Monthly Downloads: 486,586
Programming language: Elixir
License: Apache License 2.0
Tags: Testing    
Latest version: v1.0.2

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Mox is a library for defining concurrent mocks in Elixir.

The library follows the principles outlined in "Mocks and explicit contracts", summarized below:

  1. No ad-hoc mocks. You can only create mocks based on behaviours

  2. No dynamic generation of modules during tests. Mocks are preferably defined in your test_helper.exs or in a setup_all block and not per test

  3. Concurrency support. Tests using the same mock can still use async: true

  4. Rely on pattern matching and function clauses for asserting on the input instead of complex expectation rules

See the documentation for more information.


Just add mox to your list of dependencies in mix.exs:

def deps do
    {:mox, "~> 1.0", only: :test}

Mox should be automatically started unless the :applications key is set inside def application in your mix.exs. In such cases, you need to remove the :applications key in favor of :extra_applications or call Application.ensure_all_started(:mox) in your test/test_helper.exs.

Basic Usage

1) Add behaviour, defining the contract

# lib/weather_behaviour.ex
defmodule WeatherBehaviour do
  @callback get_weather(binary()) :: {:ok, map()} | {:error, binary()}

2) Add implementation for the behaviour

# lib/weather_impl.ex
defmodule WeatherImpl do
  @moduledoc """
  An implementation of a WeatherBehaviour

  @behaviour WeatherBehaviour

  @impl WeatherBehaviour
  def get_weather(city) when is_binary(city) do
    # Here you could call an external api directly with an HTTP client or use a third
    # party library that does that work for you. In this example we send a
    # request using a `httpc` to get back some html, which we can process later.


    case :httpc.request(:get, {"https://www.google.com/search?q=weather+#{city}", []}, [], []) do
      {:ok, {_, _, html_content}} -> {:ok, %{body: html_content}}
      error -> {:error, "Error getting weather: #{inspect(error)}"}

3) Add a switch

This can pull from your config/config.exs, config/test.exs, or, you can have no config as shown below and rely on a default. We also add a function to a higher level abstraction that will call the correct implementation:

# bound.ex, the main context we chose to call this function from
defmodule Bound do
  def get_weather(city) do

  defp weather_impl() do
    Application.get_env(:bound, :weather, WeatherImpl)

4) Define the mock so it is used during tests

# In your test/test_helper.exs
Mox.defmock(WeatherBehaviourMock, for: WeatherBehaviour) # <- Add this
Application.put_env(:bound, :weather, WeatherBehaviourMock) # <- Add this


5) Create a test and use expect to assert on the mock arguments

# test/bound_test.exs
defmodule BoundTest do
  use ExUnit.Case

  import Mox

  setup :verify_on_exit!

  describe "get_weather/1" do
    test "fetches weather based on a location" do
      expect(WeatherBehaviourMock, :get_weather, fn args ->
        # here we can assert on the arguments that get passed to the function
        assert args == "Chicago"

        # here we decide what the mock returns
        {:ok, %{body: "Some html with weather data"}}

      assert {:ok, _} = Bound.get_weather("Chicago")

Enforcing consistency with behaviour typespecs

Hammox is an enhanced version of Mox which automatically makes sure that calls to mocks match the typespecs defined in the behaviour. If you find this useful, see the project homepage.


Copyright 2017 Plataformatec \ Copyright 2020 Dashbit

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at


Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

*Note that all licence references and agreements mentioned in the mox README section above are relevant to that project's source code only.