R: Verify output

verify_output {testthat} | R Documentation

## Verify output

### Description

This is a regression test that records interwoven code and output into a file, in a similar way to kniting an `.Rmd` (but see caveats below).

`verify_output()` designed particularly for testing print methods and error messages, where the primary goal is to ensure that the output is helpful to a human. Obviously, you can't test that with code, so the best you can do is make the results explicit by saving them to text file. This makes the output easy to see in code reviews, and ensures that you don't change the output accidentally.

`verify_output()` is designed to be used with git: to see what has changed from the previous run, you'll need to use `git diff` or similar.

### Usage

    verify_output(
      path,
      code,
      width = 80,
      crayon = FALSE,
      unicode = FALSE,
      env = caller_env()
    )

### Arguments

`path` | Path to record results. This should usually be a call to `test_path()` to ensures that same path is used when run interactively (when the working directory is typically the project root), and when run as an autmated test (when the working directory will be `tests/testthat`).
`code` | Code to execute. This will usually be a multiline expression contained within `{}` (similarly to `test_that()` calls).
`width` | Width of console output
`crayon` | Enable crayon package colouring?
`unicode` | Enable cli package UTF-8 symbols? If you set this to `TRUE`, call `skip_if(!cli::is_utf8_output())` to disable the test on your CI platforms that don't support UTF-8 (e.g. Windows).
`env` | The environment to evaluate `code` in.

### Syntax

`verify_output()` can only capture the abstract syntax tree, losing all whitespace and comments. To mildy offset this limitation:

- Strings are converted to R comments in the output.

- Strings starting with `#` are converted to headers in the output.

### CRAN

On CRAN, `verify_output()` will never fail, even if the output changes. This avoids false positives because tests of print methods and error messages are often fragile due to implicit dependencies on other packages, and failure does not imply incorrect computation, just a change in presentation.

### Examples

    # The first argument would usually be `test_path("informative-name.txt"`)
    # but that is not permitted in examples
    path <- tempfile()
    verify_output(path, {
       head(mtcars)
       log(-10)
       "a" * 3
    })
    writeLines(readLines(path))

    # Use strings to create comments in the output
    verify_output(tempfile(), {
       "Print method"
       head(mtcars)

       "Warning"
       log(-10)

       "Error"
       "a" * 3
    })

    # Use strings starting with # to create headings
    verify_output(tempfile(), {
       "# Base functions"
       head(mtcars)
       log(-10)
       "a" * 3
    })

[Package testthat version 2.3.2 Index]