format_col/format_list_item printing generics for customization

NAMESPACE

@@ -187,3 +187,11 @@ S3method(unique, ITime)
 S3method('[<-', IDate)
 S3method(edit, data.table)
 
+# generics to support custom column formatters
+export(format_col)
+S3method(format_col, default)
+S3method(format_col, POSIXct)
+S3method(format_col, expression)
+export(format_list_item)
+S3method(format_list_item, default)
+

NEWS.md

@@ -99,6 +99,8 @@
 
 16. `fwrite()` now accepts `sep=""`, [#4817](https://github.com/Rdatatable/data.table/issues/4817). The motivation is an example where the result of `paste0()` needs to be written to file but `paste0()` takes 40 minutes due to constructing a very large number of unique long strings in R's global character cache. Allowing `fwrite(, sep="")` avoids the `paste0` and saves 40 mins. Thanks to Jan Gorecki for the request, and Ben Schwen for the PR.
 
+17. `data.table` printing now supports customizable methods for both columns and list column row items, part of [#1523](https://github.com/Rdatatable/data.table/issues/1523). `format_col` is S3-generic for customizing how to print whole columns; `format_list_item` is S3-generic for customizing how to print each row of a list column. Thanks to @mllg who initially filed [#3338](https://github.com/Rdatatable/data.table/pulls/3338) with the seed of the idea, @franknarf1 who earlier suggested the idea of providing custom formatters, @fparages who submitted a patch to improve the printing of timezones for [#2842](https://github.com/Rdatatable/data.table/issues/2842), @RichardRedding for pointing out an error relating to printing wide `expression` columns in [#3011](https://github.com/Rdatatable/data.table/issues/3011), and @MichaelChirico for implementing. See `?print.data.table` for examples.
+
 ## BUG FIXES
 
 1. `by=.EACHI` when `i` is keyed but `on=` different columns than `i`'s key could create an invalidly keyed result, [#4603](https://github.com/Rdatatable/data.table/issues/4603) [#4911](https://github.com/Rdatatable/data.table/issues/4911). Thanks to @myoung3 and @adamaltmejd for reporting, and @ColeMiller1 for the PR. An invalid key is where a `data.table` is marked as sorted by the key columns but the data is not sorted by those columns, leading to incorrect results from subsequent queries.

R/print.data.table.R

@@ -140,44 +140,11 @@ print.data.table = function(x, topn=getOption("datatable.print.topn"),
   invisible(x)
 }
 
-format.data.table = function (x, ..., justify="none", timezone = FALSE) {
+format.data.table = function (x, ..., justify="none") {
   if (is.atomic(x) && !is.null(x)) {
     stopf("Internal structure doesn't seem to be a list. Possibly corrupt data.table.")
   }
-  format.item = function(x) {
-    if (is.null(x))  # NULL item in a list column
-      ""
-    else if (is.atomic(x) || inherits(x,"formula")) # FR #2591 - format.data.table issue with columns of class "formula"
-      paste(c(format(head(x, 6L), justify=justify, ...), if (length(x) > 6L) "..."), collapse=",")  # fix for #37 - format has to be added here...
-    else
-      paste0("<", class(x)[1L], paste_dims(x), ">")
-  }
-  # FR #2842 add timezone for posix timestamps
-  format.timezone = function(col) { # paste timezone to a time object
-    tz = attr(col,'tzone', exact=TRUE)
-    if (!is.null(tz)) { # date object with tz
-      nas = is.na(col)
-      col = paste0(as.character(col)," ",tz) # parse to character
-      col[nas] = NA_character_
-    }
-    return(col)
-  }
-  # FR #1091 for pretty printing of character
-  # TODO: maybe instead of doing "this is...", we could do "this ... test"?
-  char.trunc = function(x, trunc.char = getOption("datatable.prettyprint.char")) {
-    trunc.char = max(0L, suppressWarnings(as.integer(trunc.char[1L])), na.rm=TRUE)
-    if (!is.character(x) || trunc.char <= 0L) return(x)
-    idx = which(nchar(x) > trunc.char)
-    x[idx] = paste0(substr(x[idx], 1L, as.integer(trunc.char)), "...")
-    x
-  }
-  do.call("cbind",lapply(x,function(col,...) {
-    if (!is.null(dim(col))) return("<multi-column>")
-    if(timezone) col = format.timezone(col)
-    if (is.list(col)) col = vapply_1c(col, format.item)
-    else col = format(char.trunc(col), justify=justify, ...) # added an else here to fix #37
-    col
-  },...))
+  do.call("cbind", lapply(x, format_col, ..., justify=justify))
 }
 
 mimicsAutoPrint = c("knit_print.default")
@@ -205,6 +172,55 @@ paste_dims = function(x) {
   paste0("[", paste(dims,collapse="x"), "]")
 }
 
