Elixir Behaviour Driven Development
Latest commit 07ada28 Oct 21, 2018



Build Status Hex.pm

ESpec is a BDD testing framework for Elixir.

ESpec is inspired by RSpec and the main idea is to be close to its perfect DSL.

It is NOT a wrapper around ExUnit but a completely new testing framework written from scratch.


  • Test organization with describe, context, it, and etc blocks.

  • Familiar matchers: eq, be_close_to, raise_exception, etc.

  • Possibility to add custom matchers.

  • There are two types of expectation syntax:

    • expect syntax with pipe operator expect smth1 |> to(eq smth2) or is_expected |> to(eq smth) when subject is defined;
    • should syntax: smth1 |> should(eq smth2) or should eq smth when subject is defined.
    Note: RSpec syntax expect(smth1).to eq(smth2) is deprecated and won't work with OTP 21.
  • before and finally blocks (like RSpec before and after).

  • let, let! and subject.

  • Shared examples.

  • Generated examples.

  • Async examples.

  • Mocks with Meck.

  • Doc specs.

  • HTML and JSON outputs.

  • Etc and etc.



Add espec to dependencies in the mix.exs file:

def deps do
  {:espec, "~> 1.6.3", only: :test},
  #{:espec, github: "antonmi/espec", only: :test}, to get the latest version
mix deps.get

Then run:

MIX_ENV=test mix espec.init

The task creates spec/spec_helper.exs

Set preferred_cli_env for espec in the mix.exs file:

def project do
  preferred_cli_env: [espec: :test],

Or run with MIX_ENV=test:

MIX_ENV=test mix espec

Place your _spec.exs files into spec folder. use ESpec in the 'spec module'.

defmodule SyntaxExampleSpec do
  use ESpec
  it do: expect true |> to(be_true())
  it do: (1..3) |> should(have 2)

Run specs

mix espec

Run specific spec:

mix espec spec/some_spec.exs:25

Read the help:

MIX_ENV=test mix help espec

Context blocks and tags

There are three macros with the same functionality: context, describe, and example_group.

Context can have description and tags.

defmodule ContextSpec do
  use ESpec

  example_group do
    context "Some context" do
      it do: expect "abc" |> to(match ~r/b/)

    describe "Some another context with opts", focus: true do
      it do: 5 |> should(be_between 4, 6)

Available tags are:

  • skip: true or skip: "Reason" - skips examples in the context;
  • focus: true - sets focus to run with --focus option.

There are also xcontext, xdescribe, xexample_group macros to skip example groups. And fcontext, fdescribe, fexample_group for focused groups.

'spec' module is also a context with module name as description. One can add tags for this context after use ESpec:

defmodule ContextTagsSpec do
  use ESpec, skip: "Skip all examples in the module"


example, it, and specify macros define the 'spec example'.

defmodule ExampleAliasesSpec do
  use ESpec

  example do: expect [1,2,3] |> to(have_max 3)

  it "Test with description" do
    4.2 |> should(be_close_to 4, 0.5)

  specify "Test with options", [pending: true], do: "pending"

You can use skip, pending or focus tags to control evaluation. There are also macros:

  • xit, xexample, xspecify - to skip;
  • fit, fexample, fspecify, focus - to focus;
  • pending/1, example/1, it/1, specify/1 - for pending examples.
defmodule ExampleTagsSpec do
  use ESpec

  xit "skip", do: "skipped"
  focus "Focused", do: "Focused example"

  it "pending example"
  pending "it is also pending example"


The are --only, --exclude and --string command line options.

One can tag example or context and then use --only or --exclude option to run (or exclude) tests with specific tag.

defmodule FiltersSpec do
  use ESpec

  context "context with tag", context_tag: :some_tag do
    it do: "some example"
    it "example with tag", example_tag: true do
     "another example"
mix espec spec/some_spec.exs --only context_tag:some_tag --exclude example_tag

This runs only one test "some example"

You can also filter examples by --string option which filter examples which contain given string in their nested description.

mix espec spec/some_spec.exs --string 'context with tag'

before and finally

before blocks are evaluated before the example and finally runs after the example.

