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A sane way of using mocks in Elixir. It borrows a lot from both Meck & Mox! Thanks @eproxus & @josevalim.


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

def deps do
    {:mimic, "~> 1.5", only: :test}

If :applications key is defined inside your mix.exs or you run mix test --no-start, you probably want to add Application.ensure_all_started(:mimic) in your test_helper.exs


Modules need to be prepared so that they can be used.

You must first call copy in your test_helper.exs for each module that may have the behaviour changed.



Calling copy will not change the behaviour of the module.

The user must call stub/1, stub/3, expect/4 or reject/1 so that the functions can behave differently.

Then for the actual tests one could use it like this:

use ExUnit.Case, async: true
use Mimic

test "invokes add once and mult twice" do
  |> stub(:add, fn x, y -> :stub end)
  |> expect(:add, fn x, y -> x + y end)
  |> expect(:mult, 2, fn x, y -> x * y end)

  assert Calculator.add(2, 3) == 5
  assert Calculator.mult(2, 3) == 6

  assert Calculator.add(2, 3) == :stub

Stub, Expect and Reject


stub/1 will change every module function to throw an exception if called.


** (Mimic.UnexpectedCallError) Stub! Unexpected call to Calculator.add(3, 7) from #PID<0.187.0>
     code: assert Calculator.add(3, 7) == 10

stub/3 changes a specific function to behave differently. If the function is not called no verification error will happen.


expect/4 changes a specific function and it works like a queue of operations. It has precedence over stubs and if not called a verification error will be thrown.

If the same function is called with expect/4 the order will be respected:

|> stub(:add, fn _x, _y -> :stub end)
|> expect(:add, fn _, _ -> :expected_1 end)
|> expect(:add, fn _, _ -> :expected_2 end)

assert Calculator.add(1, 1) == :expected_1
assert Calculator.add(1, 1) == :expected_2
assert Calculator.add(1, 1) == :stub

expect/4 has an optional parameter which is the amount of calls expected:

|> expect(:add, 2, fn x, y -> {:add, x, y} end)

assert Calculator.add(1, 3) == {:add, 1, 3}
assert Calculator.add(4, 5) == {:add, 4, 5}

With use Mimic, verification expect/4 function call of is done automatically on test case end. verify!/1 can be used in case custom verification timing required:

|> expect(:add, 2, fn x, y -> {:add, x, y} end)

# Will raise error because Calculator.add is not called
# ** (Mimic.VerificationError) error while verifying mocks for #PID<0.3182.0>:
#   * expected Calculator.add/2 to be invoked 1 time(s) but it has been called 0 time(s)


One may want to reject calls to a specific function. reject/1 can be used to achieved this behaviour.

assert_raise Mimic.UnexpectedCallError, fn -> Calculator.add(4, 2) end

Private and Global mode

The default mode is private which means that only the process and explicitly allowed process will see the different behaviour.

Calling allow/2 will permit a different pid to call the stubs and expects from the original process.

If you are using Task there is no need to use global mode as Tasks can see the same expectations and stubs from the calling process.

Global mode can be used with set_mimic_global like this:

setup :set_mimic_global

test "invokes add and mult" do
  |> expect(:add, fn x, y -> x + y end)
  |> expect(:mult, fn x, y -> x * y end)

  parent_pid = self()

  spawn_link(fn ->
    assert Calculator.add(2, 3) == 5
    assert Calculator.mult(2, 3) == 6

    send parent_pid, :ok

  assert_receive :ok

This means that all processes will get the same behaviour defined with expect & stub. This option is simpler but tests running concurrently will have undefined behaviour. It is important to run with async: false. One could use :set_mimic_from_context instead of using :set_mimic_global or :set_mimic_private. It will be private if async: true, global otherwise.

DSL Mode

To use DSL Mode use Mimic.DSL rather than use Mimic in your test. DSL Mode enables a more expressive api to the Mimic functionality.

  use Mimic.DSL

  test "basic example" do
    stub Calculator.add(_x, _y), do: :stub
    expect Calculator.add(x, y), do: x + y
    expect Calculator.mult(x, y), do: x * y

    assert Calculator.add(2, 3) == 5
    assert Calculator.mult(2, 3) == 6

    assert Calculator.add(2, 3) == :stub

Implementation Details & Performance

After calling Mimic.copy(MyModule), calls to functions belonging to this module will first go through an ETS table to check which pid sees what (stubs, expects or call original).

It is really fast but it won't be as fast as calling a no-op function. Here's a very simple benchmark:

defmodule Enumerator do
 def to_list(x, y), do: Enum.to_list(x..y)

Benchmarking Enumerator.to_list(1, 100) :

Name               ips        average  deviation         median         99th %
mimic         116.00 K        8.62 μs   ±729.13%           5 μs          29 μs
original       19.55 K       51.15 μs   ±302.46%          34 μs         264 μs

mimic         116.00 K
original       19.55 K - 5.93x slower

Benchmarking Enumerator.to_list(1, 250) :

Name               ips        average  deviation         median         99th %
original      131.49 K        7.61 μs   ±167.90%           7 μs          16 μs
mimic         105.47 K        9.48 μs   ±145.21%           9 μs          27 μs

original      131.49 K
mimic         105.47 K - 1.25x slower

There's a small fixed price to pay when mimic is used but it is unnoticeable for tests purposes.


Thanks to @jamesotron and @alissonsales for all the help! 🎉

Copyright and License

Copyright (c) 2016 Eduardo Gurgel

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.

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