ERT: Emacs Lisp Regression Testing

2.1 Running Tests Interactively

You can run the tests that are currently defined in your Emacs with the command M-x ert RET t RET. (For an explanation of the t argument, see Test Selectors.) ERT will pop up a new buffer, the ERT results buffer, showing the results of the tests run. It looks like this:

Selector: t
Passed:  31
Skipped: 0
Failed:  2 (2 unexpected)
Total:   33/33

Started at:   2008-09-11 08:39:25-0700
Finished.
Finished at:  2008-09-11 08:39:27-0700

FF...............................

F addition-test
    (ert-test-failed
     ((should
       (=
        (+ 1 2)
        4))
      :form
      (= 3 4)
      :value nil))

F list-test
    (ert-test-failed
     ((should
       (equal
        (list 'a 'b 'c)
        '(a b d)))
      :form
      (equal
       (a b c)
       (a b d))
      :value nil :explanation
      (list-elt 2
                (different-atoms c d))))

At the top, there is a summary of the results: we ran all tests defined in the current Emacs (Selector: t), 31 of them passed, and 2 failed unexpectedly. See Expected Failures, for an explanation of the term unexpected in this context.

The line of dots and Fs is a progress bar where each character represents one test; it fills while the tests are running. A dot means that the test passed, an F means that it failed. Below the progress bar, ERT shows details about each test that had an unexpected result. In the example above, there are two failures, both due to failed should forms. See Understanding Explanations, for more details.

The following key bindings are available in the ERT results buffer:

RET ¶

Each name of a function or macro in this buffer is a button; moving point to it and typing RET jumps to its definition.

TAB ¶

S-TAB

Cycle between buttons forward (forward-button) and backward (backward-button).

r ¶

Re-run the test near point on its own (ert-results-rerun-test-at-point).

d ¶

Re-run the test near point on its own with the debugger enabled (ert-results-rerun-test-at-point-debugging-errors).

R ¶

Re-run all tests (ert-results-rerun-all-tests).

. ¶

Jump to the definition of the test near point (ert-results-find-test-at-point-other-window). This has the same effect as RET, but does not require point to be on the name of the test.

b ¶

Show the backtrace of a failed test (ert-results-pop-to-backtrace-for-test-at-point). See Backtraces in GNU Emacs Lisp Reference Manual, for more information about backtraces.

l ¶

Show the list of should forms executed in the test (ert-results-pop-to-should-forms-for-test-at-point).

m ¶

Show any messages that were generated (with the Lisp function message) in a test or any of the code that it invoked (ert-results-pop-to-messages-for-test-at-point).

L ¶

By default, long expressions in the failure details are abbreviated using print-length and print-level. Increase the limits to show more of the expression by moving point to a test failure with this command (ert-results-toggle-printer-limits-for-test-at-point).

D ¶

Delete a test from the running Emacs session (ert-delete-test).

h ¶

Show the documentation of a test (ert-describe-test).

T ¶

Display test timings for the last run (ert-results-pop-to-timings).

M-x ert-delete-all-tests ¶

Delete all tests from the running session.

M-x ert-describe-test ¶

Prompt for a test and then show its documentation.

2.2 Running Tests in Batch Mode

ERT supports automated invocations from the command line or from scripts or makefiles. There are two functions for this purpose, ert-run-tests-batch and ert-run-tests-batch-and-exit. They can be used like this:

emacs -batch -l ert -l my-tests.el -f ert-run-tests-batch-and-exit

This command will start up Emacs in batch mode, load ERT, load my-tests.el, and run all tests defined in it. It will exit with a zero exit status if all tests passed, or nonzero if any tests failed or if anything else went wrong. It will also print progress messages and error diagnostics to standard output.

