How can I write a test to handle an expected exception?

Question

How can I test for a function that throws an expected exception? Here is the function that throws the exception:

(defn seq-of-maps?
  "Tests for a sequence of maps, and throws a custom exception if not."
  [s-o-m]
  (if-not (seq? s-o-m)
    (throw (IllegalArgumentException. "s-o-m is not a sequence"))
    (if-not (map? (first s-o-m))
      (throw (IllegalArgumentException. "s-o-m is not a sequence of maps."))
      true)))

I want to design a test like the following in which an exception is thrown and caught and then compared. The following does not work:

(deftest test-seq-of-maps
  (let [map1  {:key1 "val1"}
        empv  []
        s-o-m (list {:key1 "val1"}{:key2 "val2"})
        excp1 (try 
                (seq-of-maps? map1)
                (catch Exception e (.getMessage e)))]
    (is (seq-of-maps? s-o-m))
    (is (not (= excp1 "s-o-m is not a sequence")))))

I am getting these errors:

Testing util.test.core

FAIL in (test-seq-of-maps) (core.clj:27)
expected: (not (= excp1 "s-o-m is not a sequence"))
  actual: (not (not true))

Ran 2 tests containing 6 assertions.
1 failures, 0 errors.    

Obviously, I am missing something about writing tests. I am having trouble figuring that out. My project was set up with lein new, and I am running the tests with lein test.

Thanks.

Answer 1

The last assertion in your test is incorrect; it should be (is (= excp1 "som is not a sequence")) since map1 isn't a seq of maps.

Aside from that, it's probably clearer to use (is (thrown? ..)) or (is (thrown-with-msg? ...)) to check for thrown exceptions.

Example:

(t/is (thrown? java.lang.ClassCastException (s/valid? ::sut/int-value "x"))))

Note that you just write classname as a symbol. Use java.lang.Exception if you don't care about the exact Exception .