I don't think there is an easy answer here, and note that this is a problem
generic to mocking (i.e. not clojure or midje specific).
The usual advice applies though:
- do you really need to mock? Unit testing is about the coarseness of
granularity which is defined more by cohesion and abstractions than "one
function and only this function" (e.g. remove the problem by not overly
mocking)
- make everything fail then fix rather than fix and then upgrade (i.e.
update every instance of the call/mock to check-pw before the
implementation of check-pw).
- Clojure's lack of types means the compiler can't help. Schema or
core.typed can. This isn't *the* answer, but I have found it very helpful.
As mentioned elsewhere, mocking in general is a very powerful tool, but it
is does need wielding carefully. These problems are easier to swallow in
strongly typed languages because of IDE support (changing parameters around
in Java with IntelliJ is a matter of a few key presses for example).
Hope this helps.
On Tuesday, 6 January 2015 08:22:36 UTC, Akos Gyimesi wrote:
>
>
> On Sat, Jan 3, 2015, at 02:46 AM, Brian Marick wrote:
> >
> > > I use TDD and mocking/stubbing (conjure) to test each layer of my
> code.
> > > The problem is when I change the function signature and the tests do
> not
> > > break, because the mocks/stubs do not know when their argument lists
> no
> > > longer agree with the underlying function they are mocking. Is there
> a
> > > way to catch this? Short of a test suite that eschews stubbing in
> favor
> > > of full setup/teardown of DB data for each test?
> >
> > Could you give an example? I use mocks fairly heavily, and I don't seem
> > to have this problem. Perhaps it's because I change the tests before the
> > code?
>
> Although the subject changed a little bit, I would be still interested
> in your approach to refactoring if there is heavy use of mocking. Let me
> give you an example:
>
> Let's say I am writing a login form, trying to use the top-down approach
> you described. My approach could be the following:
>
> (unfinished check-pw)
>
> (fact "login-form succeeds if user enters the correct password"
> (login-form-success? {:username "admin" :password "secret"}) => true
> (provided
> (db/get-user "admin") => (contains (:password "my-secret-hash"))
> (check-pw "my-secret-hash" "secret") => true))
>
> (defn login-form-success? [user-input]
> (let [user (db/get-user (:username user-input))]
> (check-pw (:password user) (:password user-input))))
>
> Then I finish the check-pw function and everything works.
>
> Now, later that day I decide that I pass the whole user object to the
> check-pw function. Maybe I want to use the user ID as a salt, or maybe I
> just want to leave the possibility for checking password expiration,
> etc. So I modify the test and the implementation of check-pw so that the
> first parameter is the user object, not the password hash.
>
> Suddenly my co-worker comes to me saying "hey, I need you on a meeting
> right now!" I close my laptop, and an hour later I think "where were
> we?..." I run all the tests, and they all pass, so I commit.
>
> Except... I forgot to modify all the invocations of check-pw in both the
> test and the implementation. Every test pass, so I have no way of
> finding out the problem without careful code review or by examining the
> stack traces from the live code.
>
> While this bug is easy to catch, what if my function is mocked in
> several places, and I fail to rewrite all of them properly?
>
> Do you have any advice on what you would have done differently here to
> avoid this bug?
>
> Regards,
> Akos
>
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