jack = User(1, "jack", parser.parse("1999-01-01"))
    jill = User(2, "Jill", parser.parse("2001-06-14"))
    jane = User(3, "Jane", parser.parse("2003-01-01"))

class TestAgeAt(unittest.TestCase):
    def test_age_at_birth(self):
        self.assertEqual(age_at(Users.jack, Users.jack.date_of_birth), 0)

    def test_age_at_some_random_date_after_birth(self):
        self.assertEqual(age_at(Users.jill, parser.parse("2019-11-11")), 18)

    def test_age_at_16th_birthday(self):
        self.assertEqual(age_at(Users.jack, parser.parse("2015-01-01")), 16)

    def test_age_before_born(self):
        with self.assertRaises(AssertionError):
            age_at(Users.jack, parser.parse("1990-01-01"))

Pretty straightforward and unsurprising, right? The main thing here is that we have each test case represented by a dedicated method.

Building to this level: Getting here requires some effort to set up the testing infrastructure - e.g. configure the build tool (like sbt or maven), test runtime (like tox) and/or continuous integration tool/app (e.g. Travis CI). How much effort is required varies between languages, frameworks and build tools, but for the majority of them it’s just following the convention in the code layout or setting up the required configuration.
Pros: I won’t delve too deep into describing the advantages of actually having some automatic tests as they are very well known, but in short it makes capturing errors earlier, increases developer productivity and gives confidence in the software we build.
Cons: Major disavantage that actually gives raise to the next “level” is tediousness: single test case is a single method.
Should I get here: Absolutely, unless you’re happy in a tent.

Detour: same, but without test framework - a cabin

One other interesting approach is to save a bit on the setting up the test infra, and just use the built-in language features. In such case, the module under test exposes a separate entrypoint - a dedicated method or special combination of input parameters - that triggers module’s self check using language’s built-in assertion mechanisms, such as assert statements. This approach can even be stretched to cover “integration test” cases - just provide a separate Main class, or pass an environment variable.


def multiply(i1: int, i2: int) -> int:
    return i1 * i2

def self_test():
    assert(multiply(1, 2) == 2)
    assert(multiply(3, 4) == 12)
    print("Tests passed")

# executing tests:
import ints

This seems hacky (and indeed it is), as the test code is shipped with the production code, but it is less of a problem here, as test code doesn’t add any dependencies and require no installation actions. Moreover, some languages have certain level of support for this feature - e.g. python has doctest module that allows writing and running tests in the documentation strings.

def reverse_string(input: str) -> str:
    This is a docstring  to illustrate doctests

    >>> reverse_string("Hello!")
    >>> reverse_string("")
    >>> reverse_string('Привет!')
    return input[::-1]

if __name__ == "__main__":
    import doctest

Building to this level: the effort required here is significantly less than proper test infrastructure would need - you just use the built-in language features for everything that test framework does - from test discovery to assertions.
Pros: does not need extra tools and fast to implement (good choice for coding interviews).
Cons: test code is shipped with “production” code, limited to built-in assertions and language features, not well suited for large codebases and test suites.
Should I get here: Not really, unless you’ve overgrown the tent, but still can’t afford a proper place.

Data-driven tests - a house with two floors

Image by @pixabay Pexels license

One thing about tests that is missed way too often is that good suite of tests exercise not only “happy path” in “common case”, but also a range of cases with “normal” inputs, edge cases, and potential failures. Simply put, the test suite should define the limits of the applicability of the code under test (i.e. valid inputs and state), and then test it’s behavior:

  • inside the limits and with multiple inputs
  • at the boundary - also known as edge cases
  • outside of the applicability area, to make sure that code behavior is still reasonable (e.g. throws certain type of exception)

The issue is, “multiple good inputs” and “bad inputs” are almost always unintentionally overlooked or intentionally skipped. There aren’t much that can be done to address the unintentional overlooking - forcing the branch coverage can partially help, but it has it’s limitations2; keeping an eye on it during code reviews is an option, but prone to human error. However, the “intentionally skipped” can be mostly cured - as most if it comes from the fact that testing all that pesky corner cases is quite tedious and time-consuming.

