How do I temporarily mock a function for testing?

Question

I'd like to patch some elisp functions inside my unit tests. I've tried cl-labels and cl-flet, but neither achieve the result I want:

(defun return-number ()
  1)

(defun calls-return-number ()
  (return-number))

(defun patch-cl-labels ()
  (cl-labels ((return-number () 2))
    (calls-return-number)))

(message "After patching: %s" (patch-cl-labels))

I've looked at monkey-patching a function (but this permanently changes behaviour) and patching a third-party function (this uses advice and holds on to the old function).

Answer 1

Both macros are lexically scoped, that's why they have no effect on the "distant" call. Either use the deprecated older flet or cl-letf.

(cl-letf (((symbol-function 'return-number)
           (lambda () 2)))
  ...)