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Introduce datatable.unique.names policy for duplicate handling in setnames() #4044 #7674

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Introduce datatable.unique.names policy for duplicate handling in setnames() #4044 #7674

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c13642f
apllied the changes as requested
venom1204 Mar 20, 2026
488b873
added test
venom1204 Mar 20, 2026
471ab20
lintr
venom1204 Mar 20, 2026
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2 changes: 2 additions & 0 deletions NEWS.md
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Expand Up @@ -30,6 +30,8 @@

5. `tables()` can now optionally report `data.table` objects stored one level deep inside list objects when `depth=1L`, [#2606](https://github.com/Rdatatable/data.table/issues/2606). Thanks @MichaelChirico for the report and @manmita for the PR

6. `setnames()` now supports a global option `datatable.unique.names` to control the creation of duplicate column names. Users can choose between `"off"` (default), `"warn"`, `"error"`, or `"rename"`. This addresses long-standing ambiguity issues when duplicate names were created silently, [#4044](https://github.com/Rdatatable/data.table/issues/4044). Thanks to @venom1204 for the PR.
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@ben-schwen ben-schwen Apr 20, 2026

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I dont think we need this part since its superfluous "This addresses long-standing ambiguity issues when duplicate names were created silently"


### BUG FIXES

1. `fread()` with `skip=0` and `(header=TRUE|FALSE)` no longer skips the first row when it has fewer fields than subsequent rows, [#7463](https://github.com/Rdatatable/data.table/issues/7463). Thanks @emayerhofer for the report and @ben-schwen for the fix.
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6 changes: 6 additions & 0 deletions R/data.table.R
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Expand Up @@ -2943,6 +2943,12 @@ setnames = function(x,old,new,skip_absent=FALSE) {
if (!length(new)) return(invisible(x)) # no changes
if (length(i) != length(new)) internal_error("length(i)!=length(new)") # nocov
}

full_names = names(x)
full_names[i] = new
full_names = process_name_policy(full_names)
new = full_names[i]

# update the key if the column name being change is in the key
m = chmatch(names(x)[i], key(x))
w = which(!is.na(m))
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3 changes: 2 additions & 1 deletion R/onLoad.R
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Expand Up @@ -98,7 +98,8 @@
datatable.auto.index=TRUE, # DT[col=="val"] to auto add index so 2nd time faster
datatable.use.index=TRUE, # global switch to address #1422
datatable.prettyprint.char=NULL, # FR #1091
datatable.old.matrix.autoname=FALSE # #7145: how data.table(x=1, matrix(1)) is auto-named set to change
datatable.old.matrix.autoname=FALSE, # #7145: how data.table(x=1, matrix(1)) is auto-named set to change
datatable.unique.names = NULL
)
opts = opts[!names(opts) %chin% names(options())]
options(opts)
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24 changes: 24 additions & 0 deletions R/utils.R
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Expand Up @@ -35,6 +35,30 @@ check_duplicate_names = function(x, table_name=deparse(substitute(x))) {
table_name, brackify(duplicate_names), domain=NA)
}

process_name_policy = function(names_vec) {
policy = getOption("datatable.unique.names")
if (is.null(policy) || policy == "off") return(names_vec)
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@ben-schwen ben-schwen Apr 20, 2026

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no ultra happy with this double option for "off". I would either eliminate NULL or "off" (leaning towards eliminating NULL though)


allowed = c("warn", "error", "rename")
if (!policy %in% allowed) {
warningf("Invalid value for 'datatable.unique.names': [%s]. Falling back to 'off'. Allowed values are: 'off', 'warn', 'error', 'rename'.", as.character(policy))
return(names_vec)
}

if (anyDuplicated(names_vec)) {
dups = unique(names_vec[duplicated(names_vec)])
# Use paste0 to avoid sprintf issues with column names containing '%'
msg = paste0("Duplicate column names created: ", brackify(dups), ". This may cause ambiguity.")

switch(policy,
warn = warningf(msg),
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see comment above from previous review. this will trip with

DT = data.table("a%d" = 1, b = 2)
options(datatable.unique.names = "warn")
setnames(DT, "b", "a%d")
# Error in sprintf(gettext(fmt, domain = domain, trim = trim), ...) : 
#   too few arguments

Hence, you need to pass format strings like warningf("%s", msg)

error = stopf(msg),
rename = return(make.unique(names_vec))
)
}
names_vec
}

duplicated_values = function(x) {
# fast anyDuplicated for the typical/non-error case; second duplicated() pass for (usually) error case
if (!anyDuplicated(x)) return(vector(typeof(x)))
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20 changes: 20 additions & 0 deletions inst/tests/tests.Rraw
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Expand Up @@ -21577,3 +21577,23 @@ close(con)
file.create(f <- tempfile())
test(2367.6, fread(file(f)), data.table(), warning="Connection has size 0.")
unlink(f)

#4044
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@ben-schwen ben-schwen Apr 20, 2026

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please dont only write the issue number, but what issue is fixed. this provides information without needing to search for the issue on github.

The tests seem ok but an example of DT = data.table(a=1:2, b=c('x', 'y')) would have been easier to grasp (and also less typing/code)

DT = as.data.table(iris)
test(2368.1, names(setnames(copy(DT), "Petal.Length", "Sepal.Length")),
c("Sepal.Length", "Sepal.Width", "Sepal.Length", "Petal.Width", "Species"),
options = list(datatable.unique.names = "off"))
test(2368.2, names(setnames(copy(DT), "Petal.Length", "Sepal.Length")),
c("Sepal.Length", "Sepal.Width", "Sepal.Length", "Petal.Width", "Species"),
warning = "Duplicate column names created",
options = list(datatable.unique.names = "warn"))
test(2368.3, setnames(copy(DT), "Petal.Length", "Sepal.Length"),
error = "Duplicate column names created",
options = list(datatable.unique.names = "error"))
test(2368.4, names(setnames(copy(DT), "Petal.Length", "Sepal.Length")),
c("Sepal.Length", "Sepal.Width", "Sepal.Length.1", "Petal.Width", "Species"),
options = list(datatable.unique.names = "rename"))
test(2368.5, names(setnames(copy(DT), "Petal.Length", "Sepal.Length")),
c("Sepal.Length", "Sepal.Width", "Sepal.Length", "Petal.Width", "Species"),
warning = "Invalid value for 'datatable.unique.names'",
options = list(datatable.unique.names = "invalid_option_name"))
3 changes: 3 additions & 0 deletions man/data.table-options.Rd
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Expand Up @@ -105,6 +105,9 @@
\item{\code{datatable.enlist}}{Experimental feature. Default is \code{NULL}. If set to a function
(e.g., \code{list}), the \code{j} expression can return a \code{list}, which will then
be "enlisted" into columns in the result.}
\item{\code{datatable.unique.names}}{A character string, default \code{NULL} (same as \code{"off"}).
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@ben-schwen ben-schwen Apr 20, 2026

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we should also document the other options, e.g. warn, error etc.

Controls the behavior when operations (\bold{currently only \code{setnames}})
would result in duplicate column names.}
}
}

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