expect_s3_class_linter and expect_s4_class_linter

DESCRIPTION

@@ -65,6 +65,7 @@ Collate:
     'exclude.R'
     'expect_lint.R'
     'expect_null_linter.R'
+    'expect_s3_class_linter.R'
     'expect_type_linter.R'
     'extract.R'
     'extraction_operator_linter.R'

NAMESPACE

@@ -32,6 +32,8 @@ export(equals_na_linter)
 export(expect_lint)
 export(expect_lint_free)
 export(expect_null_linter)
+export(expect_s3_class_linter)
+export(expect_s4_class_linter)
 export(expect_type_linter)
 export(extraction_operator_linter)
 export(function_left_parentheses_linter)

NEWS.md

@@ -89,6 +89,8 @@ function calls. (#850, #851, @renkun-ken)
 * `lintr` is adopting a new set of linters provided as part of Google's extension to the tidyverse style guide (#884, @michaelchirico)
    + `expect_null_linter()` Require usage of `expect_null(x)` over `expect_equal(x, NULL)` and similar
    + `expect_type_linter()` Require usage of `expect_type(x, t)` over `expect_equal(typeof(x), t)` and similar
+   + `expect_s3_class_linter()` Require usage of `expect_s3_class(x, k)` over `expect_equal(class(x), k)` and similar
+   + `expect_s4_class_linter()` Require usage of `expect_s4_class(x, k)` over `expect_true(methods::is(x, k))`
 * `assignment_linter()` now lints right assignment (`->` and `->>`) and gains two arguments. `allow_cascading_assign` (`TRUE` by default) toggles whether to lint `<<-` and `->>`; `allow_right_assign` toggles whether to lint `->` and `->>` (#915, @michaelchirico)
 
