[R] Using a mock of an S4 class

[Martin Morgan]

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(
           methods:::C_new_object,
           getClassDef(class, where=where)
       )

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

       obj
   }
   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.

Martin


>
> Thanks in advance,
> Ramiro
>
> 	[[alternative HTML version deleted]]
>
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