The blocks can return {:shared, key: value, ...} or (like in ExUnit) {:ok, key: value, ...}, so the keyword list will be saved in the dictionary and can be accessed in other before blocks, in the example, and in finally blocks through shared. You can also use a map as a second term in returned tuple: {:shared, %{key: value, ...}}.


defmodule BeforeAndFinallySpec do
  use ESpec

  before do: {:shared, a: 1}

  context "Context" do
    before do: {:shared, %{b: shared[:a] + 1}}
    finally do: "#{shared[:b]} == 2"

    it do: shared.a |> should(eq 1)
    it do: shared.b |> should(eq 2)

    finally do: "This finally will not be run. Define 'finally' before the example"

Note, that finally blocks must be defined before the example. Also note that finally blocks are executed in reverse order. Please see 'spec/before_finally_order_spec.exs' to figure out details.

There is also a short form of 'before' macro which allows to fill in shared dictionary:

before a: 1, b: 2
# which is equivalent to
before do: {shared: a: 1, b: 2}

You can configure 'global' before and finally in spec_helper.exs:

ESpec.configure fn(config) ->
  config.before fn(tags) -> {:shared, answer: 42, tags: tags} end  #can assign values in dictionary
  config.finally fn(shared) -> shared.answer  end     #can access assigns

These functions will be called before and after each example which ESpec runs.

config.before accepts example tags as an argument. So all example tags (including tags from parent contexts) are available in config.before. This allows you to run some specific pre-configuration based on tags.

ESpec.configure fn(config) ->
  config.before fn(tags) ->
    if tags[:async] || tags[:custom_tag] == :do_like_async
    {:shared, tags: tags}

before_all and after_all

There are hooks that evaluate before and after all the examples in a module. Use this hooks for complex system setup and tear down.

defmodule BeforeAllSpec do
  use ESpec

  before_all do

  it do: ...
  it do: ...

  after_all do

Note, before_all and after_all hooks do not set shared data and do not have access to them. Also note that you can define only one before_all and one after_all hook in a spec module.

'shared' data

shared is used to share data between spec blocks. You can access data by shared.some_key or shared[:some_key]. shared.some_key will raise exception if the key 'some_key' does not exist, while shared[:some_key] will return nil.

The shared variable appears in your before, finally, in config.before and config.finally, in let and example blocks.

before and finally blocks (including 'global') can modify the dictionary when return {:shared, key: value}. The example below illustrates the life-cycle of shared:



ESpec.configure fn(config) ->
  config.before fn -> {:shared, answer: 42} end         # shared == %{answer: 42}
  config.finally fn(shared) -> IO.puts shared.answer  end    # it will print 46


defmodule SharedBehaviorSpec do
  use ESpec

  before do: {:shared, answer: shared.answer + 1}          # shared == %{answer: 43}
  finally do: {:shared, answer: shared.answer + 1}         # shared == %{answer: 46}

  context do
    before do: {:shared, answer: shared.answer + 1}        # shared == %{answer: 44}
    finally do: {:shared, answer: shared.answer + 1}       # shared == %{answer: 45}
    it do: shared.answer |> should(eq 44)

So, 'config.finally' will print 46. Pay attention to how finally blocks are defined and evaluated.

let and subject

let and let! have the same behavior as in RSpec. Both defines memoizable functions in 'spec module'. The value will be cached across multiple calls in the same example but not across examples. let is lazy-evaluated, it is not evaluated until the first time the function it defines is invoked. Use let! to force the invocation before each example. A bang version is just a shortcut for:

let :a, do: 1
before do: a()

In example below, let! :a will be evaluated just after before a: 1. But let :b will be invoked only in the last test.

defmodule LetSpec do
  use ESpec

  before a: 1
  let! :a, do: shared.a
  let :b, do: shared.a + 1

  it do: expect a() |> to(eq 1)
  it do: expect b() |> to(eq 2)

Note, The shared is available in lets but neither let nor let! can modify the dictionary.

