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A utility for defining eminently readable example-based tests in Elixir's ExUnit, inspired by example tests in Cucumber.

What are example tests?

Example tests let you define variables along a number of dimensions and re-run the same test body (including all setup) for each combination of variables.

An extremely simple (perhaps too simple!) example:

setup context do
  # context.permissions gets set by the example_test below
  permissions = Map.get(context, :permissions, nil)
  user = AccountsFixtures.user_fixture{permissions: permissions}
  %{user: user}

example_test "users can view the post regardless of permission level",
             | permissions |
             | :admin      |
             | :editor     |
             | :viewer     |
             | nil         |
             %{user: user, permissions: permissions} do
  assert Posts.can_view?(user), "User with #{permissions} permissions should be able to view"

That test will run 4 times, with the :permissions variable from the table being applied to the test's context each time (and therefore being made available to the setup handler). Thus, under the hood this generates four unique tests, equivalent to doing something like this:

setup context do
  permissions = Map.get(context, :permissions, nil)
  user = AccountsFixtures.user_fixture{permissions: permissions}
  %{user: user}

for permission <- [:admin, :editor, :viewer, nil] do
  @permission permission

  @tag permission: @permission
  test "users with #{@permission} can view the post", %{user: user} do
    assert Posts.can_view?(user)

Of course, that example only had a single variable, so you could argue the for version is cleaner. But example_test supports an arbitrary number of variables, so you can describe complex business rules like "users get free shipping if they spend more than $100, or if they buy socks, or if they have the right coupon code":

example_test "grants free shipping based on the marketing site's stated policy",
             | spending_by_category          | coupon      | gets_free_shipping? |
             | %{shoes: 19_99, pants: 29_99} |             | false               |
             | %{shoes: 59_99, pants: 49_99} |             | true                |
             | %{socks: 10_99}               |             | true                |
             | %{pants: 1_99}                | "FREE_SHIP" | true                |
               spending_by_category: spending_by_category,
               coupon: coupon,
               gets_free_shipping?: gets_free_shipping?
             } do
  shipping_cost = ShippingCalculator.calculate_shipping(spending_by_category, coupon)
  free_shipping? = shipping_cost == 0
  assert free_shipping? == gets_free_shipping?

Why example testing?

Example testing reduces toil associated with writing tests that cover a wide variety of different cases. It also localizes the test logic into a single place, so that at a glance you can see how a number of different factors affect the behavior of the system under test.

As a bonus, a table of examples (with their expected results) often matches how the business communicates the requirements of a system, both internally and to customers—for instance, in a table describing shipping costs based on how much a customer spends, where they're located, whether they've bought a promotional product, etc. This means example tests can often be initially created by pulling directly from a requirements document that your product folks provided, and the product folks can later read the tests (or at least the examples table) if they want to verify the behavior of the system.

Example tests versus property tests

Example tests cover a lot of the same ground as property-based testing. Both allow you to write fewer tests while covering more of your system's behavior. This library is not a replacement for property tests, but rather complimentary to them.

There are a few reasons you might choose to write an example test rather than a property test:

  • Ease of writing: Property tests take a lot of practice to get good at writing. They're often quite time consuming to produce, and even when you think you've adequately described the parameters to the system.
  • For communication with other stakeholders: An example test table can be made readable by non-programmers (or non-Elixir programmers), so they can be a good way of showing other people in your organization which behaviors of the system you've verified. Because they can compactly express a lot of test cases, they're much more suitable for this than saying "go read the title of every line in this file that starts with test."
  • For verifying the exact scenarios described by other stakeholders: Sometimes the edges of a particular behavior may be fuzzy—not just to you, but in the business domain as well. Hammering out hard-and-fast rules may not be necessary or worth it, so property tests that exercise the boundaries would be overkill. In contrast, when your product folks produce a document that describes the behavior of particular scenarios, you can encode that in a table and ensure that for the cases that are well-specified, the system behaves correctly.

When would you write a property test instead of an example tests?

  • When you want the absolute highest confidence in your code.
  • When the correctness of a piece of code is important enough to merit a large time investment in getting the tests right.
  • When the system's behavior at the edges is well specified.

Installation and writing your first test

  1. Add example_test to your mix.exs dependencies:

     def deps do
         {:example_test, "~> 0.0.1", only: [:test]},
  2. Run $ mix deps.get to download the package

  3. Write your first example test by adding import ExampleTest to the top of your test module, and using the example_test macro.

    You can optionally include a separator between the header and body of the table (like |--------|-------|).

    The header of your table will be parsed as atoms to pass into your test context. The body cells of the table can be any valid Elixir expression, and empty cells will produce a nil value.

    A dummy example:

     defmodule MyApp.MyModuleTest do
       use ExUnit.Case, async: true
       import ExampleTest
       example_test "behaves as expected",
                    | variable_1       | variable_2           | etc     |
                    | %{foo: :bar}     | div(19, 3)           | false   | 
                    | "bip bop"        | String.upcase("foo") | true    |
                    | ["whiz", "bang"] | :ok                  |         |
                    |                  | nil                  | "maybe" |
                      variable_1: variable_1,
                      variable_2: variable_2,
                      etc: etc
                    } do
           assert MyModule.valid_combination?(variable_1, variable_2, etc)