+format_col = function(x, ...) {
+  UseMethod("format_col")
+}
+
+format_list_item = function(x, ...) {
+  UseMethod("format_list_item")
+}
+
+format_col.default = function(x, ...) {
+  if (!is.null(dim(x))) return("<multi-column>")
+  if (is.list(x)) return(vapply_1c(x, format_list_item, ...))
+  format(char.trunc(x), ...) # relevant to #37
+}
+
+# #2842 -- different columns can have different tzone, so force usage in output
+format_col.POSIXct = function(x, ..., timezone=FALSE) {
+  if (timezone) {
+    tz = attr(x,'tzone',exact=TRUE)
+    nas = is.na(x)
+    x = paste0(as.character(x)," ",tz)
+    is.na(x) = nas
+  } else {
+    x = format(x, usetz=FALSE)
+  }
+  x
+}
+
+# #3011 -- expression columns can wrap to newlines which breaks printing
+format_col.expression = function(x, ...) format(char.trunc(as.character(x)), ...)
+
+format_list_item.default = function(x, ...) {
+  if (is.null(x))  # NULL item in a list column
+    ""
+  else if (is.atomic(x) || inherits(x, "formula")) # FR #2591 - format.data.table issue with columns of class "formula"
+    paste(c(format(head(x, 6L), ...), if (length(x) > 6L) "..."), collapse=",")  # fix for #5435 and #37 - format has to be added here...
+  else
+    paste0("<", class(x)[1L], paste_dims(x), ">")
+}
+
+# FR #1091 for pretty printing of character
+# TODO: maybe instead of doing "this is...", we could do "this ... test"?
+char.trunc <- function(x, trunc.char = getOption("datatable.prettyprint.char")) {
+  trunc.char = max(0L, suppressWarnings(as.integer(trunc.char[1L])), na.rm=TRUE)
+  if (!is.character(x) || trunc.char <= 0L) return(x)
+  idx = which(nchar(x) > trunc.char)
+  x[idx] = paste0(substr(x[idx], 1L, as.integer(trunc.char)), "...")
+  x
+}
+
 # to calculate widths of data.table for PR #4074
 # gets the width of the data.table at each column
 #   and compares it to the console width

inst/tests/tests.Rraw

@@ -34,6 +34,8 @@ if (exists("test.data.table", .GlobalEnv, inherits=FALSE)) {
   forder = data.table:::forder
   forderv = data.table:::forderv
   format.data.table = data.table:::format.data.table
+  format_col.default = data.table:::format_col.default
+  format_list_item.default = data.table:::format_list_item.default
   getdots = data.table:::getdots
   groupingsets.data.table = data.table:::groupingsets.data.table
   guess = data.table:::guess
@@ -17002,6 +17004,38 @@ DT = data.table(
            s4class(x=2L, y="yes", z=1)))
 test(2130.03, print(DT), output=c("   x             y", "1: 1 <ex_class[3]>",     "2: 2 <ex_class[3]>"))
 