 # lintr 2.0.1

R/expect_s3_class_linter.R

@@ -0,0 +1,108 @@
+#' Require usage of expect_s3_class()
+#'
+#' [testthat::expect_s3_class()] exists specifically for testing the class
+#'   of S3 objects. [testthat::expect_equal()], [testthat::expect_identical()],
+#'   and [testthat::expect_true()] can also be used for such tests,
+#'   but it is better to use the tailored function instead.
+#'
+#' @evalRd rd_tags("expect_s3_class_linter")
+#' @seealso [linters] for a complete list of linters available in lintr.
+#' @export
+expect_s3_class_linter <- function() {
+  Linter(function(source_file) {
+    if (length(source_file$parsed_content) == 0L) {
+      return(list())
+    }
+
+    xml <- source_file$xml_parsed_content
+
+    # (1) expect_{equal,identical}(class(x), C)
+    # (2) expect_true(is.<class>(x)) and expect_true(inherits(x, C))
+    is_class_call <- xp_text_in_table(c(is_s3_class_calls, "inherits"))
+    xpath <- glue::glue("//expr[
+      (
+        SYMBOL_FUNCTION_CALL[text() = 'expect_equal' or text() = 'expect_identical']
+        and following-sibling::expr[
+          expr[SYMBOL_FUNCTION_CALL[text() = 'class']]
+          and (position() = 1 or preceding-sibling::expr[STR_CONST])
+        ]
+      ) or (
+        SYMBOL_FUNCTION_CALL[text() = 'expect_true']
+        and following-sibling::expr[1][expr[SYMBOL_FUNCTION_CALL[ {is_class_call} ]]]
+      )
+    ]")
+
+    bad_expr <- xml2::xml_find_all(xml, xpath)
+    return(lapply(bad_expr, gen_expect_s3_class_lint, source_file))
+  })
+}
+
+# NB: there is no easy way to make an exhaustive list of places where an
+#   is.<x> call can be replaced by expect_s3_class(); this list was manually
+#   populated from the default R packages by inspection. For example,
+#   is.matrix(x) cannot be replaced by expect_s3_class(x, "matrix") because
+#   it is not actually an S3 class (is.object(x) is not TRUE since there
+#   is no class set for a matrix [I am not sure if this changes in R 4].
+#   Further, there are functions named is.<x> that have nothing to do with
+#   object type, e.g. is.finite(), is.nan(), or is.R().
+is_s3_class_calls <- paste0("is.", c(
+  # base
+  "data.frame", "factor", "numeric_version",
+  "ordered", "package_version", "qr", "table",
+  #      utils grDevices     tcltk    tcltk    grid    grid
+  "relistable", "raster", "tclObj", "tkwin", "grob", "unit",
+  # stats
+  "mts", "stepfun", "ts", "tskernel"
+))
+
+gen_expect_s3_class_lint <- function(expr, source_file) {
+  matched_function <- xml2::xml_text(xml2::xml_find_first(expr, "SYMBOL_FUNCTION_CALL"))
+  if (matched_function %in% c("expect_equal", "expect_identical")) {
+    lint_msg <- sprintf("expect_s3_class(x, k) is better than %s(class(x), k).", matched_function)
+  } else {
+    lint_msg <- "expect_s3_class(x, k) is better than expect_true(is.<k>(x)) or expect_true(inherits(x, k))."
+  }
+  lint_msg <- paste(lint_msg, "Note also expect_s4_class() available for testing S4 objects.")
+  xml_nodes_to_lint(expr, source_file, lint_msg, type = "warning")
+}
+
+#' Require usage of expect_s4_class(x, k) over expect_true(is(x, k))
+#'
+#' [testthat::expect_s4_class()] exists specifically for testing the class
+#'   of S4 objects. [testthat::expect_true()] can also be used for such tests,
+#'   but it is better to use the tailored function instead.
+#'
+#' @evalRd rd_tags("expect_s3_class_linter")
+#' @seealso [linters] for a complete list of linters available in lintr.
+#' @export
+expect_s4_class_linter <- function() {
+  Linter(function(source_file) {
+    if (length(source_file$parsed_content) == 0L) {
+      return(list())
+    }
+
+    xml <- source_file$xml_parsed_content
+
+    # TODO(michaelchirico): also catch expect_{equal,identical}(methods::is(x), k).
+    #   there are no hits for this on google3 as of now.
+
+    # require 2 expressions because methods::is(x) alone is a valid call, even
+    #   though the character output wouldn't make any sense for expect_true().
+    xpath <- "//expr[
+      SYMBOL_FUNCTION_CALL[text() = 'expect_true']
+      and following-sibling::expr[1][count(expr) = 3 and expr[SYMBOL_FUNCTION_CALL[text() = 'is']]]
+    ]"
+
+    bad_expr <- xml2::xml_find_all(xml, xpath)
+    return(lapply(
+      bad_expr,
+      xml_nodes_to_lint,
+      source_file = source_file,
+      message = paste(
+        "expect_s4_class(x, k) is better than expect_true(is(x, k)).",
+        "Note also expect_s3_class() available for testing S3 objects."
+      ),
+      type = "warning"
+    ))
+  })
+}

inst/lintr/linters.csv

@@ -10,6 +10,8 @@ cyclocomp_linter,style readability best_practices default configurable
 duplicate_argument_linter,correctness common_mistakes configurable
 equals_na_linter,robustness correctness common_mistakes default
 expect_null_linter,package_development best_practices
+expect_s3_class_linter,package_development best_practices
+expect_s4_class_linter,package_development best_practices
 expect_type_linter,package_development best_practices
 extraction_operator_linter,style best_practices
 function_left_parentheses_linter,style readability default

