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10. Hooks

Kaocha aims to be flexible and adaptable, so that you are able to do things with it that the authors did not anticipate. We achieve this goal by being data-driven and by providing hooks. Data-driven means that Kaocha's behavior is determined by plain data structures. By manipulating these data structures you can change how Kaocha behaves.

With hooks you supply a function which gets called at a specific point within Kaocha's process, the function gets passed some data structure, typically a config, test-plan, or testable, and can then return an updated version of that data structure, which Kaocha will then continue using. The hook function can also simply perform side effects, and return the data structure unchanged.

There are two ways to provide Kaocha with these hook functions, you can write a plugin, or you can write a plain function and use the hooks plugin to hook it up. Both approaches are equivalent, everything you can do with a plugin you can do through hooks. A plugin is nothing more than a collection of hook functions.

Plugins are great when you want to implement functionality and share it as a reusable bundle, or if you are doing something that requires hooking into multiple extension points at once. For simple project level customization of Kaocha using hook functions and the hooks plugin provides an easier alternative.

This chapter will focus on the latter, project-level customization using the hooks plugin. In Chapter 9 we talked about the data structures that Kaocha uses, and about implementing plugins. It provides some important background since to use hooks well you need to have some understanding of how Kaocha's process works. It's a good idea to read through at least the "Concepts" section, and perhaps refer to it later on.

Note that hooks and plugins are not the only way to extend Kaocha. If you want to customize how Kaocha reports test run events on the command line then the recommended way to do this is through a custom reporter.

To start using hooks first enable the hooks plugin.

{:plugins [:hooks]}

Then add your first hook:

{:plugins [:hooks]
 :kaocha.hooks/pre-load [myproject.kaocha-hooks/start-load-message]}
(ns myproject.kaocha-hooks)

(defn start-load-message [config]
  (println "About to start loading!")

In tests.edn you provide the fully qualified symbol name, and Kaocha will load the namespace and use the function as a hook.

This function will be called at the start of the load process. Notice how it returns its argument. This is very important, so that Kaocha still has something to work with after your hook is finished. As a rule hooks should always return (a possibly updated version of) their first argument.

Config / Test-plan hooks

At a high level a Kaocha run consists of two main phases, load and run, and so you get four corresponding hooks:

  • pre-load
  • post-load
  • pre-run
  • post-run

pre-load receives a config (an expanded version of what is in your tests.edn), post-load and pre-run receive a test-plan. This is like the config, but it also contains a nested tree data structure representing all the testables Kaocha found and is about to run.

post-run receives a test-result, which is like the test plan, but for each testable information got added about the result of the test.

There's also a config hook, which runs even earlier than pre-load. Here you can change config settings like :kaocha/fail-fast?, :kaocha/color? or :kaocha/bindings.

pre-test / post-test

pre-test and post-test run before/after each individual testable. Note that a "testable" in Kaocha can be an individual test (a test var in clojure.test), but also a collection of tests (like a test ns), or a full test suite. pre-test and post-test will be run for each of these.

As arguments these receive the testable in question, and the test-plan.

So say you have a single unit test suite, with two namespaces, and each has a single test var, then these hooks will be called in this order:

  • pre-test unit
    • pre-test ns-1
      • pre-test var-1
      • post-test var-1
    • pre-test ns-2
      • pre-test var-2
      • post-test var-2
    • post-test ns-2
  • post-test unit

You should use the helpers in the kaocha.hierarchy namespace to determine if a testable is a suite (top-level), group (intermediate grouping), or leaf (an individual test with assertions).

For example, a common use case of pre-test hooks is to skip certain tests based on certain conditions. Here's an example hook that skips all tests that have "foo" in their name, when running on CI.

(ns myproject.kaocha-hooks
  (require [kaocha.hierarchy :as hierarchy]))

(defn my-pre-test-hook [testable test-plan]
  (if (and (hierarchy/leaf? testable)
           (System/getenv "CI")
           (.contains (str (:kaocha.testable/id testable)) "foo"))
    (assoc testable :kaocha.testable/skip true)
{:plugins [:hooks]
 :kaocha.plugin/hooks [myproject.kaocha-hooks/my-pre-test-hook]}

pre-load-test / post-load-test

The pre-load-test / post-load-test hooks are called before and after each individual test (testable) gets loaded. They are called with the testable and a config.

Like pre-test / post-test it is up to you to filter out the testables you are interested in, possibly using kaocha.hierarchy.

These hooks will fire for each test suite, and should fire for each group and leaf testable, but this relies on the test suite type implementation yielding control back to Kaocha when loading each level of tests, which may not always be true.

Suite level hooks

Generally hooks are declared at the top level of your test configuration (tests.edn), but the testable-level hooks can also be declared on a test suite. These are pre-load-test, post-load-test, pre-test, post-test. When declared like this they will be called before/after that specific suite gets loaded or run.

{:tests [{:kaocha.hooks/pre-load-test [...]}]}

These can be used for instance with kaocha-cljs2 to prepare the ClojureScript compilation and JavaScript runtime.

Special purpose hooks


Most of Kaocha's hooks have pre- and post- variants, but we don't have "around" hooks. This can be annoying, e.g., when setting up dynamic bindings on a per-test level, in this case you can use wrap-run to "wrap" Kaocha's run function.

In particular this wraps kaocha.testable/-run, so it receives a two argument function (the arguments are testable and test-plan), and should return such a function. wrap-run also receives the test-plan directly.

(defn my-wrap-run-hook [run _test-plan]
  (fn [testable test-plan]
    (println "about to run" (:kaocha.testable/id testable))
    (run testable test-plan)))

wrap-run is used by the output capturing plugin (on by default) to rebind *out* and *err* during tests.


Kaocha reporters are based on Clojure.test reporters. They are functions that receive "events" (map with a :type key), and based on those they print out information to the terminal.

The pre-report hook allows you to modify these events before they are passed to the reporter. They receive a single argument, the event map.

An example use case is the kaocha-noyoda plugin, which changes the assertion order from (= actual expected) to (= expected actual).


This is the final hook that gets called, it runs after the finally summary has been printed (after the :summary event has fired).

107 tests, 265 assertions, 0 failures.

The profiling plugin uses this hook to print out a list of the slowest tests.

Note that this hook only runs when Kaocha is invoked through kaocha.runner, so through the command line. It does not get run when invoked via kaocha.repl or kaocha.api.

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