Suppress autoprint during := assignment on subclasses of data.table

NEWS.md

@@ -115,7 +115,9 @@ rowwiseDT(
 
 14. Added a `data.frame` method for `format_list_item()` to fix error printing data.tables with columns containing 1-column data.frames, [#6592](https://github.com/Rdatatable/data.table/issues/6592). Thanks to @r2evans for the bug report and fix.
 
-15. The auto-printing suppression in `knitr` documents is now done by implementing a method for `knit_print` instead of looking up the call stack, [#6589](https://github.com/Rdatatable/data.table/pull/6589). Thanks to @jangorecki for the report [#6509](https://github.com/Rdatatable/data.table/issues/6509) and @aitap for the fix.
+15. Auto-printing gets some substantial improvements
+   - Suppression in `knitr` documents is now done by implementing a method for `knit_print` instead of looking up the call stack, [#6589](https://github.com/Rdatatable/data.table/pull/6589). The old way was fragile and wound up broken by some implementation changes in {knitr}. Thanks to @jangorecki for the report [#6509](https://github.com/Rdatatable/data.table/issues/6509) and @aitap for the fix.
+   - `print()` methods for S3 subclasses of data.table (e.g. an object of class `c("my.table", "data.table", "data.frame")`) no longer print where plain data.tables wouldn't, e.g. `myDT[, y := 2]`, [#3029](https://github.com/Rdatatable/data.table/issues/3029). The improved detection of auto-printing scenarios has the added benefit of _allowing_ print in highly explicit statements like `print(DT[, y := 2])`, obviating our recommendation since v1.9.6 to append `[]` to signal "please print me".
 
 ## NOTES
 

R/print.data.table.R

@@ -30,7 +30,7 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
     # Other options investigated (could revisit): Cstack_info(), .Last.value gets set first before autoprint, history(), sys.status(),
     #   topenv(), inspecting next statement in caller, using clock() at C level to timeout suppression after some number of cycles
     SYS = sys.calls()
-    if (length(SYS) <= 2L ||  # "> DT" auto-print or "> print(DT)" explicit print (cannot distinguish from R 3.2.0 but that's ok)
+    if (identical(SYS[[1L]][[1L]], print) || # this is what auto-print looks like, i.e. '> DT' and '> DT[, a:=b]' in the terminal; see #3029.
         ( length(SYS) >= 3L && is.symbol(thisSYS <- SYS[[length(SYS)-2L]][[1L]]) &&
           as.character(thisSYS) == 'source') ) { # suppress printing from source(echo = TRUE) calls, #2369
       return(invisible(x))

tests/autoprint.R

@@ -10,7 +10,7 @@ DT[FALSE,a:=3L]                       # no
 DT[a==4L,a:=5L]                       # no
 DT[a %in% 4:8, a:=5L]                 # no
 DT                                    # yes
-print(DT[2,a:=4L])                    # no
+print(DT[2,a:=4L])                    # yes, as of #6631
 print(DT)                             # yes
 if (TRUE) DT[2,a:=5L]                 # no. used to print before v1.9.5
 if (TRUE) if (TRUE) DT[2,a:=6L]       # no. used to print before v1.9.5
@@ -20,7 +20,7 @@ DT                                    # yes. 2nd time needed, or solutions below
 (function(){DT[2,a:=5L];NULL})()      # print NULL
 DT[]                                  # yes. guaranteed print
 (function(){DT[2,a:=5L];NULL})()      # print NULL
-print(DT)                             # no. only DT[] is guaranteed print from v1.9.6 and R 3.2.0
+print(DT)                             # yes. restored in #6631 behavior that had changed in 1.9.6.
 (function(){DT[2,a:=5L][];NULL})()    # print NULL
 DT                                    # yes. i) function needs to add [] after last one, so that "DT" alone is guaranteed anyway
 (function(){DT[2,a:=5L];DT[];NULL})() # print NULL
@@ -29,9 +29,9 @@ DT2 = data.table(b=3:4)               # no
 (function(){DT[2,a:=6L];DT2[1,b:=7L];NULL})()
 DT                                    # yes. last := was on DT2 not DT
 {DT[2,a:=6L];invisible()}             # no
-print(DT)                             # no
+print(DT)                             # yes
 (function(){print(DT[2,a:=7L]);print(DT);invisible()})()    # yes*2
-{print(DT[2,a:=8L]);print(DT);invisible()}                  # yes*1  Not within function so as at prompt
+{print(DT[2,a:=8L]);print(DT);invisible()}                  # yes*2  as at prompt, again as of #6631
 DT[1][,a:=9L]      # no (was too tricky to detect that DT[1] is a new object). Simple rule is that := always doesn't print
 DT[2,a:=10L][1]                       # yes
 DT[1,a:=10L][1,a:=10L]                # no
@@ -51,7 +51,25 @@ local({
   writeLines(c(
     "library(data.table)",
     "DT = data.table(a = 1)",
-    "DT[,a:=1] # not auto-printed"
+    "DT[,a:=1]"                       # no
   ), f)
   source(f, local = TRUE, echo = TRUE)
 })
+
+# child class of data.table doesn't induce unintended print, #3029
+dt <- data.table(x = 1)
+class(dt) <- c("foo", "data.table", "data.frame")
+print.foo <- function(x, ...) {
+  NextMethod("print")
+}
+dt[, y := 1]                          # no
+
+# withAutoprint() testing (since R3.4.0)
+if (!exists("withAutoprint", baseenv())) {
+  q("no")
+}
+if (TRUE) withAutoprint({
+  DT                                  # yes
+  DT[1L, 1L]                          # yes
+  DT[2L, a := 10L]                    # no
+})