tests/testthat/test-expect_s3_class_linter.R

@@ -0,0 +1,97 @@
+test_that("expect_s3_class_linter skips allowed usages", {
+  # expect_s3_class doesn't have an inverted version
+  expect_lint("expect_true(!inherits(x, 'class'))", NULL, expect_s3_class_linter())
+  # NB: also applies to tinytest, but it's sufficient to test testthat
+  expect_lint("testthat::expect_s3_class(!inherits(x, 'class'))", NULL, expect_s3_class_linter())
+
+  # other is.<x> calls are not suitable for expect_s3_class in particular
+  expect_lint("expect_true(is.na(x))", NULL, expect_s3_class_linter())
+
+  # case where expect_s3_class() *could* be used but we don't enforce
+  expect_lint("expect_true(is.data.table(x))", NULL, expect_s3_class_linter())
+})
+
+test_that("expect_s3_class_linter blocks simple disallowed usages", {
+  expect_lint(
+    "expect_equal(class(x), 'data.frame')",
+    rex::rex("expect_s3_class(x, k) is better than expect_equal(class(x), k)"),
+    expect_s3_class_linter()
+  )
+
+  # expect_identical is treated the same as expect_equal
+  expect_lint(
+    "testthat::expect_identical(class(x), 'lm')",
+    rex::rex("expect_s3_class(x, k) is better than expect_identical(class(x), k)"),
+    expect_s3_class_linter()
+  )
+
+  # yoda test with string literal in first arg also caught
+  expect_lint(
+    "expect_equal('data.frame', class(x))",
+    rex::rex("expect_s3_class(x, k) is better than expect_equal(class(x), k)"),
+    expect_s3_class_linter()
+  )
+
+  # different equivalent usages
+  expect_lint(
+    "expect_true(is.table(foo(x)))",
+    rex::rex("expect_s3_class(x, k) is better than expect_true(is.<k>(x))"),
+    expect_s3_class_linter()
+  )
+  expect_lint(
+    "expect_true(inherits(x, 'table'))",
+    rex::rex("expect_s3_class(x, k) is better than expect_true(is.<k>(x))"),
+    expect_s3_class_linter()
+  )
+
+  # TODO(michaelchirico): consider more carefully which sorts of class(x) %in% . and
+  #   . %in% class(x) calls should be linted
+  # expect_lint(
+  #   "expect_true('lm' %in% class(x))",
+  #   "expect_s3_class\\(x, k\\) is better than expect_equal\\(class\\(x\\), k",
+  #   expect_s3_class_linter
+  # )
+})
+
+test_that("expect_s4_class_linter skips allowed usages", {
+  # expect_s4_class doesn't have an inverted version
+  expect_lint("expect_true(!is(x, 'class'))", NULL, expect_s4_class_linter())
+  # NB: also applies to tinytest, but it's sufficient to test testthat
+  expect_lint("testthat::expect_s3_class(!is(x, 'class'))", NULL, expect_s4_class_linter())
+})
+
+test_that("expect_s4_class blocks simple disallowed usages", {
+  expect_lint(
+    "expect_true(is(x, 'data.frame'))",
+    rex::rex("expect_s4_class(x, k) is better than expect_true(is(x, k))"),
+    expect_s4_class_linter()
+  )
+
+  # namespace qualification is irrelevant
+  expect_lint(
+    "testthat::expect_true(methods::is(x, 'SpatialPolygonsDataFrame'))",
+    rex::rex("expect_s4_class(x, k) is better than expect_true(is(x, k))"),
+    expect_s4_class_linter()
+  )
+})
+
+skip_if_not_installed("patrick")
+local({
+  # test for lint errors appropriately raised for all is.<class> calls
+  is_classes <- c(
+    "data.frame", "factor", "numeric_version",
+    "ordered", "package_version", "qr", "table",
+    "relistable", "raster", "tclObj", "tkwin", "grob", "unit",
+    "mts", "stepfun", "ts", "tskernel"
+  )
+  patrick::with_parameters_test_that(
+    "expect_true(is.<base class>) is caught",
+    expect_lint(
+      sprintf("expect_true(is.%s(x))", is_class),
+      rex::rex("expect_s3_class(x, k) is better than expect_true(is.<k>(x))"),
+      expect_s3_class_linter()
+    ),
+    .test_name = is_classes,
+    is_class = is_classes
+  )
+})