How to test for Warning using ERT?

Question

I want to check for a warning but not sure how to go about it.

My warning is:

Warning (my-func-file-not-found): /path/to/somefile.txt

This is created with display-warning 'my-func-file-not-found.

I have tried this:

(should (with-temp-buffer
            (my-func "init/nf")
            (search-forward "file-not-found" nil t)))

and this

(should-error (my-func "init/nf") :type 'my-func-file-not-found )

Answer 1

Here's some code (as babel source blocks in an Org mode file, but you can extract them and run them in an Elisp file instead) - I hope it's self-explanatory, but I have to post-and-run, so I will add some verbiage later on.

* Code                                                                                                        :noexport:

Define the function to test:

#+name: my-func
#+begin_src elisp 
  (defun my-func (file)
    (if (not (file-exists-p file))
        (display-warning 'my-func-file-not-found file)
      ;; else file exists: do something with it - here I just return the string "yes"
      "yes"))
#+end_src

#+RESULTS: my-func
: my-func

Write the test:

#+name: my-func-test
#+begin_src elisp
  (require 'ert)

  (defun check-for-warning (msg)
    (with-current-buffer "*Warnings*"
      (goto-char (point-min))
      (search-forward msg nil t)))

  (ert-deftest test-my-func/warning-produced ()
    "Call my-func with non-existent file and check that the warning is produced."
    (let ((fname "/some/non/existent/path"))
      ;;; This (killing a buffer) is something that tests are *NOT* supposed to do,
      ;;; but I can't think of a clean way to *test* a side-effect producing function like `display-warning'.
      ;; if the *Warnings* buffer exists, kill it.
      (condition-case x
        (kill-buffer "*Warnings*")
        ('error t))
      
      ;; run the function to test
      (my-func fname)
      ;; check that the side-effect has happened
      (should (check-for-warning (format "Warning (my-func-file-not-found): %s" fname)))))

#+end_src

Run the test:
#+name: run-tests
#+begin_src elisp
 (ert "test-my-func/")
#+end_src