Spy

Spy - a Clojure and ClojureScript library for stubs, spies and mocks. This library is aimed at users of clojure.test.

It records calls and responses to and from a function, allowing you to verify interactions. Terms used in this library are as follows, there are many different names for Mocks, see Test Doubles, Fakes, Mocks and Stubs for more detail.

1. Stub - function that returns a hardcoded value
2. Spy - wrapper around a function allowing verification of interactions with the function
3. Mock - function with a fake implementation to be used in place of the real thing

Documentation

• API Docs
• Introducing Spy - a Clojure / ClojureScript library for Spies, Stubs and Mocks

Usage

REPL (Clojure)

(require '[spy.core :as spy]       ;; the core library with functions returning booleans
         '[spy.assert :as assert]  ;; assertions wrapping clojure.test/is
         '[clojure.test :refer [testing is]])

(defn adder [x y] (+ x y))

(def spy-adder (spy/spy adder))

(is (= [] (spy/calls spy-adder)))
(is (= [] (spy/responses spy-adder)))

(is (true? (spy/not-called? spy-adder))) ;; spy.core/not-called? returns a boolean

(assert/not-called? spy-adder) ;; spy.assert/not-called? returns a boolean, but also wraps clojure.test/is so failures are reported

(testing "Let's see what a failure looks like..."
  (assert/called? spy-adder))

;; FAIL in () (form-init4641634702245604141.clj:37)
;; Let's see what a failure looks like...
;; Expected at least 1 call
;; Actual: 0 calls.
;; expected: (spy.core/called-at-least-n-times? spy-adder 1)
;;   actual: (not (spy.core/called-at-least-n-times? #function[clojure.lang.AFunction/1] 1))
;; false


(testing "calling the function"
  (is (= 3 (spy-adder 1 2))))

(testing "calls to the spy can be accessed via spy/calls"
  (is (= [[1 2]] (spy/calls spy-adder))))

(testing "responses from the spy can be accessed via spy/responses"
  (is (= [3] (spy/responses spy-adder))))

(testing "let's do another call"
  (is (= 42 (spy-adder 40 2))))

(testing "calls and responses are stored on the spy using metadata"
  (meta spy-adder) ;; {:calls     #atom[[(1 2)] 0x7612740d],
                   ;;  :responses #atom[[3] 0x26525904]}
  (let [{:keys [calls responses]} (meta spy-adder)]
    (is (= [[1 2] [40 2]] @calls))
    (is (= [3 42] @responses))))

(testing "but they can be access via spy/calls and spy/responses"
  (is (= [[1 2] [40 2]] (spy/calls spy-adder)))
  (is (= [3 42] (spy/responses spy-adder))))

(testing "we can check if the spy was called with some arguments"
  (is (true? (spy/called-with? spy-adder 1 2)))
  (is (false? (spy/called-with? spy-adder 1 59))))

(testing "but spy.assert gives us better error messages when our assertions don't hold true"
  (assert/called-with? spy-adder 66 99))

;; FAIL in () (form-init15061478131364358.clj:197)
;; assert gives us better error messages when our assertions don't hold true
;; Expected a call with (66 99)
;; Actual calls: [(1 2) (40 2)]
;; expected: (spy.core/called-with? spy-adder 66 99)
;;   actual: (not (spy.core/called-with? #function[clojure.lang.AFunction/1] 66 99))
;;false

Spies

spy.core/spy wraps a function and records calls to the function and responses returned by the function, this is done using an atom. Calls and responses are stored on the function itself using metadata.

(defn my-adder [x y]
  (+ x y))

(let [f (spy/spy my-adder)] ;; create a spy that wraps a simple adder function
      (is (spy/not-called? f)) ;; verify it hasn't been called yet
      (is (= 3 (f 1 2))) ;; call the function
      (is (spy/called-with? f 1 2)) ;; verify it was called with the arguments
      (is (spy/called-once? f))) ;; verify it was called only once

Stubs

A stub is a spy that wraps constantly, providing us with a function that returns a value and giving us the ability to verify calls were made to the stub.