You can also redirect the above output to a log file, say output.log, and use the ert-summarize-tests-batch-and-exit function to produce a neat summary as shown below:

emacs -batch -l ert -f ert-summarize-tests-batch-and-exit output.log

ERT attempts to limit the output size for failed tests by choosing conservative values for print-level and print-length when printing Lisp values. This can in some cases make it difficult to see which portions of those values are incorrect. Use ert-batch-print-level and ert-batch-print-length to customize that:

emacs -batch -l ert -l my-tests.el \
      --eval "(let ((ert-batch-print-level 10) \
                    (ert-batch-print-length 120)) \
                (ert-run-tests-batch-and-exit))"

Even modest settings for print-level and print-length can produce extremely long lines in backtraces, however, with attendant pauses in execution progress. Set ert-batch-backtrace-line-length to t to use the value of backtrace-line-length, nil to stop any limitations on backtrace line lengths (that is, to get full backtraces), or a positive integer to limit backtrace line length to that number.

By default, ERT in batch mode is quite verbose, printing a line with result after each test. This gives you progress information: how many tests have been executed and how many there are. However, in some cases this much output may be undesirable. In this case, set ert-quiet variable to a non-nil value:

emacs -batch -l ert -l my-tests.el \
      --eval "(let ((ert-quiet t)) (ert-run-tests-batch-and-exit))"

In quiet mode ERT prints only unexpected results and summary.

You can specify selectors to only run a subset of your tests (see Test Selectors). For example, the following would run all tests where the name of the test matches the regular expression “to-match”.

emacs -batch -l ert -l my-tests.el \
      -eval '(ert-run-tests-batch-and-exit "to-match")'

By default, ERT test failure summaries are quite brief in batch mode—only the names of the failed tests are listed. If the EMACS_TEST_VERBOSE environment variable is set and is non-empty, the failure summaries will also include the data from the failing test.

ERT can produce JUnit test reports in batch mode. If the environment variable EMACS_TEST_JUNIT_REPORT is set, ERT will produce for every test package my-tests.el a corresponding JUnit test report my-tests.xml. The function ert-summarize-tests-batch-and-exit collects all these package test reports into a new JUnit test report, with the respective name of that environment variable.

2.3 Test Selectors

Functions like ert accept a test selector, a Lisp expression specifying a set of tests. Test selector syntax is similar to Common Lisp’s type specifier syntax:

• nil selects no tests.
• t selects all tests.
• :new selects all tests that have not been run yet.
• :failed and :passed select tests according to their most recent result.
• :expected, :unexpected select tests according to their most recent result.
• A string is a regular expression that selects all tests with matching names.
• A test (i.e., an object of ert-test data type) selects that test.
• A symbol selects the test that the symbol names.
• (member tests...) selects the elements of tests, a list of tests or symbols naming tests.
• (eql test) selects test, a test or a symbol naming a test.
• (and selectors…) selects the tests that match all selectors.
• (or selectors…) selects the tests that match any of the selectors.
• (not selector) selects all tests that do not match selector.
• (tag tag) selects all tests that have tag on their tags list. (Tags are optional labels you can apply to tests when you define them.)
• (satisfies predicate) selects all tests that satisfy predicate, a function that takes a test as argument and returns non-nil if it is selected.

Selectors that are frequently useful when selecting tests to run include t to run all tests that are currently defined in Emacs, "^foo-" to run all tests in package foo (this assumes that package foo uses the prefix foo- for its test names), result-based selectors such as (or :new :unexpected) to run all tests that have either not run yet or that had an unexpected result in the last run, and tag-based selectors such as (not (tag :causes-redisplay)) to run all tests that are not tagged :causes-redisplay.

3.1 The should Macro

Test bodies can include arbitrary code; but to be useful, they need to check whether the code being tested (or code under test) does what it is supposed to do. The macro should is similar to cl-assert from the cl package (see Assertions in Common Lisp Extensions), but analyzes its argument form and records information that ERT can display to help debugging.

This test definition

(ert-deftest addition-test ()
  (should (= (+ 1 2) 4)))

will produce this output when run via M-x ert:

F addition-test
    (ert-test-failed
     ((should
       (=
        (+ 1 2)
        4))
      :form
      (= 3 4)
      :value nil))

In this example, should recorded the fact that (= (+ 1 2) 4) reduced to (= 3 4) before it reduced to nil. When debugging why the test failed, it helps to know that the function + returned 3 here. ERT records the return value for any predicate called directly within should.

