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    Sep 29, 2017

    Tampere, Helsinki, Finland


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    testit Build Status

    Midje style assertions for clojure.test

    Clojars dependency: [metosin/testit "0.2.0"]

    Note: This library is still under heavy development!

    Goals and non-goals


    • Allow writing tests in Midje style with => and =not=>
    • Uses (and extends) clojure.test
    • Allow extending functionality


    • Does not provide any compatibility with Midje
    • Does not improve output

    Quick intro for Midje users

    with Midje:

    (ns some.midje.tests 
      (:require [midje.sweet :refer :all]))
      (+ 1 2) => 3
      {:a 1 :z 1} => (contains {:a 1}))

    with testit:

    (ns example.midje-example
      (:require [clojure.test :refer :all]
                [testit.core :refer :all]))
    (deftest midje-impersonation
        (+ 1 2) => 3
        {:a 1 :z 1} =in=> {:a 1}))


    The fact macro generates a clojure.test/is form with the arrow in place of test function.

    (macroexpand-1 '(fact (+ 1 2) => 3))
    => (clojure.test/is (=> 3 (+ 1 2)) "(+ 1 2) => 3")

    The testit extends basic clojure.test/is functionality by adding appropriate assertion logic for =>, =not=>, =in=>, and =throws=> symbols.

    The facts allows grouping of multiple assertions into a single form. For example:

    (deftest group-multiple-assertions
      (facts "simple match tests"
        (+ 1 2) => 3
        (* 21 2) => 42
        (+ 623 714) => 1337))

    The => and =not=> arrows

    The left side of => arrow is a form that is evaluated once. The right side can be a simple form that is also evaluated once. Test is performed by comparing the evaluated values for equality (or non-equality in case of =not=>).

    (deftest simple-form
        (* 21 2) => 42))

    The right side of => can also be a predicate function. In that case the test is performed by passing the value of the left side to the predicate.

    (deftest function-predicate
        (* 21 2) => integer?))

    A common practice is to call function that returns a predicate. For example:

    (deftest generate-predicate
        (* 21 2) => (partial > 1337)))

    A bit more complex example:

    (defn close-enough [expected-value margin]
      (fn [result]
        (<= (- expected-value margin) result (+ expected-value margin))))
    (deftest function-generating-predicate
        (* 21 2) => (close-enough 40 5)))

    Testing exceptions with the =throws=> arrow

    The =throws=> arrow can be used to assert that the evaluation of the left side throws an exception. The right side of =throws=> can be:

    • A class extending java.lang.Throwable
    • An object extending java.lang.Throwable
    • A predicate function
    • A seq of the above

    If the right side is a class, the assertion is made to ensure that the left side throws an exception that is, or extends, the class on the right side.

    (fact "Match exception class"
      (/ 1 0) =throws=> java.lang.ArithmeticException)

    You can also use an exception object. This ensures that the exception is of correct type, and also that the message of the exception equals that of the message on right side.

    (fact "Match exception class and message"
      (/ 1 0) =throws=> (java.lang.ArithmeticException. "Divide by zero"))

    Most flexible case is to use a predicate function.

    (fact "Match against predicate"
      (/ 1 0) =throws=> #(-> % .getMessage (str/starts-with? "Divide")))

    Finally, =throws=> supports a sequence. The thrown exception is tested against the first element from the seq, and it’s cause to the second, and so on. This is very handy when the actual exception you are interested is wrapped into another exception, for example to java.util.concurrent.ExecutionException.

    This example creates an java.lang.ArithmeticException, wrapped into a java.lang.RuntimeException, wrapped into a java.util.concurrent.ExecutionException.

