Elisp Unit Testing with ERT

Emacs 24 comes with a unit testing library, ERT (Emacs Lisp Regression Testing). I learned about it after watching Extending Emacs Rocks! and I’ve been using it ever since. It’s been a pleasant experience; enough so that I made a key binding for it so that I can effortlessly run tests at any time. When I recently made a major overhaul to my Emacs web server I added a small test suite using ERT.

Emacs also comes with the ERT manual so it’s easy to start learning, but here’s the gist of it. There are essentially two macros to worry about: ert-deftest and should. The first is used to create tests and the second behaves like assert but with nicer behavior. Here’s an example,

(ert-deftest example-test ()
  (should (= (+ 9 2) 11)))

ert-deftest is what you’d expect from every other def*. The empty parameter list does nothing at the moment other than to make it feel like writing a defun. The body is evaluated as normal. This is all turned into an anonymous function which is stuffed in the plist of the symbol example-test. When it comes time to running tests, they are found by searching the plists of every interned symbol.

The other macro, should, takes one argument: a form that should evaluate to true. There is also a should-not and a should-error, which do what you would expect.

Tests are run with M-x ert. It will ask for a test selector, where t selects all defined tests. There are many ways to select a subset of all tests (:new, :passed, :failed, etc.) but I usually just run all of them (as my key binding makes obvious). The results are displayed in a separate pop-up buffer which, as usual, can be dismissed with q.

Running ERT

What makes should special is error reporting. When tests fail you will be provided with the forms that failed and their return values. For example, if we modify the test above to fail.

(ert-deftest example-test ()
  (should (= (+ 9 2) 100)))

Then run the test and it will note the failure. There is also some red coloring not captured here.

F example-test
        (+ 9 2)
      (= 11 100)
      :value nil))

Displayed are the forms we were comparing — (+ 9 2) and 100 — and what they evaluated to: (= 11 100). If I put the point at the test result and type . it will take me to the test definition so that I can start looking further. Or I can press b to see a backtrace, m to see all output messages from that test, or, if I’m in disbelief, r to rerun that test.


Elisp’s dynamic bindings really come in handy when functions need to be mocked. For example, say I have a function that, at some point, needs to check whether or not a particular file exists. This would be done using file-exists-p. Creating or removing the file in the filesystem before the test isn’t a well-contained unit test. Tests running in parallel could interfere and there are a number of ways something could go wrong.

Instead I’ll temporarily override the definition of file-exists-p with a mock function using let’s cousin, flet. Note that file-exists-p is a C source function but I can still override it as if it was any regular lisp function.

(defun determine-next-action ()
  (if (file-exists-p "death-star-plans.org")

(ert-deftest file-check-test ()
  (flet ((file-exists-p (file) t))
    (should (eq (determine-next-action) 'bring-him-the-passengers)))
  (flet ((file-exists-p (file) nil))
    (should (eq (determine-next-action) 'tear-this-ship-apart))))

This is a very simple mock. For a real unit test I might want the mock to return t for some filename patterns and nil for others. There’s an extension to ERT, el-mock.el, which assists in creating more complex mocks, but I haven’t used or needed it yet.

Since it’s so convenient I’m going to be using ERT more and more until it becomes second-nature.

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Chris Wellons

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