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Copyright © 2008, 2010–2015 Free Software Foundation, Inc.

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INFO-DIR-SECTION Emacs misc features
START-INFO-DIR-ENTRY
* ERT: (ert).                   Emacs Lisp regression testing tool.
END-INFO-DIR-ENTRY


File: ert.info,  Node: Top,  Next: Introduction,  Up: (dir)

ERT: Emacs Lisp Regression Testing
**********************************

Copyright © 2008, 2010–2015 Free Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.3 or any later version published by the Free Software
     Foundation; with no Invariant Sections, with the Front-Cover Texts
     being “A GNU Manual,” and with the Back-Cover Texts as in (a)
     below.  A copy of the license is included in the section entitled
     “GNU Free Documentation License”.

     (a) The FSF’s Back-Cover Text is: “You have the freedom to copy and
     modify this GNU manual.”

   ERT is a tool for automated testing in Emacs Lisp.  Its main features
are facilities for defining tests, running them and reporting the
results, and for debugging test failures interactively.

   ERT is similar to tools for other environments such as JUnit, but has
unique features that take advantage of the dynamic and interactive
nature of Emacs.  Despite its name, it works well both for test-driven
development (see <http://en.wikipedia.org/wiki/Test-driven_development>)
and for traditional software development methods.

* Menu:

* Introduction::                A simple example of an ERT test.
* How to Run Tests::            Run tests in Emacs or from the command line.
* How to Write Tests::          How to add tests to your Emacs Lisp code.
* How to Debug Tests::          What to do if a test fails.
* Extending ERT::               ERT is extensible in several ways.
* Other Testing Concepts::      Features not in ERT.
* GNU Free Documentation License::  The license for this documentation.

 — The Detailed Node Listing —

How to Run Tests

* Running Tests Interactively::  Run tests in your current Emacs.
* Running Tests in Batch Mode::  Run tests in emacs -Q.
* Test Selectors::               Choose which tests to run.

How to Write Tests

* The ‘should’ Macro::          A powerful way to express assertions.
* Expected Failures::           Tests for known bugs.
* Tests and Their Environment:: Don’t depend on customizations; no side effects.
* Useful Techniques::           Some examples.

How to Debug Tests

* Understanding Explanations::  How ERT gives details on why an assertion failed.
* Interactive Debugging::       Tools available in the ERT results buffer.

Extending ERT

* Defining Explanation Functions::  Teach ERT about more predicates.
* Low-Level Functions for Working with Tests::  Use ERT’s data for your purposes.

Other Testing Concepts

* Mocks and Stubs::           Stubbing out code that is irrelevant to the test.
* Fixtures and Test Suites::  How ERT differs from tools for other languages.

Appendix

* GNU Free Documentation License:: The license for this documentation.



File: ert.info,  Node: Introduction,  Next: How to Run Tests,  Prev: Top,  Up: Top

1 Introduction
**************

ERT allows you to define _tests_ in addition to functions, macros,
variables, and the other usual Lisp constructs.  Tests are simply Lisp
code: code that invokes other code and checks whether it behaves as
expected.

   ERT keeps track of the tests that are defined and provides convenient
commands to run them to verify whether the definitions that are
currently loaded in Emacs pass the tests.

   Some Lisp files have comments like the following (adapted from the
package ‘pp.el’):

     ;; (pp-to-string '(quote quote))          ; expected: "'quote"
     ;; (pp-to-string '((quote a) (quote b)))  ; expected: "('a 'b)\n"
     ;; (pp-to-string '('a 'b))                ; same as above

   The code contained in these comments can be evaluated from time to
time to compare the output with the expected output.  ERT formalizes
this and introduces a common convention, which simplifies Emacs
development, since programmers no longer have to manually find and
evaluate such comments.

   An ERT test definition equivalent to the above comments is this:

     (ert-deftest pp-test-quote ()
       "Tests the rendering of `quote' symbols in `pp-to-string'."
       (should (equal (pp-to-string '(quote quote)) "'quote"))
       (should (equal (pp-to-string '((quote a) (quote b))) "('a 'b)\n"))
       (should (equal (pp-to-string '('a 'b)) "('a 'b)\n")))

   If you know ‘defun’, the syntax of ‘ert-deftest’ should look
familiar: This example defines a test named ‘pp-test-quote’ that will
pass if the three calls to ‘equal’ all return non-‘nil’.

   ‘should’ is a macro with the same meaning as ‘cl-assert’ but better
error reporting.  *Note The ‘should’ Macro::.

   Each test should have a name that describes what functionality it
tests.  Test names can be chosen arbitrarily—they are in a namespace
separate from functions and variables—but should follow the usual Emacs
Lisp convention of having a prefix that indicates which package they
belong to.  Test names are displayed by ERT when reporting failures and
can be used when selecting which tests to run.

