A mocking library for ExUnit inspired by RSpec and based on meck.




Placebo is a mocking library based on meck. It is inspired by RSpec and Mock. All the functionality covered below is provided by meck. Placebo is just a pretty wrapper around the features of meck.

To enable just use Placebo in your ExUnit tests

defmodule SomeTests do
  use ExUnit.Case
  use Placebo

  ## some awesome tests


To ignore parentheses around mocks when running mix format, import your :placebo dependency in .formatter.exs:

  inputs: ["mix.exs", "{config,lib,test}/**/*.{ex,exs}"],
  import_deps: [:placebo]


allow Some.Module.hello("world"), return: "some value"

# or

allow(Some.Module.hello("world")) |> return("some value")

This will mock the module "Some.Module" and stub out the hello function with an argument of "world" to return "some value" when called. Any Elixir term can be returned using this syntax.

If you want more dyanmic behavior you can have a function executed when the mock is called.

allow Some.Module.hello(any()), exec: &String.upcase/1
allow Some.Module.hello(any()), exec: fn arg -> String.upcase(arg) end

# or

allow(Some.Module.hello(any())) |> exec(&String.upcase/1)
allow(Some.Module.hello(any())) |> exec(fn arg -> String.upcase(arg) end)

If you pass no arguments in the allow section the arguments to the anonymous function will be used for matching.

allow Some.Module.hello, exec: fn 1 -> "One"
                                  2 -> "Two"
                                  _ -> "Everything else" end

# or

allow(Some.Module.hello) |> exec(fn 1 -> "One"
                                    2 -> "Two"
                                    _ -> "Everything else" end)

To return different values on subsequent calls use seq or loop

allow Some.Module.hello(any()), seq: [1,2,3,4]
allow Some.Module.hello(any()), loop: [1,2,3,4]

# or

allow(Some.Module.hello(any())) |> seq([1,2,3,4])
allow(Some.Module.hello(any())) |> loop([1,2,3,4])

seq will return the last value for every call after the last one is called loop will continue to loop around the list after the last one is called

Argument Matching

Any term passed in as an argument will be matched exactly. Placebo.Matchers provides several argument matchers to be used for more dynamic scenarios. If you need something custom you can pass a function to the is/1 matcher.

allow Some.Module.hello(is(fn arg -> rem(arg,2) == 0 end)), return: "Even"

# or

allow(Some.Module.hello(is(fn arg -> rem(arg,2) == 0 end))) |> return("Even")

all mocks created by Placebo are automaticaly created with :merge_expects option. So multiple allow statements can be made for the same function and will match in the order defined.

allow Some.Module.hello(is(fn arg -> rem(arg,2) == 0 end))), return: "Even"
allow Some.Module.hello(any()), return: "Odd"

# or

allow(Some.Module.hello(is(fn arg -> rem(arg,2) == 0 end)))) |> return("Even")
allow(Some.Module.hello(any())) |> return("Odd")


Verification is done with assert_called and refute_called. All argument matchers also work in the verification step.

assert_called Some.Module.hello("world")
assert_called Some.Module.hello("world"), once()
assert_called Some.Module.hello("world"), times(2)

if you use expect instead of allow for any scenario. The interaction will automatically be verified at the end of the test.

expect Some.Module.hello("world"), return: "some value"

# or

expect(Some.Module.hello("world")) |> return("some value")

Failed verifications will automatically print out all recorded interactions with the mocked module.


If available in Hex, the package can be installed by adding placebo to your list of dependencies in mix.exs:

def deps do
    {:placebo, "~> 1.2", only: :test}

Documentation can be generated with ExDoc and published on HexDocs. Once published, the docs can be found at