The cure for the itch is called data-driven tests3 or parameterized tests. The idea is simple - quite often the behavior can be described as a collection of reference inputs and expected outputs/behaviors. Instead of having each test case pick single input case, let’s instead write the test case as a function that accept inputs/outputs as arguments and assert on expected behavior. This makes creating large suits of test cases (1) much less tedious (which we programmers hate) and (2) much more creative and expressive (which we love).

Most of the mainstream test frameworks either provide parameterized tests feature (e.g. JUnit, NUnit, Boost.Test), have 3rd-party plugin libraries that provide that feature (e.g. python ddt, scalacheck) or can leverage on existing language features (e.g. Go, Scala, Ruby).

So, here’s our test suite extended to use data-driven test (full listing):

class TestAgeAt(unittest.TestCase):
        (Users.jack, Users.jack.date_of_birth, 0),
        (Users.jill, parser.parse("2019-11-11"), 18),
        (Users.jack, parser.parse("2015-01-01"), 16),
        (Users.jane, parser.parse("2203-01-01"), 200),
    def test_age_at(self, user, date, expected_age):
        self.assertEqual(age_at(user, date), expected_age)

        (Users.jack, parser.parse("1990-01-01")),
        (Users.jane, Users.jane.date_of_birth - timedelta(seconds=1)),
    def test_age_before_born(self, user, date):
        with self.assertRaises(AssertionError):
            age_at(user, date)

This style makes it really easy to add more cases if necessary, so handling multiple “normal” cases and edge cases becomes trivial. This testing style literally forces to think of edge cases and applicability limits.

One caveat is that handling different behaviors (e.g. normal output vs. exception) still better be done via adding more test methods. While sometimes it might make sense to have one data-driven test that covers entire range of behaviors of code under test, quite often it can only be done via loops or conditionals inside the test body - and having logic in the test body is usually discouraged. If you weren’t yet bitten by the complex logic in the tests - just trust me, it always happens sooner or later - I’m myself guilty of that “crime” and have done my “sentence” already :smile:.

One thing to emphasize - there’s a significant difference between data-driven test and “classical” test method that does multiple assertions passing different inputs to the code under test. The difference is that “classical” code will fail on the first error/failure, while data-driven test will still execute all of cases - so data-driven test gives more information to the developer to figure out what went wrong. Simply put:

def bad_reciprocal(arg): return 1 / arg

class TestFoo(unittest.TestCase):
    # GOOD - shows three tests, only one of which fails
    @ddt.unpack, 1), (1, 1), (2, 0.5))
    def test_add_one(self, arg, expected_output): 
        self.assertEqual(bad_reciprocal(arg), expected_output)
    # BAD
    def test_add_one_bad(self):
        self.assertEqual(bad_reciprocal(0), 1)  # fails here and does not execute other cases
        self.assertEqual(bad_reciprocal(1), 1)
        self.assertEqual(bad_reciprocal(2), 0.5)

Building to this level: effort is quite different between if you’re building a new test suite and decide to go to this level straight away, or re-building an existing codebase - I’d actually suggest letting the existing test be as they are, and only gradually migrate when changing the application code itself. In any case, building to this level requires a small,but sensible shift in thinking - one should start thinking of code under test explicitly in terms of inputs, outputs, behaviors and invariants.
Pros: Much less tedious and more creative, “forces” to think about applicability limits and edge cases.
Cons: Some test frameworks/languages require additional dependencies (albeit those dependencies are usually lightweight); somewhat incentivizes complex tests that has logic in them; requires some change in thinking.
Should I get here: I would strongly recommend - data-driven tests are a very useful tool that multiplies developer productivity in writing tests, while also improving the quality of test suite (i.e. better coverage, easier to read&understand, etc.)


In this section we’ve covered the more “lightweight” approaches to testing. In the next part, we’ll continue to a more sophisticated - and powerful - techniques that require significant change in thinking about tests.

  1. That is, something that is acceptable short-term and someone will rectify the issues/tech debt if this piece of software suddenly becomes very important or long-living. ↩︎

  2. How many branches does this implementation have def string_size_in_bytes(a): return len(a)? And how many corner cases? And how many bugs (spoiler: utf-8)? ↩︎

  3. Fun fact - acronym of data-driven test (DDT) is the reverse of test-driven development (TDD). ↩︎

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