You can pass a keyword list to let or let! to define several 'lets' at once:

defmodule LetSpec do
  use ESpec

  let a: 1, b: 2

  it do: expect a() |> to(eq 1)
  it do: expect b() |> to(eq 2)

Note, subject and subject! are just aliases for let :subject, do: smth and let! :subject, do: smth. You can use is_expected macro (or a simple should expression) when subject is defined.

defmodule SubjectSpec do
  use ESpec

  subject(1 + 1)
  it do: is_expected() |> to(eq 2)
  it do: should(eq 2)

  context "with block" do
    subject do: 2 + 2
    it do: is_expected() |> to_not(eq 2)
    it do: should_not(eq 2)

There are helpers that can help you assign values from expressions that return {:ok, result} or {:error, result} tuples. For example, File.read\1 returns {:ok, binary} or {:error, reason}.

There are let_ok (let_ok!) and let_error (let_error!) functions that allow you assign values easily:

let_ok :file_binary, do: File.read("file.txt")
let_error :error_reason, do: File.read("error.txt")

Shared Examples

One can reuse the examples defined in spec module.

defmodule SharedSpec do
  use ESpec, shared: true

  subject shared.hello
  it do: should eq("world!")

shared: true marks examples in the module as shared, so the examples will be skipped until you reuse them. You can use the examples with it_behaves_like or its alias include_examples macro:

defmodule UseSharedSpec do
  use ESpec

  before hello: "world!"

You can also use let variables from parent module in shared examples. Use let_overridable macro to define let which will be overridden. You can pass single atom, list of atoms, or keyword with default values. See examples below.

defmodule SharedSpec do
  use ESpec, shared: true, async: true

  let_overridable a: 10, b: 20
  let_overridable [:c, :d]
  let_overridable :e

  let :internal_value, do: :shared_spec

  it "will be overridden" do
    expect a() |> to(eq 1)
    expect c() |> to(eq 3)
    expect e() |> to(eq 5)

  it "returns defaults" do
    expect b() |> to(eq 20)
    expect d() |> to(eq nil)

  it "does not override internal 'lets'" do
    expect internal_value() |> to(eq :shared_spec)

defmodule LetOverridableSpec do
  use ESpec, async: true

  let :internal_value, do: :some_spec

  it_behaves_like(SharedSpec, a: 1, c: 3, e: 5)

Shared Examples in separate files

In case you want to add some "global" shared specs which you want to use in multiple specs, ESpec has you covered. Simply add these files to your spec/shared folder. The place where mix espec.init generates you a placeholder folder and file.

By default ESpec loads all files contained in <your_spec_paths>/shared which match your spec_pattern. The Configuration and options chapter contains details on how to control this behaviour.

In case you already use Code.require_file/1 in your spec_helper.exs don't sweat. ESpec makes sure to require each file only once, it ignores files which already have been included.

Generated examples

Examples can be generated from code "templates". This should help with making the code more DRY:

defmodule GeneratedExamplesSpec do
  use ESpec, async: true


  Enum.map 2..4, fn(n) ->
    it "is divisible by #{n}" do
      expect rem(subject(), unquote(n)) |> to(eq 0)

Please mind the unquote call above - if you forget to unquote the n variable the compiler will show some warnings about it missing and eventually stop with an error: undefined function n/0.

Async examples

There is an async: true option you can set for the context or for the individual example:

defmodule AsyncSpec do
  use ESpec, async: true
  it do: "async example"

  context "Sync", async: false do
    it do: "sync example"

    it "async again", async: true do

The examples will be partitioned into two queries. Examples in asynchronous query will be executed in parallel in different processes.

