More helpful error when trying to write out data frames with list col…

…umns (#1009)

Fixes #938

NAMESPACE

@@ -12,6 +12,7 @@ S3method(format,col_spec)
 S3method(output_column,POSIXt)
 S3method(output_column,default)
 S3method(output_column,double)
+S3method(output_column,list)
 S3method(print,col_spec)
 S3method(print,collector)
 S3method(print,date_names)

NEWS.md

@@ -8,6 +8,7 @@
   than signaling an error when given a connection for the `file` argument that
   contains no data. This makes the behavior consistent as when called with an
   empty file (@pralitp, #963).
+
 * It is now possible to generate a column specification from any tibble (or
   data.frame) with `as.col_spec()` and convert any column specification to a
   short representation with `as.character()`
@@ -24,6 +25,8 @@
     as.character(s)
     #> [1] "ddddf"
 
+* More helpful error when trying to write out data frames with list columns (@ellessenne, #938)
+
 # readr 1.3.1
 
 * Column specifications are now coloured when printed. This makes it easy to

R/write.R

@@ -63,7 +63,7 @@ write_delim <- function(x, path, delim = " ", na = "NA", append = FALSE,
                         col_names = !append, quote_escape = "double") {
   stopifnot(is.data.frame(x))
 
-  x[] <- lapply(x, output_column)
+  x[] <- lapply(names(x), function(i) output_column(x[[i]], i))
   stream_delim(x, path, delim = delim, col_names = col_names, append = append,
     na = na, quote_escape = quote_escape)
 
@@ -187,26 +187,31 @@ format_tsv <- function(x, na = "NA", append = FALSE, col_names = !append, quote_
 #' # converted to ISO8601.
 #' x <- parse_datetime("2016-01-01")
 #' str(output_column(x))
-output_column <- function(x) {
+output_column <- function(x, name) {
   UseMethod("output_column")
 }
 
 #' @export
-output_column.default <- function(x) {
+output_column.default <- function(x, name) {
   if (!is.object(x)) return(x)
   as.character(x)
 }
 
 #' @export
-output_column.double <- function(x) {
+output_column.double <- function(x, name) {
   x
 }
 
 #' @export
-output_column.POSIXt <- function(x) {
+output_column.POSIXt <- function(x, name) {
   format(x, "%Y-%m-%dT%H:%M:%OSZ", tz = "UTC", justify = "none")
 }
 
+#' @export
+output_column.list <- function(x, name) {
+  stop("Flat files can't store the list column `", name, "`", call. = FALSE)
+}
+
 stream_delim <- function(df, path, append = FALSE, bom = FALSE, ..., quote_escape) {
   quote_escape <- standardise_escape(quote_escape)
 

tests/testthat/test-write.R

@@ -163,3 +163,8 @@ test_that("hms NAs are written without padding (#930)", {
   df <- data.frame(x = hms::as.hms(c(NA, 34.234)))
   expect_equal(format_tsv(df), "x\nNA\n00:00:34.234\n")
 })
+
+test_that("More helpful error when trying to write out data frames with list columns (#938)", {
+  df <- tibble::tibble(id = seq(1), list = list(1))
+  expect_error(write_csv(x = df, path = tempfile()), "Flat files can't store the list column")
+})