   The empty parentheses ‘()’ in the first line don’t currently have any
meaning and are reserved for future extension.  They also make the
syntax of ‘ert-deftest’ more similar to that of ‘defun’.

   The docstring describes what feature this test tests.  When running
tests interactively, the first line of the docstring is displayed for
tests that fail, so it is good if the first line makes sense on its own.

   The body of a test can be arbitrary Lisp code.  It should have as few
side effects as possible; each test should be written to clean up after
itself, leaving Emacs in the same state as it was before the test.
Tests should clean up even if they fail.  *Note Tests and Their
Environment::.


File: ert.info,  Node: How to Run Tests,  Next: How to Write Tests,  Prev: Introduction,  Up: Top

2 How to Run Tests
******************

You can run tests either in the Emacs you are working in, or on the
command line in a separate Emacs process in batch mode (i.e., with no
user interface).  The former mode is convenient during interactive
development, the latter is useful to make sure that tests pass
independently of your customizations; and it allows you to invoke tests
from makefiles, and to write scripts that run tests in several different
Emacs versions.

* Menu:

* Running Tests Interactively::  Run tests in your current Emacs.
* Running Tests in Batch Mode::  Run tests in emacs -Q.
* Test Selectors::               Choose which tests to run.


File: ert.info,  Node: Running Tests Interactively,  Next: Running Tests in Batch Mode,  Up: How to Run Tests

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,
*note 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.  *Note Expected Failures::, for an explanation of
the term _unexpected_ in this context.

   The line of dots and ‘F’s 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.  *Note Understanding Explanations::, for more details.

   In the ERT results buffer, ‘TAB’ and ‘S-TAB’ cycle between buttons.
Each name of a function or macro in this buffer is a button; moving
point to it and typing ‘RET’ jumps to its definition.

   Pressing ‘r’ re-runs the test near point on its own.  Pressing ‘d’
re-runs it with the debugger enabled.  ‘.’ jumps to the definition of
the test near point (‘RET’ has the same effect if point is on the name
of the test).  On a failed test, ‘b’ shows the backtrace of the failure.

   ‘l’ shows the list of ‘should’ forms executed in the test.  If any
messages were generated (with the Lisp function ‘message’) in a test or
any of the code that it invoked, ‘m’ will show them.

   By default, long expressions in the failure details are abbreviated
using ‘print-length’ and ‘print-level’.  Pressing ‘L’ while point is on
a test failure will increase the limits to show more of the expression.


File: ert.info,  Node: Running Tests in Batch Mode,  Next: Test Selectors,  Prev: Running Tests Interactively,  Up: How to Run Tests

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.

   If ERT is not part of your Emacs distribution, you may need to use
‘-L /path/to/ert/’ so that Emacs can find it.  You may need additional
‘-L’ flags to ensure that ‘my-tests.el’ and all the files that it
requires are on your ‘load-path’.


File: ert.info,  Node: Test Selectors,  Prev: Running Tests in Batch Mode,  Up: How to Run Tests

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 SELECTOR.
   • ‘(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’.


File: ert.info,  Node: How to Write Tests,  Next: How to Debug Tests,  Prev: How to Run Tests,  Up: Top

3 How to Write Tests
********************

ERT lets you define tests in the same way you define functions.  You can
type ‘ert-deftest’ forms in a buffer and evaluate them there with
‘eval-defun’ or ‘compile-defun’, or you can save the file and load it,
optionally byte-compiling it first.

   Just like ‘find-function’ is only able to find where a function was
defined if the function was loaded from a file, ERT is only able to find
where a test was defined if the test was loaded from a file.

* Menu:

* The ‘should’ Macro::          A powerful way to express assertions.
* Expected Failures::           Tests for known bugs.
* Tests and Their Environment:: Don’t depend on customizations; no side effects.
* Useful Techniques::           Some examples.


File: ert.info,  Node: The ‘should’ Macro,  Next: Expected Failures,  Up: How to Write Tests

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 (*note (cl)Assertions::), 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.

   *Note Understanding Explanations::, for more details on what ‘should’
reports.


File: ert.info,  Node: Expected Failures,  Next: Tests and Their Environment,  Prev: The ‘should’ Macro,  Up: How to Write Tests

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)
       ...)


File: ert.info,  Node: Tests and Their Environment,  Next: Useful Techniques,  Prev: Expected Failures,  Up: How to Write Tests

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.


File: ert.info,  Node: Useful Techniques,  Prev: Tests and Their Environment,  Up: How to Write Tests

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

   This test calls the function ‘ert--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.


File: ert.info,  Node: How to Debug Tests,  Next: Extending ERT,  Prev: How to Write Tests,  Up: Top

4 How to Debug Tests
********************

This section describes how to use ERT’s features to understand why a
test failed.