Don't use async: true if you change the global state in your specs!



expect actual |> to(eq expected)  # passes if actual == expected
expect actual |> to(eql expected) # passes if actual === expected
expect actual |> to(be_close_to expected, delta)
expect actual |> to(be_between hard_place, rock)


Can be used with :>, :<, :>=, :<=, and etc.

expect actual |> to(be operator, value)

Passes if apply(Kernel, operator, [actual, value]) == true


expect actual |> to(match_pattern {:ok, _}) # {:ok, _} = actual

It's not possible to call functions in the pattern and use the return value as pattern ({:ok, function()}), this obviously means no let functions. If you neeed to use the return value of a function, use a variable:

value = function()

expect actual |> to(match_pattern {:ok, ^value})


expect actual |> to(be_true())
expect actual |> to(be_truthy())
expect actual |> to(be_false())
expect actual |> to(be_falsy())

Regular expressions

expect actual |> to(match ~r/expression/)
expect actual |> to(match "string")


There are many helpers to test enumerable collections:

expect collection |> to(be_empty()) # Enum.count(collection) == 0
... have value                      # Enum.member?(collection, value)
... have_all func                   # Enum.all?(collection, func)
... have_any func                   # Enum.any?(collection, func)
... have_at position, value         # Enum.at?(collection, position) == value
... have_count value                # Enum.count(collection) == value
... have_size value                 # alias
... have_length value               # alias
... have_count_by func, value       # Enum.count(collection, func) == value
... have_max value                  # Enum.max(collection) == value
... have_max_by func, value         # Enum.max_by(collection, fun) == value
... have_min value                  # Enum.min(collection) == value
... have_min_by func, value         # Enum.min_by(collection, fun) == value


expect list |> to(have_first value)  # List.first(list) == value
... have_last value                  # List.last(list) == value
... have_hd value                    # hd(list) == value
... have_tl value                    # tl(list) == value
... contain_exactly value            # Keyword.equals?(list, value)
... match_list value                 # alias for contain_exactly


expect binary |> to(have_byte_size value) # byte_size(binary) == value


expect string |> to(have_first value)   # String.first(string) == value
... have_last value                     # String.last(string) == value
... start_with value                    # String.starts_with?(string, value)
... end_with value                      # String.end_with?(string, value)
... have value                          # String.contains?(string, value)
... have_at pos, value                  # String.at(string, pos) == value
... have_length value                   # String.length(string) == value
... have_size value                     # alias
... have_count value                    # alias
... be_valid_string()                   # String.valid?(string)
... be_printable()                      # String.printable?(string)
... be_blank()                          # String.length(string) == 0
... be_empty()                          # String.length(string) == 0


expect map |> to(have foo: "bar")     # Map.get(map, :foo) == "bar"
expect map |> to(have {:foo, "bar"})  # Map.get(map, :foo) == "bar"
expect map |> to(have {"foo", "bar"}) # Map.get(map, :foo) == "bar"
expect map |> to(have_key value)      # Map.has_key?(map, value)
expect map |> to(have_value value)    # Enum.member?(Map.values(map), value)


expect pid |> to(be_alive) # Process.alive?(pid)

have also works for Structs.

Type checking

expect :espec |> to(be_atom)  #is_atom(:espec) == true
... be_binary()
... be_bitstring()
... be_boolean()
... ...
... ...
... be_tuple()
... be_function()
... be_function arity
... be_struct()
... be_struct StructExample


expect function |> to(raise_exception())
expect function |> to(raise_exception ErrorModule)
expect function |> to(raise_exception ErrorModule, "message")


expect function |> to(throw_term())
expect function |> to(throw_term term)

Change function's return value

Test if call of function1 change the function2 returned value to smth or from to smth

expect function1 |> to(change function2)
expect function1 |> to(change function2, to)
expect function1 |> to(change function2, from, to)
expect function1 |> to(change function2, by: value)

