clojure test that assertion throws

Question

I have a function defined as:

(defn strict-get
  [m key]
  {:pre [(is (contains? m key))]}
  (get m key))

And then I have a test for it:

(is (thrown? java.lang.AssertionError (strict-get {} :abc)))

However this test fails:

  ;; FAIL in () (myfile.clj:189)
  ;; throws exception when key is not present
  ;; expected: (contains? m key)
  ;; actual: (not (contains? {} :abc))

What is needed to check that the assertion would throw an error?

Isn't there redundant `is` in the body of the precondition? try change it to `{:pre [(contains? m key)]}` — OlegTheCat, Nov 30 '16 at 20:45
@OlegTheCat well spotted! I am using this trick from there: http://stackoverflow.com/a/24836592/1327651 I guess I have to choose between tests and useful REPL errors. — nha, Nov 30 '16 at 21:42
From Clojure 1.9 you don't have to chose. See http://stackoverflow.com/questions/37885542/meaningful-error-message-for-clojure-spec-validation-in-pre — NielsK, Dec 01 '16 at 10:22

Leon Grapenthin · Accepted Answer · 2016-11-30T21:56:34.117

The reason your assertion fails because you are nesting two is. The inner is already catches the exception so the outer is test then fails because nothing is thrown.

(defn strict-get
  [m key]
  {:pre [(contains? m key)]} ;; <-- fix
  (get m key))

(is (thrown? java.lang.AssertionError (strict-get {} nil)))
;; does not throw, but returns exception object for reasons idk

(deftest strict-get-test
  (is (thrown? java.lang.AssertionError (strict-get {} nil))))

(strict-get-test) ;; passes

clojure test that assertion throws

1 Answers1