+# format_col and format_list_item printing helpers/generics
+## Use case: solve #2842 by defining format_col.POSIXct to have usetz = TRUE
+DT = data.table(
+  t1 = as.POSIXct('2018-05-01 12:34:56', tz = 'UTC'),
+  t2 = as.POSIXct('2018-05-01 12:34:56', tz = 'Asia/Singapore')
+)
+test(2130.101, print(DT, timezone=TRUE), output='UTC')
+test(2130.102, print(DT, timezone=FALSE), notOutput='UTC')
+
+# default expression printing can break format_col.default, #3011
+test(2130.11, print(data.table(e = expression(1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13))), output = '1 + 2 + 3')
+
+# format_col generic is used
+format_col.complex = function(x, ...) sprintf('(%.1f, %.1fi)', Re(x), Im(x))
+registerS3method("format_col", "complex", format_col.complex)
+# this registerS3method does seem to be necessary to work within the test.data.table() environment
+# assigning the method using <<- probably works too, but we don't want to write to user's environment at all
+x = data.table(z = c(1 + 3i, 2 - 1i, pi + 2.718i))
+test(2130.12, x, output = '(1.0, 3.0i)')
+rm(format_col.complex)
+registerS3method("format_col", "complex", format_col.default) 
+# otherwise it remains registered after test.data.table() and causes test 1610.1 to fail on the next run for example, and user display if they have complex data
+# haven't found a way to unregister an S3 method (tried registering NULL but there's an error that NULL isn't a function)
+
+# format_list_item() generic is used
+format_list_item.myclass <- function(x, ...) paste0("<", class(x)[1L], ":", x$id, ">")
+registerS3method("format_list_item", "myclass", format_list_item.myclass)
+DT = data.table(row = 1:2, objs = list(structure(list(id = "foo"), class = "myclass"),  structure(list(id = "bar"), class = "myclass")))
+test(2130.13, print(DT), output = "myclass:foo.*myclass:bar")
+rm(format_list_item.myclass)
+registerS3method("format_list_item", "myclass", format_list_item.default)
+
 # .SD from grouping should be unlocked, part of #4159
 x = data.table(a=1:3, b=4:6)
 test(2131.1, lapply(x[ , list(dt = list(.SD)), by = a]$dt, attr, '.data.table.locked'),

man/print.data.table.Rd

@@ -1,10 +1,18 @@
 \name{print.data.table}
 \alias{print.data.table}
+\alias{format_col}
+\alias{format_col.default}
+\alias{format_col.POSIXct}
+\alias{format_col.expression}
+\alias{format_list_item}
+\alias{format_list_item.default}
 \title{ data.table Printing Options }
 \description{
   \code{print.data.table} extends the functionalities of \code{print.data.frame}.
 
   Key enhancements include automatic output compression of many observations and concise column-wise \code{class} summary.
+
+  \code{format_col} and \code{format_list_item} generics provide flexibility for end-users to define custom printing methods for generic classes.
 }
 \usage{
   \method{print}{data.table}(x,
@@ -17,6 +25,14 @@
     trunc.cols=getOption("datatable.print.trunc.cols"), # default: FALSE
     quote=FALSE,
     timezone=FALSE, \dots)
+
+  format_col(x, \dots)
+  \method{format_col}{default}(x, \dots)
+  \method{format_col}{POSIXct}(x, \dots, timezone=FALSE)
+  \method{format_col}{expression}(x, \dots)
+
+  format_list_item(x, \dots)
+  \method{format_list_item}{default}(x, \dots)
 }
 \arguments{
   \item{x}{ A \code{data.table}. }
@@ -31,8 +47,17 @@
   \item{timezone}{ If \code{TRUE}, time columns of class POSIXct or POSIXlt will be printed with their timezones (if attribute is available). }
   \item{\dots}{ Other arguments ultimately passed to \code{format}. }
 }
+\value{
+  \code{print.data.table} returns \code{x} invisibly.
+
+  \code{format_col} returns a \code{length(x)}-size \code{character} vector.
+
+  \code{format_list_item} returns a length-1 \code{character} scalar.
+}
 \details{
   By default, with an eye to the typically large number of observations in a \code{data.table}, only the beginning and end of the object are displayed (specifically, \code{head(x, topn)} and \code{tail(x, topn)} are displayed unless \code{nrow(x) < nrows}, in which case all rows will print).
+
+  \code{format_col} is applied at a column level; for example, \code{format_col.POSIXct} is used to tag the time zones of \code{POSIXct} columns. \code{format_list_item} is applied to the elements (rows) of \code{list} columns; see Examples.
 }
 \seealso{\code{\link{print.default}}}
 \examples{
@@ -72,5 +97,15 @@
                    thing_61 = vector("complex", 3))
   print(DT, trunc.cols=TRUE)
   options(old_width)
+
+  # Formatting customization
+  format_col.complex = function(x, ...) sprintf('(\%.1f, \%.1fi)', Re(x), Im(x))
+  x = data.table(z = c(1 + 3i, 2 - 1i, pi + 2.718i))
+  print(x)
+
+  iris = as.data.table(iris)
+  iris_agg = iris[ , .(reg = list(lm(Sepal.Length ~ Petal.Length))), by = Species]
+  format_list_item.lm = function(x, ...) sprintf('<lm:\%s>', format(x$call$formula))
+  print(iris_agg)
 }