Check result

Test if function returns {:ok, result} or {:error, reason} tuple

expect {:ok, :the_result} |> to(be_ok_result())
expect {:error, :an_error} |> to(be_error_result())

assert and refute

If you are missing ExUnit assert and refute, ESpec has such functions as aliases to be_truthy and be_falsy

defmodule AssertAndRefuteSpec do
  use ESpec

  it "asserts" do
    assert "ESpec"
    #expect "ESpec" |> to(be_truthy())

  it "refutes" do
    refute nil
    #expect nil |> to(be_falsy())

assert_receive and refute_receive

assert_receive (assert_received) and refute_receive (refute_received) work identically to ExUnit ones.

assert_receive asserts that a message matching pattern was or is going to be received within timeout. assert_received asserts that a message was received and is in the current process mailbox. It is the same as assert_receive with 0 timeout.

refute_receive asserts that a message matching pattern was not received and won’t be received within the timeout. refute_received asserts that a message was not received (refute_receive with 0 timeout).

The default timeout for assert_receive and refute_receive is 100ms. You can pass custom timeout as a second argument.

defmodule AssertReceviveSpec do
  use ESpec

  it "demonstrates assert_received" do
    send(self(), :hello)
    assert_received :hello

  it "demonstrates assert_receive with custom timeout" do
    parent = self()
    spawn(fn -> :timer.sleep(200); send(parent, :hello) end)
    assert_receive(:hello, 300)

  it "demonstrates refute_receive" do
    send(self(), :another_hello)
    refute_receive :hello_refute

capture_io and capture_log

capture_io and capture_log are just copied from ExUnit and designed to test IO or Logger output:

defmodule CaptureSpec do
  use ESpec

  it "tests capture_io" do
    message = capture_io(fn -> IO.write "john" end)
    message |> should(eq "john")

  it "tests capture_log" do
    message = capture_log(fn -> Logger.error "log msg" end)
    expect message |> to(match "log msg")

Custom matchers

You can define your own matchers! The only functions you should implement is match/2, success_message/4, and error_message. Read the wiki page for detailed instructions. There is an example in custom_assertion_spec.exs.


There are community supported projects with sets of mathers:


If you keep the naming convention 'module TheModuleSpec is spec for TheModule' you can access tested module by described_module() helper.

defmodule TheModule do
  def fun, do: :fun

defmodule TheModuleSpec do
  use ESpec
  it do: expect described_module().fun |> to(eq :fun)


ESpec uses Meck to mock functions. You can mock the module with 'allow accept':

defmodule MocksSpec do
  use ESpec
  context "with old syntax" do
    before do: allow SomeModule |> to(accept(:func, fn(a, b) -> a + b end))
    it do: expect SomeModule.func(1, 2) |> to(eq 3)

  context "with new syntax" do
    before do: allow SomeModule |> to(accept :func, fn(a, b) -> a + b end)
    it do: expect SomeModule.func(1, 2) |> to(eq 3)

If you don't specify the function to return ESpec creates stubs with arity 0 and 1: fn -> end and fn(_) -> end, which return nil.

defmodule DefaultMocksSpec do
  use ESpec
  before do: allow SomeModule |> to(accept :func)
  it do: expect SomeModule.func |> to(be_nil())
  it do: expect SomeModule.func(42) |> to(be_nil())

You can also use pattern matching in your mocks:

defmodule PatternMockSpec do
  use ESpec
  before do
    args = {:some, :args}
    allow SomeModule |> to(accept :func, fn(^args) -> {:ok, :success} end)

  it do: expect SomeModule.func({:some, :args}) |> to(be_ok_result())

  it "raises exception when does not match" do
    expect(fn -> SomeModule.func({:wrong, :args}) end)
    |> to(raise_exception FunctionClauseError)

Behind the scenes 'allow accept' makes the following:

:meck.new(module, [:non_strict, :passthrough])
:meck.expect(module, name, function)

Find the explanation about the :non_strict and :passthrough options here. The default options ([:non_strict, :passthrough]) can be overridden:

allow SomeModule |> to(accept :func, fn(a,b) -> a + b end, [:non_strict, :unstick])

All the mocked modules are unloaded with :meck.unload(modules) after each example.