* Menu:

* Understanding Explanations::  How ERT gives details on why an assertion failed.
* Interactive Debugging::       Tools available in the ERT results buffer.


File: ert.info,  Node: Understanding Explanations,  Next: Interactive Debugging,  Up: How to Debug Tests

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.  *Note Defining Explanation
Functions::.


File: ert.info,  Node: Interactive Debugging,  Prev: Understanding Explanations,  Up: How to Debug Tests

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
     ‘defun’s 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.


File: ert.info,  Node: Extending ERT,  Next: Other Testing Concepts,  Prev: How to Debug Tests,  Up: Top

5 Extending ERT
***************

There are several ways to add functionality to ERT.

* Menu:

* Defining Explanation Functions::  Teach ERT about more predicates.
* Low-Level Functions for Working with Tests::  Use ERT’s data for your purposes.


File: ert.info,  Node: Defining Explanation Functions,  Next: Low-Level Functions for Working with Tests,  Up: Extending ERT

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.


File: ert.info,  Node: Low-Level Functions for Working with Tests,  Prev: Defining Explanation Functions,  Up: Extending ERT

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.


File: ert.info,  Node: Other Testing Concepts,  Next: GNU Free Documentation License,  Prev: Extending ERT,  Up: Top

6 Other Testing Concepts
************************

For information on mocks, stubs, fixtures, or test suites, see below.

* Menu:

* Mocks and Stubs::           Stubbing out code that is irrelevant to the test.
* Fixtures and Test Suites::  How ERT differs from tools for other languages.


File: ert.info,  Node: Mocks and Stubs,  Next: Fixtures and Test Suites,  Up: Other Testing Concepts

6.1 Other Tools for Emacs Lisp
==============================

Stubbing out functions or using so-called _mocks_ can make it easier to
write tests.  See <http://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 <http://www.emacswiki.org/emacs/el-mock.el>) offers mocks
for Emacs Lisp and can be used in conjunction with ERT.


File: ert.info,  Node: Fixtures and Test Suites,  Prev: Mocks and Stubs,  Up: Other Testing Concepts

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.


File: ert.info,  Node: GNU Free Documentation License,  Prev: Other Testing Concepts,  Up: Top

Appendix A GNU Free Documentation License
*****************************************

                     Version 1.3, 3 November 2008

     Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
     <http://fsf.org/>

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

  0. PREAMBLE

     The purpose of this License is to make a manual, textbook, or other
     functional and useful document "free" in the sense of freedom: to
     assure everyone the effective freedom to copy and redistribute it,
     with or without modifying it, either commercially or
     noncommercially.  Secondarily, this License preserves for the
     author and publisher a way to get credit for their work, while not
     being considered responsible for modifications made by others.

     This License is a kind of “copyleft”, which means that derivative
     works of the document must themselves be free in the same sense.
     It complements the GNU General Public License, which is a copyleft
     license designed for free software.

     We have designed this License in order to use it for manuals for
     free software, because free software needs free documentation: a
     free program should come with manuals providing the same freedoms
     that the software does.  But this License is not limited to
     software manuals; it can be used for any textual work, regardless
     of subject matter or whether it is published as a printed book.  We
     recommend this License principally for works whose purpose is
     instruction or reference.

  1. APPLICABILITY AND DEFINITIONS

     This License applies to any manual or other work, in any medium,
     that contains a notice placed by the copyright holder saying it can
     be distributed under the terms of this License.  Such a notice
     grants a world-wide, royalty-free license, unlimited in duration,
     to use that work under the conditions stated herein.  The
     “Document”, below, refers to any such manual or work.  Any member
     of the public is a licensee, and is addressed as “you”.  You accept
     the license if you copy, modify or distribute the work in a way
     requiring permission under copyright law.

     A “Modified Version” of the Document means any work containing the
     Document or a portion of it, either copied verbatim, or with
     modifications and/or translated into another language.

     A “Secondary Section” is a named appendix or a front-matter section
     of the Document that deals exclusively with the relationship of the
     publishers or authors of the Document to the Document’s overall
     subject (or to related matters) and contains nothing that could
     fall directly within that overall subject.  (Thus, if the Document
     is in part a textbook of mathematics, a Secondary Section may not
     explain any mathematics.)  The relationship could be a matter of
     historical connection with the subject or with related matters, or
     of legal, commercial, philosophical, ethical or political position
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     The “Invariant Sections” are certain Secondary Sections whose
     titles are designated, as being those of Invariant Sections, in the
     notice that says that the Document is released under this License.
     If a section does not fit the above definition of Secondary then it
     is not allowed to be designated as Invariant.  The Document may
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     The “Cover Texts” are certain short passages of text that are
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     A “Transparent” copy of the Document means a machine-readable copy,
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  2. VERBATIM COPYING