    The fact then tests that the left side throws an exception that extends java.lang.Exception and has a message "1", and that is caused by another exception, also extending java.lang.Exception with message "2", that is caused yet another exception, this time with message "3":

      (->> (java.lang.ArithmeticException. "3")
           (java.lang.RuntimeException. "2")
           (java.util.concurrent.ExecutionException. "1")
      =throws=> [(Exception. "1")
                 (Exception. "2")
                 (Exception. "3")])

    A common pattern with Clojure code is to generate exceptions with clojure.core/ex-info. To help testing these kind of exceptions, testit provides a function testit.core/ex-info?. The function accepts a message (string or a predicate) and a data (map or a predicate), and returns a predicate that tests given exception type (must extend clojure.lang.IExceptionInfo), message and data.

    (fact "Match ex-info exceptions"
      (throw (ex-info "oh no" {:reason "too lazy"}))
      (ex-info? "oh no" {:reason "too lazy"}))

    The above test ensures that the left side throws an ex-info exception with expected message and data.

    Helper predicates


    The any is a predicate that matches anything. It’s implemented like this:

    ; in ns testit.core:
    (def any (constantly true))

    truthy and falsey

    Other helper predicate are testit.core/truthy and testit.core/falsey which test given values for clojure ‘truthines’ and ‘falsines’ respectively.

    The =eventually=> arrow

    You can use the =eventually=> arrow to test async code.

    (let [a (atom -1)]
        (Thread/sleep 100)
        (reset! a 1))
        (deref a) =eventually=> pos?))

    On the code above, the result of evaluating the (deref a) is initially -1. The test does not match the expected predicate pos?. How ever, the =eventually=> will keep repeating the test until the test matches, or a timeout occurs. Eventually the future resets the atom to 1 and the test passes.

    By default the =eventually=> keeps evaluating and testing every 50 ms and the timeout is 1 sec. You can change these by binding testit.core/*eventually-polling-ms* and testit.core/*eventually-timeout-ms*. For example, code below sets the timeout to 2 sec.

    (testing "You can change the timeout from it's default of 1sec"
      (binding [*eventually-timeout-ms* 2000]
        (let [a (atom -1)]
            (Thread/sleep 1500)
            (reset! a 1))
            (deref a) =eventually=> pos?))))


    The =in=> is for more relaxed equality tests, like contains in Midje. For example:

      {:a 1 :b 2} => {:a 1})  ;=> FAILS
      {:a 1 :b 2} =in=> {:a 1})  ;=> Ok

    This ensures that the actual (left side of =in=>) contains everything the expected (right side) contains.

    Testing maps with contains

    When given a map, =in=> checks that all given keys are found and they match expectations. Expected map can be nested and can contain predicates. For example:

      {:a 1
       :b {:c 42
           :d {:e "foo"}}} 
      {:b {:c pos?
           :d {:e string?}}})

    A very common use-case for this kind of testing is testing the HTTP responses. Here’s an example.

    (ns example.http-example
      (:require [clojure.test :refer :all]
                [testit.core :refer :all]
                [clojure.string :as str]
                [clj-http.client :as http]))
    (deftest test-google-response
        (http/get "")
        {:status 200
         :headers {"Content-Type" #(str/starts-with? % "text/html")}
         :body string?}))

    Testing sequentials with =in=>

    When given a vector, =in=> checks that the expected value contains matches for each expected value, where the expected values can be basic values or predicates. For example:

      [1 2 3] =in=> [1 pos? integer?])

    The matching is recursive, so this works too:

      [{:a 1, :b 1}
       {:a 2, :b 2}
       {:a 3, :b 3}]
      [{:a 1}, map?, {:b pos?}])

    …and there can be more

    If the expectation vector ends with symbol ..., the actual vector can contain more elements, they are just ignored. For example, these tests all pass:

      [1 2 3] =in=> [1 2 3 ...]
      [1 2 3 4] =in=> [1 2 3 ...]
      [1 2 3 4 5] =in=> [1 2 3 ...])

    This does not pass:

      [1 2] =in=> [1 2 3 ...])  ;=> FAILS

    When the order is not important

    If the expected vector has metadata :in-any-order, the =in=> checks that all expected elements are found in actual, but they are allowed to be in any order.