You can also pass a list of atom-function pairs to the accept function:

allow SomeModule |> to(accept f1: fn -> :f1 end, f2: fn -> :f2 end)

One can use passthrough/1 function to call the original function:

  before do
    allow SomeModule |> to(accept(:fun, fn
      :mocked -> "mock!"
      _ -> passthrough([args])

  it do: expect SomeModule.fun(:mocked) |> to(eq "mock!")
  it do: expect SomeModule.fun(2) |> to(eq 3)

The passthrough/1 just calls the :meck.passthrough/1 from the :meck module.

There is also an expectation to check if the module accepted a function call:

accepted(func, args \\ :any, opts \\ [pid: :any, count: :any])

So, the options are:

  • test if the function is called with some particular arguments of with any;
  • specify the pid of the process which called the function;
  • test the count of function calls.
defmodule MockOptionsSpec do
  use ESpec
  before do
    allow SomeModule |> to(accept :func, fn(a,b) -> a + b end)
    SomeModule.func(1, 2)

  it do: expect SomeModule |> to(accepted :func)
  it do: expect SomeModule |> to(accepted :func, [1,2])

  describe "with options" do
    defmodule Server do
      def call(a, b) do
        ESpec.SomeModule.func(a, b)
        ESpec.SomeModule.func(a, b)

    before do
      pid = spawn(Server, :call, [1, 2])
      {:ok, pid: pid}

    it do: expect ESpec.SomeModule |> to(accepted :func, [1,2], pid: shared.pid, count: 2)

accepted assertion checks :meck.history(SomeModule). See meck documentation.

Don't use async: true when using mocks!

Datetime Comparison

ESpec has comparison support for Elixir's date(time) related structs. Specifically, it has support for Date, Time, NaiveDateTime, and DateTime structs using ESpec's be_close_to and be assertions. It allows you to compare using the lowest-level granularity available in the struct. For example, since the lowest level of granularity available in a NaiveDateTime is the microsecond, you can compare how close to NaiveDateTime structs are with respect to microseconds.

Datetime be assertion(s) syntax

For the be assertions, ESpec supports a syntax with a granularity tuple (or keyword list) or a syntax without it. The following examples are shown with a Date struct.

Be assertion syntax without granularity
it do: expect ~D[2020-08-07] |> to(be :>=, ~D[2017-08-07])
Be assertion syntax with granularity
it do: expect ~D[2020-08-07] |> to(be :>=, ~D[2017-08-07], {:years, 3})

# or alternatively, you can do:
it do: expect ~D[2020-08-07]) |> to(be :>=, ~D[2017-08-07], years: 3)

Datetime be_close_to asssertion(s) syntax

Date Struct Comparison Example(s)
expect ~D[2017-08-07] |> to(be_close_to(~D[2018-08-07], {:years, 1}))

# or alternatively, you can do:
it do: expect ~D[2017-08-07] |> to(be_close_to(~D[2020-08-07], {:years, 3}))
NaiveDateTime Struct Comparison Example
expect ~N[2017-08-07 01:10:10.000001] |> to(be_close_to(~N[2017-08-07 01:10:10.000003], {:microseconds, 2}))

# or alternatively, you can do:
it do: expect ~N[2017-08-07 01:10:10.000001] |> to(be_close_to(~N[2017-08-07 01:10:10.000003], {:microseconds, 2}))
Time Struct Comparison Example
expect ~T[01:10:10] |> to(be_close_to(~T[01:50:10], {:minutes, 40}))

# or alternatively, you can do:
it do: expect ~T[01:10:10] |> to(be_close_to(~T[01:50:10], {:minutes, 40}))
DateTime Struct Comparison Example

Note the example shows a DateTime comparison with utc and std offsets for time zone differences. It is up to the user to be aware of the time zone utc and std offsets.