In addition to should, ERT provides should-not, which checks that the predicate returns nil, and should-error, which checks that the form called within it signals an error. An example use of should-error:

(ert-deftest test-divide-by-zero ()
  (should-error (/ 1 0)
                :type 'arith-error))

This checks that dividing one by zero signals an error of type arith-error. The :type argument to should-error is optional; if absent, any type of error is accepted. should-error returns an error description of the error that was signaled, to allow additional checks to be made. The error description has the format (ERROR-SYMBOL . DATA).

There is no should-not-error macro since tests that signal an error fail anyway, so should-not-error is effectively the default.

See Understanding Explanations, for more details on what should reports.

3.2 Expected Failures

Some bugs are complicated to fix, or not very important, and are left as known bugs. If there is a test case that triggers the bug and fails, ERT will alert you of this failure every time you run all tests. For known bugs, this alert is a distraction. The way to suppress it is to add :expected-result :failed to the test definition:

(ert-deftest future-bug ()
  "Test `time-forward' with negative arguments.
Since this functionality isn't implemented, the test is known to fail."
  :expected-result :failed
  (time-forward -1))

ERT will still display a small f in the progress bar as a reminder that there is a known bug, and will count the test as failed, but it will be quiet about it otherwise.

An alternative to marking the test as a known failure this way is to delete the test. This is a good idea if there is no intent to fix it, i.e., if the behavior that was formerly considered a bug has become an accepted feature.

In general, however, it can be useful to keep tests that are known to fail. If someone wants to fix the bug, they will have a very good starting point: an automated test case that reproduces the bug. This makes it much easier to fix the bug, demonstrate that it is fixed, and prevent future regressions.

ERT displays the same kind of alerts for tests that pass unexpectedly as it displays for unexpected failures. This way, if you make code changes that happen to fix a bug that you weren’t aware of, you will know to remove the :expected-result clause of that test and close the corresponding bug report, if any.

Since :expected-result evaluates its argument when the test is loaded, tests can be marked as known failures only on certain Emacs versions, specific architectures, etc.:

(ert-deftest foo ()
  "A test that is expected to fail on Emacs 23 but succeed elsewhere."
  :expected-result (if (string-match "GNU Emacs 23[.]" (emacs-version))
                       :failed
                     :passed)
  ...)

3.3 Tests and Their Environment

Sometimes, it doesn’t make sense to run a test due to missing preconditions. A required Emacs feature might not be compiled in, the function to be tested could call an external binary which might not be available on the test machine, you name it. In this case, the macro skip-unless could be used to skip the test:

(ert-deftest test-dbus ()
  "A test that checks D-BUS functionality."
  (skip-unless (featurep 'dbusbind))
  ...)

The outcome of running a test should not depend on the current state of the environment, and each test should leave its environment in the same state it found it in. In particular, a test should not depend on any Emacs customization variables or hooks, and if it has to make any changes to Emacs’s state or state external to Emacs (such as the file system), it should undo these changes before it returns, regardless of whether it passed or failed.

Tests should not depend on the environment because any such dependencies can make the test brittle or lead to failures that occur only under certain circumstances and are hard to reproduce. Of course, the code under test may have settings that affect its behavior. In that case, it is best to make the test let-bind all such setting variables to set up a specific configuration for the duration of the test. The test can also set up a number of different configurations and run the code under test with each.

Tests that have side effects on their environment should restore it to its original state because any side effects that persist after the test can disrupt the workflow of the programmer running the tests. If the code under test has side effects on Emacs’s current state, such as on the current buffer or window configuration, the test should create a temporary buffer for the code to manipulate (using with-temp-buffer), or save and restore the window configuration (using save-window-excursion), respectively. For aspects of the state that can not be preserved with such macros, cleanup should be performed with unwind-protect, to ensure that the cleanup occurs even if the test fails.

An exception to this are messages that the code under test prints with message and similar logging; tests should not bother restoring the *Message* buffer to its original state.

The above guidelines imply that tests should avoid calling highly customizable commands such as find-file, except, of course, if such commands are what they want to test. The exact behavior of find-file depends on many settings such as find-file-wildcards, enable-local-variables, and auto-mode-alist. It is difficult to write a meaningful test if its behavior can be affected by so many external factors. Also, find-file has side effects that are hard to predict and thus hard to undo: It may create a new buffer or reuse an existing buffer if one is already visiting the requested file; and it runs find-file-hook, which can have arbitrary side effects.