     You may copy and distribute the Document in any medium, either
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  3. COPYING IN QUANTITY

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     If the required texts for either cover are too voluminous to fit
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     If you publish or distribute Opaque copies of the Document
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     It is requested, but not required, that you contact the authors of
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  4. MODIFICATIONS

     You may copy and distribute a Modified Version of the Document
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     Modified Version filling the role of the Document, thus licensing
     distribution and modification of the Modified Version to whoever
     possesses a copy of it.  In addition, you must do these things in
     the Modified Version:

       A. Use in the Title Page (and on the covers, if any) a title
          distinct from that of the Document, and from those of previous
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          History section of the Document).  You may use the same title
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       B. List on the Title Page, as authors, one or more persons or
          entities responsible for authorship of the modifications in
          the Modified Version, together with at least five of the
          principal authors of the Document (all of its principal
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          from this requirement.

       C. State on the Title page the name of the publisher of the
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       D. Preserve all the copyright notices of the Document.

       E. Add an appropriate copyright notice for your modifications
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       F. Include, immediately after the copyright notices, a license
          notice giving the public permission to use the Modified
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       G. Preserve in that license notice the full lists of Invariant
          Sections and required Cover Texts given in the Document’s
          license notice.

       H. Include an unaltered copy of this License.

       I. Preserve the section Entitled “History”, Preserve its Title,
          and add to it an item stating at least the title, year, new
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          previous sentence.

       J. Preserve the network location, if any, given in the Document
          for public access to a Transparent copy of the Document, and
          likewise the network locations given in the Document for
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          “History” section.  You may omit a network location for a work
          that was published at least four years before the Document
          itself, or if the original publisher of the version it refers
          to gives permission.

       K. For any section Entitled “Acknowledgements” or “Dedications”,
          Preserve the Title of the section, and preserve in the section
          all the substance and tone of each of the contributor
          acknowledgements and/or dedications given therein.

       L. Preserve all the Invariant Sections of the Document, unaltered
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       M. Delete any section Entitled “Endorsements”.  Such a section
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       N. Do not retitle any existing section to be Entitled
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       O. Preserve any Warranty Disclaimers.

     If the Modified Version includes new front-matter sections or
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     You may add a section Entitled “Endorsements”, provided it contains
     nothing but endorsements of your Modified Version by various
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     been approved by an organization as the authoritative definition of
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     You may add a passage of up to five words as a Front-Cover Text,
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     The author(s) and publisher(s) of the Document do not by this
     License give permission to use their names for publicity for or to
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  5. COMBINING DOCUMENTS

     You may combine the Document with other documents released under
     this License, under the terms defined in section 4 above for
     modified versions, provided that you include in the combination all
     of the Invariant Sections of all of the original documents,
     unmodified, and list them all as Invariant Sections of your
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     their Warranty Disclaimers.

     The combined work need only contain one copy of this License, and
     multiple identical Invariant Sections may be replaced with a single
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     In the combination, you must combine any sections Entitled
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     Entitled “History”; likewise combine any sections Entitled
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     must delete all sections Entitled “Endorsements.”

  6. COLLECTIONS OF DOCUMENTS

     You may make a collection consisting of the Document and other
     documents released under this License, and replace the individual
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  7. AGGREGATION WITH INDEPENDENT WORKS

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  8. TRANSLATION

     Translation is considered a kind of modification, so you may
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  9. TERMINATION

     You may not copy, modify, sublicense, or distribute the Document
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  10. FUTURE REVISIONS OF THIS LICENSE

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  11. RELICENSING

     “Massive Multiauthor Collaboration Site” (or “MMC Site”) means any
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     The operator of an MMC Site may republish an MMC contained in the
     site under CC-BY-SA on the same site at any time before August 1,
     2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents
====================================================

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.3
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
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   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the “with…Texts.” line with this:

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   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
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   If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
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their use in free software.



Tag Table:
Node: Top847
Node: Introduction3754
Node: How to Run Tests6693
Node: Running Tests Interactively7455
Node: Running Tests in Batch Mode10299
Node: Test Selectors11378
Node: How to Write Tests13537
Node: The ‘should’ Macro14424
Node: Expected Failures16526
Node: Tests and Their Environment18763
Node: Useful Techniques22348
Node: How to Debug Tests26166
Node: Understanding Explanations26565
Node: Interactive Debugging28481
Node: Extending ERT30664
Node: Defining Explanation Functions31021
Node: Low-Level Functions for Working with Tests31959
Node: Other Testing Concepts32788
Node: Mocks and Stubs33198
Node: Fixtures and Test Suites33783
Node: GNU Free Documentation License36199

End Tag Table


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