tinysnapshot: Snapshots for unit tests in R using the tinytest framework.

tinytest is a “lightweight, no-dependency, but full-featured package for unit testing in R created by Mark van der Loo.

tinysnapshot extends tinytest with expectations to test plots (base R or ggplot2) and print() output. In particular, tinysnapshot allows:

  1. Taking snapshots of known “target” plots or print() output.
  2. Testing if the “current” plot or print() output matches the target.
  3. Displaying a visual “diff” to facilitate comparison when a test fails.

Under the hood, tinysnapshot uses the magick package by Jeroen Ooms to read and compare images, and the diffobjpackage by Brodie Gaslam to compare printed output.



Install the development version of tinysnapshot:


You may also want to install additional packages to benefit from extra features:

install.packages(c("rsvg", "ragg", "svglite"))

Visual expectations: expect_snapshot_plot()

To test a visual expectation, we create an R script, give it a name which starts with “test”, and save it in the inst/tinytest/ directory of our package.

Each test script with visual expectations must include these two lines at the top:


When users run the tinytest suite, the expect_snapshot_plot() and expect_snapshot_print() expectations are executed and three main states can arise:


In this example script, we test two ggplot2 objects:


p1 <- ggplot(mtcars, aes(mpg, wt)) + geom_point()
p2 <- ggplot(mtcars, aes(mpg, hp)) + geom_point()

# On first run: fail and save a snapshot
# On subsequent runs: pass
expect_snapshot_plot(p1, label = "ggplot2_example")

# Always fails
expect_snapshot_plot(p2, label = "ggplot2_example")

Base R graphics

Testing a Base R plot is slightly different: we need to supply a function which prints the plot:


p1 <- function() plot(mtcars$hp, mtcars$wt)
p2 <- function() plot(mtcars$hp, mtcars$mpg)

# On first run: fail and save a snapshot
# On subsequent runs: pass
expect_snapshot_plot(p1, label = "base_example")

# Always fails
expect_snapshot_plot(p2, label = "base_example")

Options and arguments

expect_snapshot_plot supports 4 graphics devices: png and ragg for PNG, and svg and svglite for SVG. It can set different values for the height and width of the pictures (pixels for PNG and inches for SVG). Most of the arguments can also be fixed globally using options:

options(tinysnapshot_device = "svglite")
options(tinysnapshot_height = 7) # inches
options(tinysnapshot_width = 7)
options(tinysnapshot_tol = 200) # pixels

Visual diff

When (not “if”) tests fail, tinysnapshot will save diff files in the inst/tinytest/_tinysnapshot_review/ folder. Diff files for plots look like this:

First, we save this script in inst/tinytest/test-print.R:


mod1 <- lm(mpg ~ hp + factor(gear), mtcars)
expect_snapshot_print(summary(mod1), label = "print-lm_summary")

mod2 <- lm(mpg ~ factor(gear), mtcars)
expect_snapshot_print(summary(mod2), label = "print-lm_summary")

Then, we run the tests.


The first time we run the test, it fails and saves a reference file. The second time we run it, there is already a reference text file, so only one of the tests fails. This is the expected result.

When tests fail, tinytest will return a diff like this one:

    test-print.R..................    2 tests 2 fails 0.3s
    ----- FAILED[]: test-print.R<12--12>
    call| expect_snapshot_print(summary(mod1), label = "print-lm_summary")
    diff| Missing reference file.
    info| diffobj::printDiff()
    ----- FAILED[]: test-print.R<15--15>
    call| expect_snapshot_print(summary(mod2), label = "print-lm_summary")
    diff| < ref                                                           
    diff| > x                                                             
    diff| @@ 1,21 / 1,20 @@                                               
    diff| Call:                                                         
    diff| < lm(formula = mpg ~ hp + factor(gear), data = mtcars)          
    diff| > lm(formula = mpg ~ factor(gear), data = mtcars)               
    diff| Residuals:                                                    
    diff| Min      1Q  Median      3Q     Max                       
    diff| < -4.4937 -2.3586 -0.8277  2.2753  7.7287                       
    diff| > -6.7333 -3.2333 -0.9067  2.8483  9.3667                       
    diff| Coefficients:                                                 
    diff| Estimate Std. Error t value Pr(>|t|)            
    diff| < (Intercept)   27.88193    2.10908  13.220 1.47e-13 ***        
    diff| < hp            -0.06685    0.01105  -6.052 1.59e-06 ***        
    diff| > (Intercept)     16.107      1.216  13.250 7.87e-14 ***        
    diff| < factor(gear)4  2.63486    1.55164   1.698 0.100575            
    diff| > factor(gear)4    8.427      1.823   4.621 7.26e-05 ***        
    diff| < factor(gear)5  6.57476    1.64268   4.002 0.000417 ***        
    diff| > factor(gear)5    5.273      2.431   2.169   0.0384 *          
    diff| ---                                                           
    diff| Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
    diff| < Residual standard error: 3.154 on 28 degrees of freedom       
    diff| > Residual standard error: 4.708 on 29 degrees of freedom       
    diff| < Multiple R-squared:  0.7527,    Adjusted R-squared:  0.7262   
    diff| > Multiple R-squared:  0.4292,    Adjusted R-squared:  0.3898   
    diff| < F-statistic: 28.41 on 3 and 28 DF,  p-value: 1.217e-08        
    diff| > F-statistic:  10.9 on 2 and 29 DF,  p-value: 0.0002948        
    info| diffobj::printDiff()
    Warning message:
    Creating reference file: _tinysnapshot/print-lm_summary.txt 

