testingelixirhttpoisonmox

How to mock HTTPoison with Mox?

Background

I have library that uses HTTPoison for some functionality I need to test. To achieve this I am using Mox, which I believe is the universal mocking library for Elixir (even though there are others this one has the seal of approval of José Valim)

Problem

Everything is going fine, I define my mocks in the test_helpers.exs:

    ExUnit.start()
    Mox.defmock(HTTPoisonMock, for: HTTPoison)

And I set up my dummy tests:

defmodule Alfred.Test.Http.Test do
  use ExUnit.Case, async: true
  import Mox
  # Make sure mocks are verified when the test exits
  setup :verify_on_exit!

  describe "get" do
    test "works on OK" do
      HTTPoisonMock
      |> get(:get, fn _ -> 1 end)

      assert HTTPoisonMock.get(1) == 1
    end
  end
end

The problem here is I can't run them:

module HTTPoison is not a behaviour, please pass a behaviour to :for

Mock Contracts, not implementations

Now, I know José Valim supports this ideology, thus everything we should mock should have a contract. But HTTPoison is not mine and it doesn't have one. So this brings me to the following question:

Solution

  • HTTPoison uses the behaviour HTTPoison.Base, so in this particular case, you are all set.

    How can I mock 3rd libraries that don’t offer behaviours using Mox?

    Usually, we do that with dependency injection. Whenever you need your 3rd-party, you simply pass it as a parameter and pass another module / mock in the test environment.