compare output to output from previous version of package with testthat

[teebusch]

I'm working on a package in which a function was written by an expert who has since left the project. I myself am not an expert and thus want to make sure that I never break the original algorithm without noticing.

Thus, I'd like to test the output of the current version of a function in my package against that of the same function in a previous version of the package, identifiable by a git commit, branch, or similar in the package repository.

Lets say, for example, I would like to compare the output of foo() from the dev branch against foo() from a certain commit on the main branch.

test_that("old foo matches new foo", {
  
  foo_new <- foo()
  foo_old <- ... # install and load earlier version of foo somehow
  
  expect_equal(foo_old("bar"), foo_new("bar"))
})

Can I somehow install the old version of the package temporarily to run the test and how can I avoid messing up the namespace?

[maelle]

Why not write tests in the old version of the package first (maybe snapshot tests, or any other unit test) and then do your edits (possibly in a git branch) and see if the tests still pass?

The question " I’m interested in your thoughts on Jenny Bryan’s question (first in the list). If you’ve got a package already, where can one start with writing tests or how should one proceed then?" towards the end of https://docs.google.com/document/d/1TLj44dNbJHeANcgV4f0JZF1-0j76EsxlxDjGBWb5rzc/ might also be useful.

[teebusch]

Thank you! I think snapshot tests will do the trick, although it will be a bit of a hassle to retroactively add a snapshot for the function. I should have done that when I started making changes

I suppose I have to ...

• branch from the old commit using git checkout -b add_snapshot_test <SHA of commit>
• create a snapshot test in the branch, e.g.

testthat("foo never changes", {  
    expect_snapshot_output(foo("bar")) 
})

• run the test once to create a snapshot
• merge the branch with the new snapshot test (and the snapshot) into the dev branch
• (possibly: change the foo() function call in the test to reflect changes in the function signature)

I will give it a try!

[maelle]

@teebusch

I should have done that when I started making changes

Well, hindsight is 20/20. Good luck!