When there are too many failures, tinytest will not always print the full diff. In those cases, you can save the tinytest object and print it out manually while specifying the nlong argument:

results <- tinytest::run_test_dir()
print(results, nlong = Inf)

Updating snapshots

To update the snapshot for a test, simply delete the relevant snapshot from the inst/tinytest/_tinysnapshot folder and run the test suite again. As when we ran the suite for the very first time, this will report a failure but generate a new snapshot.

CRAN, continuous integration, and deterministic plots

The images produced by R are not deterministic, in the sense that they can vary slightly based on the operating system, graphics device, R version, etc. Unfortunately, this means that visual expectations will often fail on CRAN, where tests are run on many different platforms.

Here are some steps you can take to make testing images more portable:

  1. Use the svglite graphics device.
  2. Use a pre-defined font.
  3. Run continuous integration tests on the same Operating System where you generated the original snapshot files.
  4. Skip visual expectations on CRAN.

From tinysnapshot 0.0.3 (or using the development version from Github), many of these steps can be taken automatically by setting a few options at the top of your test scripts:


options(tinysnapshot_os = "Darwin") # see Sys.info()["sysname"]
options(tinysnapshot_device = "svglite")
options(tinysnapshot_device_args = list(user_fonts = fontquiver::font_families("Liberation")))

Other packages like vdiffr ship with an embedded version svglite and their own fonts to ensure deterministic plots, but tinysnapshot does not do that (yet).

Minimal package example

We now create a minimal R package to illustrate how to use tinysnapshot in the “real world.”

Create a temp directory and use the pkgKitten package to create an ultra-minimalist package (an alternative would be the usethis package):

kitten(name = "testpkg")
    Creating directories ...
    Creating DESCRIPTION ...
    Creating NAMESPACE ...
    Adding pkgKitten overrides.
    >> added .gitignore file
    >> added .Rbuildignore file
    >> added tinytest support

    Consider reading the documentation for all the packaging details.
    A good start is the 'Writing R Extensions' manual.

    And run 'R CMD check'. Run it frequently. And think of those kittens.

Download an example test script from the tinysnapshot repository:

    url = "https://raw.githubusercontent.com/vincentarelbundock/tinysnapshot/main/inst/tinytest/test-png.R",
    destfile = "testpkg/inst/tinytest/test-png.R",
    quiet = TRUE)

Our package now includes 7 tests: 1 created by default by the puppy() function, and 6 tests in the test-png.R script. When we run tinytest the first time, the 6 test-png.R tests fail, but some generate snapshots in PNG format:

    test_testpkg.R................    1 tests OK 22ms
    test-png.R..................    6 tests 6 fails 0.8s
    ----- FAILED[]: test-png.R<15--15>
    call| expect_snapshot_plot(p1, "base")
    diff| 0
    info| pixels
    ----- FAILED[]: test-png.R<18--18>
    call| **expect_snapshot_plot**(p2, "base")
    diff| 3232
    info| pixels
    ----- FAILED[]: test-png.R<25--25>
    call| expect_snapshot_plot(p1, "ggplot2_variable")
    diff| 0
    info| pixels
    FAILED[]: test-png.R<28--28> expect_snapshot_plot(p2, "ggplot2_variable")
    FAILED[]: test-png.R<31--31> expect_snapshot_plot(p3, "ggplot2_theme")
    FAILED[]: test-png.R<34--34> expect_snapshot_plot(p4, "ggplot2_theme")
    Showing 6 out of 7 results: 6 fails, 1 passes (0.8s)
    Warning messages:
    1: Creating reference file: _tinysnapshot/base.png 
    2: Creating reference file: _tinysnapshot/ggplot2_variable.png 
    3: Creating reference file: _tinysnapshot/ggplot2_theme.png 

The second time we run the test suite, only 3 of the test-png.R tests fail:

    test_testpkg.R................    1 tests OK 6ms
    test-png.R....................    6 tests 3 fails 0.6s
    ----- FAILED[]: test-png.R<18--18>
    call| expect_snapshot_plot(p2, "base")
    diff| 3232
    info| pixels
    ----- FAILED[]: test-png.R<28--28>
    call| expect_snapshot_plot(p2, "ggplot2_variable")
    diff| 33536
    info| pixels
    ----- FAILED[]: test-png.R<34--34>
    call| expect_snapshot_plot(p4, "ggplot2_theme")
    diff| 191955
    info| pixels
    Showing 3 out of 7 results: 3 fails, 4 passes (0.7s)