(let [f (spy/stub 42)] ;; create a stub that returns a hardcoded value
      (is (spy/not-called? f)) ;; verify the stub has not been called yet
      (f) ;; call the stub
      (is (spy/called? f))
      (is (spy/called-once? f))
      (f) ;; call it for a second time
      (f) ;; call if for a third time
      (is (spy/called-n? 3 f))) ;; verify it was called 3 times

Mocks (also known as Fakes / Test Doubles)

To implement a mock you just need to implement a function that has the same contract as the one you're replacing, the best person to do this is you! For convenience this library provides s/mock which is an alias for s/spy, it's up to you to write the function that mocks the behaviour:

(let [f (spy/mock (fn [x] (if (= 1 x)
                            :one
                            :something-else)))]
      (is (= :one (f 1)))
      (is (spy/called-once? f))
      (is (= :something-else (f 42))))

Exceptions

If you spy on a function that throws an exception then Spy will catch your exception, record it in the responses, then re-throw the original exception, thus enabling you to test that the exception was thrown. A stub-throws helper function is provided.

Clojure

(let [f (spy/stub-throws (Exception. "Goodbye World!"))]
      (is (thrown? Exception (f)))
      (is (= 1 (count (spy/responses f))))
      (is (contains? (spy/first-response f) :thrown))
      (is (= "Goodbye World!" (-> (spy/first-response f) :thrown :cause))))

ClojureScript

(let [f (spy/stub-throws (js/Error "Goodbye World!"))]
      (is (thrown? js/Object (f)))
      (is (= 1 (count (spy/responses f))))
      (is (contains? (spy/first-response f) :thrown)))

Using with-redefs to replace functions with spies

If you're testing synchronous code then you can replace functions using with-redefs, if you're testing async code then it's safer to pass the functions in using dependency injection. If you want to see more examples of this checkout this excellent blog post about TDD in Clojure.

(ns spy-example.core-test
  (:require [clojure.test :refer [deftest testing is]]
            [spy.core :as spy]))

(def beatle->email
  {:john   "john.lennon@beatles.com"
   :paul   "paul.mccartney@beatles.com"
   :george "george.harrison@beatles.com"
   :ringo  "ringo.starr@beatles.com"})

(defn lookup-email [beatle-id]
  (get beatle->email beatle-id))

(defn send-message [email message]
  (println (str "Sending " message " to " email))
  nil)

(defn email-beatle [beatle-id message]
  (when-let [email (lookup-email beatle-id)]
    (send-message email message)))

(deftest email-beatle-test
  (testing "A message is sent to a Beatle"
    ;; example 1 - wrap the original fn (so it is still called)
    (with-redefs [send-message (spy/spy send-message)]
      (email-beatle :ringo "Hello Ringo!")
      (is (spy/called-once? send-message))
      (is (spy/called-with? send-message "ringo.starr@beatles.com" "Hello Ringo!"))))

  (testing "A message is not sent to a Rolling Stone"
    ;; example 2 - call spy without passing a fn (to avoid sending the email)
    (with-redefs [send-message (spy/spy)]
      (email-beatle :mick "Hello Mr Jagger!")
      (is (spy/not-called? send-message)))))

Spying on Protocols (Experimental)

Currently only Clojure is supported, I intend to make this work for ClojureScript too but it's a little trickier. I'm open to suggestions on how to improve this and support ClojureScript, all contributions are welcome!

See test/clj/spy/protocol_test.clj for examples.

Contributing

Pull requests are welcome. Please run the test suite and check that all tests pass prior to submission.

Tests:

$ lein test

Code coverage:

$ lein cloverage

Docs:

$ lein codox

License

MIT License

Copyright (c) 2019 Alexander James King

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