Mock

A mocking library for the Elixir language.

We use the Erlang meck library to provide module mocking functionality for Elixir. It uses macros in Elixir to expose the functionality in a convenient manner for integrating in Elixir tests.

See the full reference documentation.

Installation

First, add mock to your mix.exs dependencies:

def deps do
  [{:mock, "~> 0.3.0", only: :test}]
end

and run $ mix deps.get.

Example

The Mock library provides the with_mock macro for running tests with mocks.

For a simple example, if you wanted to test some code which calls HTTPotion.get to get a webpage but without actually fetching the webpage you could do something like this.

defmodule MyTest do
  use ExUnit.Case, async: false

  import Mock

  test "test_name" do
    with_mock HTTPotion, [get: fn(_url) -> "<html></html>" end] do
      HTTPotion.get("http://example.com")
      # Tests that make the expected call
      assert_called HTTPotion.get("http://example.com")
    end
  end
end

You can also assert for number of counts for a call to happen

assert called 1, HTTPotion.get("http://example.com")
assert_called 1, HTTPotion.get("http://example.com")

And you can mock up multiple modules with with_mocks.

opts List of optional arguments passed to meck. :passthrough will passthrough arguments to the original module. Pass [] as opts if you don't need this.

defmodule MyTest do
  use ExUnit.Case, async: false

  import Mock

  test "multiple mocks" do
    with_mocks([
      {Map,
       [],
       [get: fn(%{}, "http://example.com") -> "<html></html>" end]},
      {String,
       [],
       [reverse: fn(x) -> 2*x end,
        length: fn(_x) -> :ok end]}
    ]) do
      assert Map.get(%{}, "http://example.com") == "<html></html>"
      assert String.reverse(3) == 6
      assert String.length(3) == :ok
    end
  end
end

You can mock functions that return different values depending on the input:

defmodule MyTest do
  use ExUnit.Case, async: false

  import Mock

  test "mock functions with multiple returns" do
    with_mocks(HTTPotion, [
      get: fn
        "http://example.com" -> "<html>Hello from example.com</html>"
        "http://example.org" -> "<html>example.org says hi</html>"
      end
    ]) do
      assert HTTPotion.get("http://example.com") == "<html>Hello from example.com</html>"
      assert HTTPotion.get("http://example.org") == "<html>example.org says hi</html>"
    end
  end
end

You can mock functions in the same module with different arity. The same way you could mock function with optional args.

defmodule MyTest do
  use ExUnit.Case, async: false

  import Mock

  test "mock functions with different arity" do
    with_mock String,
      [slice: fn(string, range)      -> string end,
       slice: fn(string, range, len) -> string end]
    do
      assert String.slice("test", 1..3) == "test"
      assert String.slice("test", 1, 3) == "test"
    end
  end
end

An additional convenience macro test_with_mock is supplied which internally delegates to with_mock. Allowing the above test to be written as follows:

defmodule MyTest do
  use ExUnit.Case, async: false

  import Mock

  test_with_mock "test_name", HTTPotion,
    [get: fn(_url) -> "<html></html>" end] do
    HTTPotion.get("http://example.com")
    assert_called HTTPotion.get("http://example.com")
  end
end

The test_with_mock macro can also be passed a context argument allowing the sharing of information between callbacks and the test

defmodule MyTest do
  use ExUnit.Case, async: false

  import Mock

  setup do
    doc = "<html></html>"
    {:ok, doc: doc}
  end

  test_with_mock "test_with_mock with context", %{doc: doc}, HTTPotion, [],
    [get: fn(_url, _headers) -> doc end] do

    HTTPotion.get("http://example.com", [foo: :bar])
    assert_called HTTPotion.get("http://example.com", :_)
  end
end

The with_mock creates a mock module. The keyword list provides a set of mock implementation for functions we want to provide in the mock (in this case just get). Inside with_mock we exercise the test code and we can check that the call was made as we expected using called and providing the example of the call we expected (the second argument :_ has a special meaning of matching anything).

You can also pass the option :passthrough to retain the original module functionality. For example

defmodule MyTest do
  use ExUnit.Case, async: false
  import Mock

  test_with_mock "test_name", IO, [:passthrough], [] do
    IO.puts "hello"
    assert_called IO.puts "hello"
  end
end

The setup_with_mocks mocks up multiple modules prior to every single test along with calling the provided setup block. It is simply an integration of the with_mocks macro available in this module along with the setup macro defined in elixir's ExUnit.

defmodule MyTest do
  use ExUnit.Case, async: false
  import Mock

  setup_with_mocks([
    {Map, [], [get: fn(%{}, "http://example.com") -> "<html></html>" end]}
  ]) do
    foo = "bar"
    {:ok, foo: foo}
  end

  test "setup_with_mocks" do
    assert Map.get(%{}, "http://example.com") == "<html></html>"
  end
end

The behaviour of a mocked module within the setup call can be overridden using any of the methods above in the scope of a specific test. Providing this functionality by setup_all is more difficult, and as such, setup_all_with_mocks is not currently supported.

Currently, mocking modules cannot be done asynchronously, so make sure that you are not using async: true in any module where you are testing.

Also, because of the way mock overrides the module, it must be defined in a separate file from the test file.

Tips

The use of mocking can be somewhat controversial. I personally think that it works well for certain types of tests. Certainly, you should not overuse it. It is best to write as much as possible of your code as pure functions which don't require mocking to test. However, when interacting with the real world (or web services, users etc.) sometimes side-effects are necessary. In these cases, mocking is one useful approach for testing this functionality.

Also, note that Mock has a global effect so if you are using Mocks in multiple tests set async: false so that only one test runs at a time.

Help

Open an issue.

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