/ˈniːmiː/ - Snapshot testing utilities for Elixir

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https://gist.github.com/assets/503938/57073c2c-6243-4b17-a91f-b705f4524dc9

Mneme augments ExUnit.Assertions with a set of assertions that know how to update themselves. This is sometimes called snapshot, approval, or golden master testing.

With Mneme, you write something like...

auto_assert my_function()

...and next time you run your tests, Mneme:

  1. runs the code,
  2. generates a pattern from the result,
  3. prints a diff,
  4. and asks if you'd like the test updated.

When you say yes, your test now looks like this:

auto_assert %MyAwesomeValue{so: :cool} <- my_function()

This lets you quickly write lots of tests, and like ordinary tests, you'll see when they fail. But, unlike ordinary tests, Mneme asks if you'd like the test updated for the new value.

Features:

Getting started

  1. Add :mneme as a dependency in your mix.exs. 

    defp deps do
  [
    {:mneme, ">= 0.0.0", only: :test}
  ]
end

  2. Fetch dependencies and run mix mneme.install with MIX_ENV=test. You're prompted with a diff before any files are saved. 

    $ mix deps.get

# If MIX_ENV=test is not set, you will see a "task could not be found" error
$ MIX_ENV=test mix mneme.install

  3. Add use Mneme wherever you use ExUnit.Case. 

    defmodule MyTest do
  use ExUnit.Case, async: true
  use Mneme

  test "arithmetic" do
    auto_assert 2 + 2
  end
end

  4. Run mix test and type y<ENTER> when prompted. The auto_assert call should be updated to: 

    auto_assert 4 <- 2 + 2

Interactive tour

If you'd like to see Mneme in action, you can download and run examples/tour_mneme.exs, a standalone tour that only requires that you have Elixir installed. Give it a try without installing Mneme into your own project.

$ curl -o tour_mneme.exs https://raw.githubusercontent.com/zachallaun/mneme/main/examples/tour_mneme.exs

$ elixir tour_mneme.exs

Requirements

Elixir

Mneme requires Elixir version 1.14 or later.

Formatter

If you do not use a formatter, the first auto-assertion will reformat the entire file, introducing unrelated formatting changes. Mneme rewrites your test scripts when updating an assertion using the formatter configuration for your project.

It's highly recommended to configure your editor to format Elixir files on-save.

Supported formatters:

Command line interface

Auto-assertions are run with your normal tests when you run mix test and a terminal prompt is used whenever one needs to be updated. Here's what that might look like:

Screenshot of Mneme CLI

Whenever that happens, you have a few options:

Key Action Description
y Accept Accept the proposed change. The assertion will be re-run and should pass.
n Reject Reject the proposed change and fail the test.
s Skip Skip this assertion. The test will not fail, but the mix test process will exit with 1.
k Next If multiple patterns have been generated, cycle to the next one.
K Last If multiple patterns have been generated, cycle to the last one.
j Previous If multiple patterns have been generated, cycle to the previous one.
J First If multiple patterns have been generated, cycle to the first one.

Note that the CLI is not available when tests are run in a CI environment.

Continuous Integration

In a CI environment, Mneme will not attempt to prompt and update any assertions, but will instead fail any tests that would update. This behavior is enabled by the CI environment variable, which is set by convention by many continuous integration providers.

export CI=true

Links and acknowledgements

Special thanks to: