Skip to content


Repository files navigation

Buttercup — Behavior-Driven Emacs Lisp Testing

Build and test NonGNU ELPA MELPA Stable

Ranculus repens, photo by sannse

Buttercup is a behavior-driven development framework for testing Emacs Lisp code. It allows to group related tests so they can share common set-up and tear-down code, and allows the programmer to “spy” on functions to ensure they are called with the right arguments during testing.

The framework is heavily inspired by Jasmine.


Full article: Writing Tests

A simple test looks like this. Note that lexical-binding: t is required in files defining buttercup tests.

;;; lexical-binding is required -*- lexical-binding: t; -*-
(describe "A suite"
  (it "contains a spec with an expectation"
    (expect t :to-be t)))

Installation and Usage

Full article: Running Tests

You can install buttercup from NonGNU ELPA or MELPA Stable. Add the following to your init.el or .emacs file:

(require 'package)
;; Available as a default in GNU Emacs from version 28.1
(add-to-list 'package-archives
             ("nongnu" . "") t)
(add-to-list 'package-archives
             '("melpa-stable" . "") t)

This should allow you to M-x package-install RET buttercup RET.

Alternatively, users of Debian 9 or later or Ubuntu 16.10 or later may simply apt-get install elpa-buttercup.

Now create a file called test-feature.el with these contents:

;;; -*- lexical-binding: t; -*-
(describe "A suite"
  (it "contains a spec with an expectation"
    (expect t :to-be t)))

You can now use buttercup to run this test:

$ emacs -batch -f package-initialize -L . -f buttercup-run-discover
Running 1 specs.

A suite
  contains a spec with an expectation

Ran 1 specs, 0 failed, in 0.0 seconds.

Congratulations, you ran your first test!

Feature List

  • Shared set-up and tear-down sections to reduce code repetition and share a common environment among tests.
  • Easy to read and extensible expect macro to describe expected behavior.
  • Powerful mocking framework, called “spies,” to both cause them to return expected values or throw errors as needed by the test, as well as to ensure functions are called with expected arguments during tests.
  • Built to be run within a Continuous Integration environment, including test runners to discover and execute tests in a directory tree.

Why not ERT?

Emacs comes with a testing framework, ERT. Buttercup was written to address some shortcomings of that framework.

  • ERT deliberately leaves it up to the programmer to define set-up and tear-down code, which requires a lot of boiler-plate code for every set-up function. Buttercup makes this easy and seamless.
  • ERT has no good way of being run in a continuous integration environment. There are external projects to make this less of a pain instead. Once all is said and done, you installed six external packages your project does not need just to run your own tests. And that does not include a mocking library.
  • ERT has no way of grouping related tests, requiring every test name to share the same prefix, making names long and hard to read.

Nonetheless, ERT is a great project. It introduced testing to Emacs, and Buttercup learned a lot from its code to record a stack trace for error display. Even though Buttercup tries to be a better testing framework than ERT, we do wish ERT and the ERT maintainers all the best and hope both frameworks can continue to benefit from each other.