Instead, it is better to use lower-level mechanisms with simple and predictable semantics like with-temp-buffer, insert or insert-file-contents-literally, and to activate any desired mode by calling the corresponding function directly, after binding the hook variables to nil. This avoids the above problems.

3.4 Useful Techniques when Writing Tests

Testing simple functions that have no side effects and no dependencies on their environment is easy. Such tests often look like this:

(ert-deftest ert-test-mismatch ()
  (should (eql (cl-mismatch "" "") nil))
  (should (eql (cl-mismatch "" "a") 0))
  (should (eql (cl-mismatch "a" "a") nil))
  (should (eql (cl-mismatch "ab" "a") 1))
  (should (eql (cl-mismatch "Aa" "aA") 0))
  (should (eql (cl-mismatch '(a b c) '(a b d)) 2)))

This test calls the function cl-mismatch several times with various combinations of arguments and compares the return value to the expected return value. (Some programmers prefer (should (eql EXPECTED ACTUAL)) over the (should (eql ACTUAL EXPECTED)) shown here. ERT works either way.)

Here’s a more complicated test:

(ert-deftest ert-test-record-backtrace ()
  (let ((test (make-ert-test :body (lambda () (ert-fail "foo")))))
    (let ((result (ert-run-test test)))
      (should (ert-test-failed-p result))
      (with-temp-buffer
        (ert--print-backtrace (ert-test-failed-backtrace result))
        (goto-char (point-min))
        (end-of-line)
        (let ((first-line (buffer-substring-no-properties
                           (point-min) (point))))
          (should (equal first-line
                         "  signal(ert-test-failed (\"foo\"))")))))))

This test creates a test object using make-ert-test whose body will immediately signal failure. It then runs that test and asserts that it fails. Then, it creates a temporary buffer and invokes ert--print-backtrace to print the backtrace of the failed test to the current buffer. Finally, it extracts the first line from the buffer and asserts that it matches what we expect. It uses buffer-substring-no-properties and equal to ignore text properties; for a test that takes properties into account, buffer-substring and ert-equal-including-properties could be used instead.

The reason why this test only checks the first line of the backtrace is that the remainder of the backtrace is dependent on ERT’s internals as well as whether the code is running interpreted or compiled. By looking only at the first line, the test checks a useful property—that the backtrace correctly captures the call to signal that results from the call to ert-fail—without being brittle.

This example also shows that writing tests is much easier if the code under test was structured with testing in mind.

For example, if ert-run-test accepted only symbols that name tests rather than test objects, the test would need a name for the failing test, which would have to be a temporary symbol generated with make-symbol, to avoid side effects on Emacs’s state. Choosing the right interface for ert-run-tests allows the test to be simpler.

Similarly, if ert--print-backtrace printed the backtrace to a buffer with a fixed name rather than the current buffer, it would be much harder for the test to undo the side effect. Of course, some code somewhere needs to pick the buffer name. But that logic is independent of the logic that prints backtraces, and keeping them in separate functions allows us to test them independently.

A lot of code that you will encounter in Emacs was not written with testing in mind. Sometimes, the easiest way to write tests for such code is to restructure the code slightly to provide better interfaces for testing. Usually, this makes the interfaces easier to use as well.

3.5 erts files

Many relevant Emacs tests depend on comparing the contents of a buffer before and after executing a particular function. These tests can be written the normal way—making a temporary buffer, inserting the “before” text, running the function, and then comparing with the expected “after” text. However, this often leads to test code that’s pretty difficult to read and write, especially when the text in question is multi-line.

So ert provides a function called ert-test-erts-file that takes two parameters: the name of a specially-formatted erts file, and (optionally) a function that performs the transform.

These erts files can be edited with the erts-mode major mode.

An erts file is divided into sections by the (‘=-=’) separator.