      [-1 0 +1] =in=> ^:in-any-order [0 +1]
      [-1 0 +1] =in=> ^:in-any-order [pos? neg?]))

    Error messages with =in=>

    The =in=> tries to provide informative error messages. For example:

      {:a {:b {:c -1}}} =in=> {:a {:b {:c pos?}}})
    FAIL in (failing-test) (in_example.clj:52)
    {:a {:b {:c -1}}} =in=> {:a {:b {:c pos?}}}:
      in [:a :b :c]:
      (pos? -1) => false
        expected: pos?
          actual: -1
    expected: {:a {:b {:c pos?}}}
      actual: {:a {:b {:c -1}}}

    The in [:a :b :c] in above error message shows the path to nested element that failed the test.

    Here’s another example with sequential values:

      [0 1 2 3] =in=> [0 1 42 3])
    FAIL in (failing-test) (in_example.clj:54)
    [0 1 2 3] =in=> [0 1 42 3]:
      in [2]:
      (= 42 2) => false
        expected: 42
          actual: 2
    expected: [0 1 42 3]
      actual: [0 1 2 3]

    Strings have special reporting:

      "foodar" => "foobar")
    FAIL in (failing-test) (in_example.clj:56)
    foodar =in=> foobar:
        (= "foobar" "foodar") => false
        expected: "foobar"
          actual: "foodar"
            diff:     ^^^
    expected: "foobar"
      actual: "foodar"

    Here’s an example with nested structures:

      {:a {:b [0 1 2 {:c "foodar"}]}}
      {:a {:b [0 neg? 2 {:c "foobar"}]}})
    FAIL in (failing-test) (in_example.clj:58)
    {:a {:b [0 1 2 {:c "foodar"}]}} =in=> {:a {:b [0 neg? 2 {:c "foobar"}]}}:
      in [:a :b 1]:
      (neg? 1) => false
        expected: neg?
          actual: 1      
      in [:a :b 3 :c]:
      (= "foobar" "foodar") => false
        expected: "foobar"
          actual: "foodar"
            diff:     ^^^
    expected: {:a {:b [0 neg? 2 {:c "foobar"}]}}
      actual: {:a {:b [0 1 2 {:c "foodar"}]}}

    Note that the above message reports two errors, first at [:a :b 1] and the second at [:a :b 3 :c].

    Extend via functions

    The testit is designed to be easily extendable using plain old functions. You can provide your own predicates, and combine your predicates with those provided by clojure.core and testit.

    Here’s an example that combines contains and ex-info?:

    (fact "Match ex-info exceptions with combine"
      (throw (ex-info "oh no" {:reason "too lazy"}))
      (ex-info? any (contains {:reason string?})))

    This tests that the left side throws an ex-info exception, with any message and a data that contains at least a :reason that must be a string.

    Extending testit with your own arrows

    You can add your custom arrows using clojure.test/assert-expr. For example, here’s a simple extension that asserts that the test is completed within 1 sec.

    (ns testit.your-own-arrow
      (:require [clojure.test :refer :all]
                [testit.core :refer :all]))
    (declare =quickly=>)
    (defmethod assert-expr '=quickly=> [msg [_ & body]]
      (assert-expr msg `(let [d# (future ~@body)
                              r# (deref d# 1000 ::timeout)]
                          (if (= r# ::timeout)

    And then you use it like this:

    (deftest things-must-be-fast-tests
        (do (Thread/sleep 200) 42) =quickly=> 42)    ;=> PASS
        (do (Thread/sleep 2000) 42) =quickly=> 42))  ;=> FAIL

    In the example above, the first fact passes but the second fails after 1 sec.


    • [x] Implement =eventually=> for async tests
    • [x] Add support to comparing seq’s and lists
    • [ ] Detect and complain about common mistakes like using multile => forms with fact
    • [ ] Provide better error messages when right side is a predicate
    • [ ] Figure out a way to support humane-test-output style output


    Copyright © 2017 Metosin Oy

    Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.