[R] Using a mock of an S4 class

Ramiro Barrantes ramiro at precisionbioassay.com
Fri Feb 3 16:51:51 CET 2017

Thank you so much!!  This is very helpful.

On 02/01/2017 02:46 PM, Ramiro Barrantes wrote:
> Hello,
> I have a function that applies to an S4 object which contains a slot called @analysis:
> function calculation(myObject) {
>   tmp <- myObjects at analysis
>   result <- ...operations on analysis...
>   return result
> }
> I am writing a unit test for this function.  So I was hoping to create a mock object but I can't figure out how to do it:
> test_that("test calculation function", {
>   mockMyObject<- mock(?????)  #I am not sure what to put here
>   r<-calculation(mockMyObject)
>   expect_true(r,0.83625)
> })
> How can I create a mock S4 object??

I don't know of a convenient way to create a mock with functionality
like mocks in other languages. But here's a class

   .A = setClass("A", contains="integer")

This creates an instance that might be used as a mock

    mock = .A()  # same as new("A")

but maybe you have an initialize method (initialize methods are very
tricky to get correct, and many people avoid them, using
plain-old-functions to form an API around object creation; the
plain-old-function finishes by calling the constructor .A() or new("A"))
that has side effects that are inappropriate for your test, mimicked
here with stop()

   setMethod("initialize", "A", function(.Object, ...) stop("oops"))

our initial attempts are thwarted

 > .A()
Error in initialize(value, ...) : oops

but we could reach into our bag of hacks and try

   mock = .Call(methods:::C_new_object, getClassDef("A"))

You would still need to populate slots / data used in your test, e.g.,

   slot(mock, ".Data") = 1:4

This is robust to any validity method, since the validity method is not
invoked on direct slot assignment

   setValidity("A", function(object) {
       if (all(object > 0)) TRUE else "oops2"

   slot(mock, ".Data") = 0:4  # still works

So something like

   mockS4object = function(class, ..., where=topenv(parent.frame())) {
       obj <- .Call(
           getClassDef(class, where=where)

       args = list(...)
       for (nm in names(args))
           slot(obj, nm) = args[[nm]]

   mockS4object("A", .Data=1:4)

Mock objects typically have useful testing properties, like returning
the number of times a slot (field) is accessed. Unfortunately, I don't
have anything to offer for that.


> Thanks in advance,
> Ramiro
>       [[alternative HTML version deleted]]
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

This email message may contain legally privileged and/or confidential information.  If you are not the intended recipient(s), or the employee or agent responsible for the delivery of this message to the intended recipient(s), you are hereby notified that any disclosure, copying, distribution, or use of this email message is prohibited.  If you have received this message in error, please notify the sender immediately by e-mail and delete this email message from your computer. Thank you.

More information about the R-help mailing list