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Using a mock of an S4 class

6 messages · Ramiro Barrantes, David Winsemius, Jeff Newmiller +1 more

Ramiro Barrantes
# 
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??

Thanks in advance,
Ramiro
David Winsemius
# 
I'm not seeing a class definition for any "S4"-classed object. I would expect you to have used `setClass`. I believe that once the class is defined that you should have access to the `new` constructor function.
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David Winsemius
Alameda, CA, USA
Ramiro Barrantes
# 
Yes, so are you suggesting that I create an instance of my S4 class in order to test my function.

My understanding is that ideally the test should not depend on any code besides the one that I am testing.

I just thought that you could perhaps define a mock class so that I would not need to invoke my external constructor function, hence making this test focus exclusively on the function that I am trying to test.  Is it possible to do this in R?
On 2/1/17, 6:57 PM, "David Winsemius" <dwinsemius at comcast.net> wrote:

            

      
      
      
    

  
  


  


      

    

    

      
      

        


  

        

      Jeff Newmiller
    

    

      
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You seem like you have painted yourself into a corner... you want to access slots in an S4 object yet you don't want to create one. Are you going to do this test without the benefit of an operating system either? 

Beware of getting too meta... David didn't say you had to use an S4 object defined elsewhere... you invented that. But you do have to use the setClass function to create an S4 class with which you can create an object. No class, no object, mock or otherwise.

Me, I like S3.

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Martin Morgan
    

    

      
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On 02/01/2017 02:46 PM, Ramiro Barrantes wrote:

            

      
      
      
    

        
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

            

      
      
      
    

        

      
      
    

        
This email message may contain legally privileged and/or...{{dropped:2}}

  
  


  


      

    

    

      
      

        


  

        

      Ramiro Barrantes
    

    

      
      #
    

  


  

      
Thank you so much!!  This is very helpful.

        
On 02/01/2017 02:46 PM, Ramiro Barrantes wrote:

            

      
      
      
    

        
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

            

      
      
      
    

        

      
      
    

        
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