StreamData Property Testing -

I was recently lamenting Elixir’s lack of Pytest-like parameterized testing (or at least my inability to find such).

I’d found several examples including ExUnit.Parameterized, but I could not find a solution that output the actual parameters from among hundreds that caused the failed assert.

Maciej Lukksepp (icejam_@hachyderm.io) responded that StreamData property testing should solve the problem, and provided a useful starting point.

An example test for JCB card brand using StreamData property tests:

defmodule SimpleCardBrandJCB do
  @moduledoc """
  JCB tests.
  """

  use ExUnit.Case, async: true
  use ExUnitProperties

  property "JCB" do
    check all(
            fragment <- StreamData.integer(3528..3589),
            pan_length <- StreamData.integer(16..19)
          ) do
      assert SimpleCardBrand.card_brand(Integer.to_string(fragment), pan_length) == {:ok, :jcb}
    end
  end
end

Which provides an explicit description of an error:

➜  simplecardbrand git:(main) ✗ mix test
.......

  1) property JCB (SimpleCardBrandJCB)
     test/jcb_test.exs:9
     Failed with generated values (after 6 successful runs):

         * Clause:    fragment <- StreamData.integer(3528..3589)
           Generated: 3530

         * Clause:    pan_length <- StreamData.integer(16..19)
           Generated: 16

     Assertion with == failed
     code:  assert SimpleCardBrand.card_brand(Integer.to_string(fragment), pan_length) == {:ok, :jcb}
     left:  {:ok, :rupay}
     right: {:ok, :jcb}
     stacktrace:
       test/jcb_test.exs:14: anonymous fn/3 in SimpleCardBrandJCB."property JCB"/1
       (stream_data 0.6.0) lib/stream_data.ex:2367: StreamData.shrink_failure/6
       (stream_data 0.6.0) lib/stream_data.ex:2327: StreamData.check_all/7
       test/jcb_test.exs:10: (test)

......................................
Finished in 0.1 seconds (0.1s async, 0.00s sync)
8 doctests, 1 property, 37 tests, 1 failure

Thanks Maciej.