Here’s an example file containing two tests:

Name: flet

=-=
(cl-flet ((bla (x)
(* x x)))
(bla 42))
=-=
(cl-flet ((bla (x)
            (* x x)))
  (bla 42))
=-=-=

Name: defun

=-=
(defun x ()
  (print (quote ( thingy great
                  stuff))))
=-=-=

A test starts with a line containing just ‘=-=’ and ends with a line containing just ‘=-=-=’. The test may be preceded by freeform text (for instance, comments), and also name/value pairs (see below for a list of them).

If there is a line with ‘=-=’ inside the test, that designates the start of the “after” text. Otherwise, the “before” and “after” texts are assumed to be identical, which you typically see when writing indentation tests.

ert-test-erts-file puts the “before” section into a temporary buffer, calls the transform function, and then compares with the “after” section.

Here’s an example usage:

(ert-test-erts-file "elisp.erts"
                    (lambda ()
                      (emacs-lisp-mode)
                      (indent-region (point-min) (point-max))))

A list of the name/value specifications that can appear before a test follows. The general syntax is ‘Name: Value’, but continuation lines can be used (along the same lines as in mail—subsequent lines that start with a space are part of the value).

Name: foo
Code: (indent-region
        (point-min) (point-max))

‘Name’

All tests should have a name. This name will appear in ERT output if the test fails, and helps to identify the failing test.

‘Code’

This is the code that will be run to do the transform. This can also be passed in via the ert-test-erts-file call, but ‘Code’ overrides that. It’s used not only in the following test, but in all subsequent tests in the file (until overridden by another ‘Code’ specification).

‘No-Before-Newline’

‘No-After-Newline’

These specifications say whether the “before” or “after” portions have a newline at the end. (This would otherwise be impossible to specify.)

‘Point-Char’

Sometimes it’s useful to be able to put point at a specific place before executing the transform function. ‘Point-Char: |’ will make ert-test-erts-file place point where ‘|’ is in the “before” form (and remove that character), and will check that it’s where the ‘|’ character is in the “after” form (and issue a test failure if that isn’t the case). (This is used in all subsequent tests, unless overridden by a new ‘Point-Char’ spec.)

‘Skip’

If this is present and value is a form that evaluates to a non-nil value, the test will be skipped.

If you need to use the literal line single line ‘=-=’ in a test section, you can quote it with a ‘\’ character.

4.1 Understanding Explanations

Failed should forms are reported like this:

F addition-test
    (ert-test-failed
     ((should
       (=
        (+ 1 2)
        4))
      :form
      (= 3 4)
      :value nil))

ERT shows what the should expression looked like and what values its subexpressions had: The source code of the assertion was (should (= (+ 1 2) 4)), which applied the function = to the arguments 3 and 4, resulting in the value nil. In this case, the test is wrong; it should expect 3 rather than 4.

If a predicate like equal is used with should, ERT provides a so-called explanation:

F list-test
    (ert-test-failed
     ((should
       (equal
        (list 'a 'b 'c)
        '(a b d)))
      :form
      (equal
       (a b c)
       (a b d))
      :value nil :explanation
      (list-elt 2
                (different-atoms c d))))

In this case, the function equal was applied to the arguments (a b c) and (a b d). ERT’s explanation shows that the item at index 2 differs between the two lists; in one list, it is the atom c, in the other, it is the atom d.

In simple examples like the above, the explanation is unnecessary. But in cases where the difference is not immediately apparent, it can save time:

F test1
    (ert-test-failed
     ((should
       (equal x y))
      :form
      (equal a a)
      :value nil :explanation
      (different-symbols-with-the-same-name a a)))

ERT only provides explanations for predicates that have an explanation function registered. See Defining Explanation Functions.