tests/autoprint.Rout.save

@@ -1,7 +1,7 @@
 
-R version 4.3.2 (2023-10-31) -- "Eye Holes"
-Copyright (C) 2023 The R Foundation for Statistical Computing
-Platform: x86_64-pc-linux-gnu (64-bit)
+R Under development (unstable) (2024-12-01 r87412) -- "Unsuffered Consequences"
+Copyright (C) 2024 The R Foundation for Statistical Computing
+Platform: x86_64-pc-linux-gnu
 
 R is free software and comes with ABSOLUTELY NO WARRANTY.
 You are welcome to redistribute it under certain conditions.
@@ -44,7 +44,11 @@ Index: <a>
    <int>
 1:     1
 2:     3
-> print(DT[2,a:=4L])                    # no
+> print(DT[2,a:=4L])                    # yes, as of #6631
+       a
+   <int>
+1:     1
+2:     4
 > print(DT)                             # yes
        a
    <int>
@@ -69,7 +73,11 @@ NULL
 2:     5
 > (function(){DT[2,a:=5L];NULL})()      # print NULL
 NULL
-> print(DT)                             # no. only DT[] is guaranteed print from v1.9.6 and R 3.2.0
+> print(DT)                             # yes. restored in #6631 behavior that had changed in 1.9.6.
+       a
+   <int>
+1:     1
+2:     5
 > (function(){DT[2,a:=5L][];NULL})()    # print NULL
 NULL
 > DT                                    # yes. i) function needs to add [] after last one, so that "DT" alone is guaranteed anyway
@@ -93,7 +101,11 @@ NULL
 1:     1
 2:     6
 > {DT[2,a:=6L];invisible()}             # no
-> print(DT)                             # no
+> print(DT)                             # yes
+       a
+   <int>
+1:     1
+2:     6
 > (function(){print(DT[2,a:=7L]);print(DT);invisible()})()    # yes*2
        a
    <int>
@@ -103,7 +115,11 @@ NULL
    <int>
 1:     1
 2:     7
-> {print(DT[2,a:=8L]);print(DT);invisible()}                  # yes*1  Not within function so as at prompt
+> {print(DT[2,a:=8L]);print(DT);invisible()}                  # yes*2  as at prompt, again as of #6631
+       a
+   <int>
+1:     1
+2:     8
        a
    <int>
 1:     1
@@ -143,7 +159,7 @@ NULL
 +   writeLines(c(
 +     "library(data.table)",
 +     "DT = data.table(a = 1)",
-+     "DT[,a:=1] # not auto-printed"
++     "DT[,a:=1]"                       # no
 +   ), f)
 +   source(f, local = TRUE, echo = TRUE)
 + })
@@ -154,6 +170,34 @@ NULL
 
 > DT[, `:=`(a, 1)]
 > 
+> # child class of data.table doesn't induce unintended print, #3029
+> dt <- data.table(x = 1)
+> class(dt) <- c("foo", "data.table", "data.frame")
+> print.foo <- function(x, ...) {
++   NextMethod("print")
++ }
+> dt[, y := 1]                          # no
+> 
+> # withAutoprint() testing (since R3.4.0)
+> if (!exists("withAutoprint", baseenv())) {
++   q("no")
++ }
+> if (TRUE) withAutoprint({
++   DT                                  # yes
++   DT[1L, 1L]                          # yes
++   DT[2L, a := 10L]                    # no
++ })
+> DT
+       a
+   <int>
+1:    10
+2:    10
+> DT[1L, 1L]
+       a
+   <int>
+1:    10
+> DT[2L, `:=`(a, 10L)]
+> 
 > proc.time()
    user  system elapsed 
-  0.223   0.016   0.231 
+  0.182   0.056   0.246