testing

Elixir Testing with ExUnit

This skill activates when writing, organizing, or improving tests for Elixir applications using ExUnit and related testing tools.

ExUnit Basics

Test Module Structure

defmodule MyApp.MathTest do
  use ExUnit.Case, async: true

  describe "add/2" do
    test "adds two positive numbers" do
      assert Math.add(2, 3) == 5
    end

    test "adds negative numbers" do
      assert Math.add(-1, -1) == -2
    end

    test "adds zero" do
      assert Math.add(5, 0) == 5
    end
  end

  describe "divide/2" do
    test "divides two numbers" do
      assert Math.divide(10, 2) == 5.0
    end

    test "returns error for division by zero" do
      assert Math.divide(10, 0) == {:error, :division_by_zero}
    end
  end
end

Assertion patterns and the full setup/setup_all/conditional-setup catalog live in references/exunit-reference.md.

Test Organization

Using describe blocks

Group related tests:

defmodule MyApp.UserTest do
  use ExUnit.Case

  describe "create_user/1" do
    test "creates user with valid attributes" do
      # ...
    end

    test "returns error with invalid email" do
      # ...
    end
  end

  describe "update_user/2" do
    test "updates user attributes" do
      # ...
    end
  end
end

Test tags

Categorize and filter tests:

@moduletag :integration

@tag :slow
test "expensive operation" do
  # ...
end

@tag :external
test "calls external API" do
  # ...
end

# Run only tagged tests
# mix test --only slow
# mix test --exclude external

Setup and teardown forms (setup, setup_all, describe-scoped setup, conditional setup) live in references/exunit-reference.md. Every setup that starts an owned resource or mutates shared state pairs it with on_exit(fn -> ... end) to restore it — the mechanism behind this workspace's binding test-isolation rules (restore Application.put_env, clean up Process.put keys, remove temp dirs).

Database Testing