context "Success with DateTime with utc and std offsets to represent time zone differences" do
  let :datetime_pst, do: %DateTime{year: 2017, month: 3, day: 15, hour: 1, minute: 30, second: 30, microsecond: {1, 6}, std_offset: 1*3600, utc_offset: -8*3600, zone_abbr: "PST", time_zone: "America/Los_Angeles"}
  let :datetime_est, do: %DateTime{year: 2017, month: 3, day: 15, hour: 6, minute: 30, second: 30, microsecond: {1, 6}, std_offset: 1*3600, utc_offset: -5*3600, zone_abbr: "EST", time_zone: "America/New_York"}

  it do: expect datetime_pst() |> to(be_close_to(datetime_est(), {:hours, 2}))


Meck has trouble mocking certain modules, such as erlang, os, and timer.

Also, meck does not track module-local calls. For example, this will not be tracked:

defmodule SomeModule
  def some_func, do: another_func()

  def another_func, do: nil

But this will:

defmodule SomeModule
  def some_func, do: __MODULE__.another_func()

  def another_func, do: nil

It is recommended to prefix module-local calls with __MODULE__ when using them with meck.

See this section in the meck README for a more detailed explanation.

Doc specs

ESpec has functionality similar to ExUnit.DocTest. Read more about docs syntax here The functionality is implemented by two modules: ESpec.DocExample parses module documentation and ESpec.DocTest creates 'spec' examples for it. ESpec.DocExample functions are just copy-pasted from ExUnit.Doctest parsing functionality. ESpec.DocTest implements doctest macro which is identical to ExUnit.

defmodule DoctestSpec do
  use ESpec
  doctest MySuperModule

There are three options (similar to ExUnit.DocTest):

:except - generate specs for all functions except those listed (list of {function, arity} tuples).

defmodule DoctestOptionsSpec do
  use ESpec
  doctest MySuperModule, except: [fun: 1, func: 2]

:only — generate specs only for functions listed (list of {function, arity} tuples).

And :import to test a function defined in the module without referring to the module name.Default is false. Use this option with care because you can clash with other modules.

There are three types of specs can be generated based on docs.

  • Examples where input and output can be evaluated. For example:
@doc """
iex> Enum.map [1, 2, 3], fn(x) ->
...>   x * 2
...> end

Such examples will be converted to:

it "Example description" do
  expect input |> to(eq output)
  • Examples which return complex structure so Elixir prints it as #Name<...>.:
@doc """
iex> Enum.into([a: 10, b: 20], HashDict.new)
#HashDict<[b: 20, a: 10]>

The examples will be converted to:

it "Example description" do
  expect inspect(input) |> to(eq output)
  • Examples with exceptions:
@doc """
iex(1)> String.to_atom((fn() -> 1 end).())
** (ArgumentError) argument error

The examples will be tested as:

it "Example description" do
  expect fn -> input end |> to(raise_exception error_module, error_message)

Configuration and options

`MIX_ENV=test mix help espec`

Spec paths and pattern

You can change (in the mix.exs file) the folder where your specs are and the pattern to match these files.

 def project do
  spec_paths: ["my_specs", "espec"],
  spec_pattern: "*_espec.exs",

Shared spec paths and pattern

In addition to specifying the spec paths you can also tell ESpec where to find your SharedSpecs.

  def project do
    shared_spec_paths: ["my_specs/shared", "espec/my_shared"],
    shared_spec_pattern: "*_shared.exs",


One can run specs with coverage:

mix espec --cover

Find the results in /cover folder. ESpec, like ExUnit, uses very simple wrapper around OTP's cover. But you can override this.

Take a look to coverex as a perfect example.


