@iago-pssjd Thank you for your report. What would be even more better is to include minimal example (so we can just copy copy to observe issue you are describing) and your session info (to check if you might be using older version of DT and issue might have been fixed already).

Thank you @jangorecki . I updated the issue and removed the reference to data.table() which i toke just after look at the SO question, but I didn't check it. Now I see that this function does what I asked for, but setDT yet not.

Thank you.
We can define an action point for this issue to add check for POSIXlt inside setDT.

It looks like a problem of print.data.table() to me and not setDT(). print.data.table() should work with a data.table object that contains POSIXlt vectors.

In my opinion, generally, we should not change the vector value of the object by setDT(), unless there're very strong reasons.

@shrektan is not only a problem of print.data.table(), since if you want to do other operations on the object, it gives an error too (let me remember which specific operations, but I believe I was trying to create some variable, and I believe it had nothing to do with the time variable, bot sure also if using by = ...)

If we change POSIXlt to POSIXct it won't be by reference anymore so I think the best is to raise error. Other operations will also warn on POSIXlt input.

library(data.table)
x <- data.table(g=Sys.time(), a=1)
x[["g"]] <- as.POSIXlt(Sys.time())
str(x)
#> Classes 'data.table' and 'data.frame':   11 obs. of  2 variables:
#>  $ g: POSIXlt, format: "2020-11-12 02:01:56"
#>  $ a: num 1
#>  - attr(*, ".internal.selfref")=<externalptr>
x[, sum(a), g]
#> Error in `[.data.table`(x, , sum(a), g): column or expression 1 of 'by' or 'keyby' is type list. Do not quote column names. Usage: DT[,sum(colC),by=list(colA,month(colB))]

@jangorecki The example is not suitable as the warning is thrown from data.table(). However, I did some test (and edited your example) and realized that your conclusion is correct, the POSIXlt is indeed special - it's a named list - so it's supposed to not work with data.table other operations well.

@iago-pssjd you're right, POSIXlt is indeed different and we should take action in setDT().

Just an addition on my initial message and example that I found now. In there, I did

> x <- as.data.frame(tibble(now))
> mdt <- data.table(id=1:3, d=strptime(c("06:02:36", "06:02:48", "07:03:12"), "%H:%M:%S"))
Warning message:
In as.data.table.list(x, keep.rownames = keep.rownames, check.names = check.names,  :
  POSIXlt column type detected and converted to POSIXct. We do not recommend use of POSIXlt at all because it uses 40 bytes to store one date.

But if I do then:

str(as.data.table(x))
Classes ‘data.table’ and 'data.frame':	11 obs. of  1 variable:
 $ now: POSIXlt, format: "2021-02-26 16:07:19"
 - attr(*, ".internal.selfref")=<externalptr> 

So as.data.table.list converts POSIXlt column to POSIXct, but the S3 method for class data.frame doesn't.

I guess we could just reuse setdt_nrows from setDT for list input from #5427? WDYT @MichaelChirico, if so, I can fill the PR later

I think since #6299, we start to get nicer behavior of POSIXlt columns (?).

Anyway, throwing a warning() from setDT() (e.g. encouraging as.data.table() instead) looks appropriate. I think an error is too strong.

Just tested example in initial post, after data.table::update_dev_pkg():

> library(tibble)
> library(data.table)
> now <- as.POSIXlt(Sys.time())
> x <- as.data.frame(tibble(now))
> mdt <- data.table(id=1:3, d=strptime(c("06:02:36", "06:02:48", "07:03:12"), "%H:%M:%S"))
Warning message:
In as.data.table.list(x, keep.rownames = keep.rownames, check.names = check.names,  :
  POSIXlt column type detected and converted to POSIXct. We do not recommend use of POSIXlt at all because it uses 40 bytes to store one date.
> # previous is fine; but next...
> str(as.data.table(x))
Classes 'data.table' and 'data.frame':	11 obs. of  1 variable:
 $ now: POSIXlt, format: Error en format.POSIXlt(object): a valid "POSIXlt" object has names
> setDT(x)
> head(x)
Error en format.POSIXlt(x, ...): 
  a valid "POSIXlt" object is a list of at least 9 elements

I guess we could just reuse setdt_nrows from setDT for list input from #5427? WDYT @MichaelChirico, if so, I can fill the PR later

Ah, I guess I see what you mean now. We already error for list input to setDT():

setDT(list(as.POSIXlt(Sys.time())))
# Error in setDT(list(as.POSIXlt(Sys.time()))) : 
#   Column 1 has class 'POSIXlt'....

I still suspect erroring is not the best choice, so I propose moving to warning() in both cases (list or data.frame input). User will not make it far with POSIXlt columns, but they can at least have agency on the fix instead of having setDT() blocked.

Found the error has been there since #646.

Also related: #5981

OK I have some progress in branch setdt-posixlt:

https://github.com/Rdatatable/data.table/tree/setdt-posixlt

After some effort, I think it's really too much work to get setDT() to support POSIXlt input vs. the benefit.

So rather than warn() for list input, I will move to error for data.frame input. I don't think this will be a back-compatibility issue because I can't imagine anything useful/remotely functional has come out of setDT(data.frame(<POSIXlt>)).

Hmm just discovered another inconsistency of setDT() on list vs. data.frame input:

M = list(matrix(1:6,ncol=2L))
setDT(M)[]
#       V1    V2
#    <int> <int>
# 1:     1     4
# 2:     2     5
# 3:     3     6

DF = data.frame(row.names=letters[1:3])
DF$M = matrix(1:6, ncol=2L)
setDT(DF) # issues warning
DF
# Error in dimnames(x) <- dn : 
#   length of 'dimnames' [1] not equal to array extent

IMO the behavior of setDT(<list>) here is a bit problematic...

@MichaelChirico

I can't imagine anything useful/remotely functional has come out of setDT(data.frame(<POSIXlt>))

Actually, note that the structure setDT(data.frame(<POSIXlt>)) would work, since data.frame makes the transformation from POSIXlt to POSIXct. It is due to this that I used the library tibble in the example/

now <- as.POSIXlt(Sys.time())
str(data.frame(now))
'data.frame':	1 obs. of  1 variable:
 $ now: POSIXct, format: "2024-12-12 09:02:40"
str(data.frame(x=now))
'data.frame':	1 obs. of  1 variable:
 $ x: POSIXct, format: "2024-12-12 09:02:40"
str(as.data.frame(now))
'data.frame':	1 obs. of  1 variable:
 $ now: POSIXct, format: "2024-12-12 09:02:40"