4.2 Interactive Debugging

Debugging failed tests essentially works the same way as debugging any other problems with Lisp code. Here are a few tricks specific to tests:

• Re-run the failed test a few times to see if it fails in the same way each time. It’s good to find out whether the behavior is deterministic before spending any time looking for a cause. In the ERT results buffer, r re-runs the selected test.
• Use . to jump to the source code of the test to find out exactly what it does. Perhaps the test is broken rather than the code under test.
• If the test contains a series of should forms and you can’t tell which one failed, use l, which shows you the list of all should forms executed during the test before it failed.
• Use b to view the backtrace. You can also use d to re-run the test with debugging enabled, this will enter the debugger and show the backtrace as well; but the top few frames shown there will not be relevant to you since they are ERT’s own debugger hook. b strips them out, so it is more convenient.
• If the test or the code under testing prints messages using message, use m to see what messages it printed before it failed. This can be useful to figure out how far it got.
• You can instrument tests for debugging the same way you instrument defuns for debugging: go to the source code of the test and type C-u C-M-x. Then, go back to the ERT buffer and re-run the test with r or d.
• If you have been editing and rearranging tests, it is possible that ERT remembers an old test that you have since renamed or removed: renamings or removals of definitions in the source code leave around a stray definition under the old name in the running process (this is a common problem in Lisp). In such a situation, hit D to let ERT forget about the obsolete test.

5.1 Defining Explanation Functions

The explanation function for a predicate is a function that takes the same arguments as the predicate and returns an explanation. The explanation should explain why the predicate, when invoked with the arguments given to the explanation function, returns the value that it returns. The explanation can be any object but should have a comprehensible printed representation. If the return value of the predicate needs no explanation for a given list of arguments, the explanation function should return nil.

To associate an explanation function with a predicate, add the property ert-explainer to the symbol that names the predicate. The value of the property should be the symbol that names the explanation function.

5.2 Low-Level Functions for Working with Tests

Both ert-run-tests-interactively and ert-run-tests-batch are implemented on top of the lower-level test handling code in the sections of ert.el labeled “Facilities for running a single test”, “Test selectors”, and “Facilities for running a whole set of tests”.

If you want to write code that works with ERT tests, you should take a look at this lower-level code. Symbols that start with ert-- are internal to ERT, whereas those that start with ert- are meant to be usable by other code. But there is no mature API yet.

Contributions to ERT are welcome.

6.1 Other Tools for Emacs Lisp

Stubbing out functions or using so-called mocks can make it easier to write tests. See https://en.wikipedia.org/wiki/Mock_object for an explanation of the corresponding concepts in object-oriented languages.

ERT does not have built-in support for mocks or stubs. The package el-mock (see https://www.emacswiki.org/emacs/el-mock.el) offers mocks for Emacs Lisp and can be used in conjunction with ERT.

6.2 Fixtures and Test Suites

In many ways, ERT is similar to frameworks for other languages like SUnit or JUnit. However, two features commonly found in such frameworks are notably absent from ERT: fixtures and test suites.

Fixtures are mainly used (e.g., in SUnit or JUnit) to provide an environment for a set of tests, and consist of set-up and tear-down functions.

While fixtures are a useful syntactic simplification in other languages, this does not apply to Lisp, where higher-order functions and unwind-protect are available. One way to implement and use a fixture in ERT is

(defun my-fixture (body)
  (unwind-protect
      (progn [set up]
             (funcall body))
    [tear down]))

(ert-deftest my-test ()
  (my-fixture
   (lambda ()
     [test code])))

(Another way would be a with-my-fixture macro.) This solves the set-up and tear-down part, and additionally allows any test to use any combination of fixtures, so it is more flexible than what other tools typically allow.

If the test needs access to the environment the fixture sets up, the fixture can be modified to pass arguments to the body.

These are well-known Lisp techniques. Special syntax for them could be added but would provide only a minor simplification.

(If you are interested in such syntax, note that splitting set-up and tear-down into separate functions, like *Unit tools usually do, makes it impossible to establish dynamic let bindings as part of the fixture. So, blindly imitating the way fixtures are implemented in other languages would be counter-productive in Lisp.)

The purpose of test suites is to group related tests together.

The most common use of this is to run just the tests for one particular module. Since symbol prefixes are the usual way of separating module namespaces in Emacs Lisp, test selectors already solve this by allowing regexp matching on test names; e.g., the selector "^ert-" selects ERT’s self-tests.

Other uses include grouping tests by their expected execution time, e.g., to run quick tests during interactive development and slow tests less often. This can be achieved with the :tag argument to ert-deftest and tag test selectors.