Closes #6592, nested frames break if any are just 1-column

Author: r2evans

NAMESPACE

@@ -201,6 +201,7 @@ S3method(format_col, POSIXct)
 S3method(format_col, expression)
 export(format_list_item)
 S3method(format_list_item, default)
+S3method(format_list_item, data.frame)
 
 export(fdroplevels, setdroplevels)
 S3method(droplevels, data.table)

NEWS.md

@@ -115,6 +115,8 @@ rowwiseDT(
 
 15. `DT[1, on=NULL]` now works for returning the first row, [#6579](https://github.com/Rdatatable/data.table/issues/6579). Thanks to @Kodiologist for the report and @tdhock for the PR.
 
+16. Using `print.data.table()` with nested dataframes no longer errors when one of the nested dataframes has only one column, [#6592](https://github.com/Rdatatable/data.table/issues). Thanks to @r2evans for the but report and fix.
+
 ## NOTES
 
 1. Tests run again when some Suggests packages are missing, [#6411](https://github.com/Rdatatable/data.table/issues/6411). Thanks @aadler for the note and @MichaelChirico for the fix.

R/print.data.table.R

@@ -237,6 +237,11 @@ format_list_item.default = function(x, ...) {
   }
 }
 
+# #6592 -- nested 1-column frames breaks printing
+format_list_item.data.frame = function(x, ...) {
+  paste0("<", class1(x), paste_dims(x), ">")
+}
+
 # FR #1091 for pretty printing of character
 # TODO: maybe instead of doing "this is...", we could do "this ... test"?
 # Current implementation may have issues when dealing with strings that have combinations of full-width and half-width characters,

inst/tests/tests.Rraw

@@ -20596,3 +20596,18 @@ test(2295.3, is.data.table(d2))
 
 # #6588: .checkTypos used to give arbitrary strings to stopf as the first argument
 test(2296, d2[x %no such operator% 1], error = '%no such operator%')
+
+# #6592: nested single-column frames break in print.data.table
+test(2297.01, format_list_item(data.frame(a=1)),                     output = "<data.frame\\[1x1]>")
+test(2297.02, format_list_item(data.frame(a=1)[0,,drop=FALSE]),      output = "<data.frame\\[0x1]>")
+test(2297.03, format_list_item(data.frame(a=1)[,0]),                 output = "<data.frame\\[1x0]>")
+test(2297.04, format_list_item(data.frame(a=1, b=2)[0,,drop=FALSE]), output = "<data.frame\\[0x2]>")
+test(2297.05, format_list_item(data.frame(a=1, b=2)[,0]),            output = "<data.frame\\[1x0]>")
+test(2297.06, format_list_item(data.table(a=1)),                     output = "<data.table\\[1x1]>")
+test(2297.07, format_list_item(data.table(a=1)[0,]),                 output = "<data.table\\[0x1]>")
+test(2297.08, format_list_item(data.table(a=1)[,0]),                 output = "<data.table\\[0x0]>")
+test(2297.09, format_list_item(data.table(a=1, b=2)[0,]),            output = "<data.table\\[0x2]>")
+test(2297.10, format_list_item(data.table(a=1, b=2)[,0]),            output = "<data.table\\[0x0]>")
+test(2297.11, data.table(a=1),                                       output = " *a\n1: *1")
+test(2297.12, data.table(a=list(data.frame(b=1))),                   output = " *a\n1: <data.frame\\[1x1]>")
+test(2297.13, data.table(a=list(data.table(b=1))),                   output = " *a\n1: <data.table\\[1x1]>")