There are three formatters in ESpec:

  • ESpec.Formatters.Doc
  • ESpec.Formatters.Json
  • ESpec.Formatters.Html

The Doc formatter tries to help you read the failed tests results by doing diffs between the expected and actual values in some cases (the eq and eql assertions, for example). If you don't want this you can disable it like this:

ESpec.configure fn(config) ->
  config.formatters [
    {ESpec.Formatters.Doc, %{diff_enabled?: false}}

By default ESpec uses 'Doc' with empty options. In order to use another one, you must specify formatters in 'ESpec.config'

For example:

ESpec.configure fn(config) ->
  config.formatters [
    {ESpec.Formatters.Json, %{out_path: "results.json"}},
    {ESpec.Formatters.Html, %{out_path: "results.html"}},
    {ESpec.YouCustomFormatter, %{a: 1, b: 2}},

ESpec design allows custom formatters of test results. The custom formatter is a module which use ESpec.Formatters.Base and implement 3 functions:

  • init/1
  • handle_cast/2 for example_finished event
  • handle_cast/2 for final_result event

Take a look at lib/espec/formatters and spec_formatters folders to see examples

Other formatters

There are community supported formatters:


  • 0.2.0:
    • Basic functionality (contexts, 'before' and 'let', mocking, basic matchers)
  • 0.3.0:
    • Tags for examples and contexts
    • 'config.before' and 'config.finally'
  • 0.4.0:
    • Lots of internal changes
    • Shared examples
  • 0.5.0:
    • 'count', 'pid' and 'args' options in 'accepted' assertion
    • 'async' option for parallel execution
  • 0.6.0:
    • String and Dictionary matchers
    • Doctests
  • 0.7.0:
    • Mocking options
    • Html and Json outputs
    • capture_io
  • 0.8.0:
    • 'only' and 'exclude' options
    • 'double_underscore' replaced by 'shared'
  • 1.0.0:
    • 'let' implementation rewritten completely
    • 'assert_receive' and 'refute_receive'
    • 'let_overridable' for shared examples
    • 'let_ok' and 'let_error'
    • new syntax with pipe
  • 1.1.0:
    • capture_log
    • 'let' and 'before' with keyword
  • 1.1.1:
    • Fix 'finally' execution order
  • 1.1.2:
    • Added support for unicode characters in example names
  • 1.2.0:
    • before_all and after_all callbacks
  • 1.2.1:
    • removed module name duplication in example description
    • fix statistic output for async examples
  • 1.2.2:
    • Elixir 1.4.0 warnings ware fixed
  • 1.3.0
    • Formatters were refactored to support custom ones
  • 1.3.1
    • Structure diffs were added to 'eq' matcher
  • 1.3.2
    • Generated examples were added
  • 1.3.3
    • Bug fix structure diff were fixed
  • 1.3.4
    • Line number option for contexts
  • 1.4.0
    • Pretty diffs for failed specs
    • 'let' agent fix
    • 'contain_exactly', 'match_list' and 'change_by' assertions
    • Elixir 1.2 is no longer supported
  • 1.4.1
    • Configurable timeouts for output formatters
  • 1.4.2
    • Fix Elixir 1.5.0 issues
  • 1.4.3
    • Fix options issues
  • 1.4.4
    • Stacktrace for failed examples
  • 1.4.5
    • Update 'meck' to fix issue with elrang 20 mocks
    • Fixed options passing
  • 1.4.6
    • Fix doctests (allow "strings")
    • allow keywords in let_ok and let_error
    • Fix before to ignore not enumerables
  • 1.5.0
    • Add be and be_close_to assertions for Date, Time, NaiveDateTime, and DateTime
    • 'have' matcher for Map
    • 'let' works for generated examples
    • 'match_pattern' macro
  • 1.5.1
    • Code formatting
    • Improve have and eq assertions
    • Fix let caching for shared examples
  • 1.6.0
    • Compatibility with OTP 21
  • 1.6.1
    • Doctest fix for Elixir >= 1.7
  • 1.6.2
    • Fix 'let' caching in shared specs
  • 1.6.3
    • Use 'Task.async_stream' for async examples


Contributions are welcome and appreciated!

Request a new feature by creating an issue.

Create a pull request with new features or fixes.

ESpec is tested using ExUnit and ESpec. So run:

mix test
mix espec