Semantically invalid name not recognized when passed to keyby

[HughParsonage]

Reprex:

library(data.table)
DT <- data.table(x = 1:5, y = letters[1:5])
setnames(DT, "x", "x y")
DT[, y := "j", keyby = "x y"]
#> Warning in `[.data.table`(DT, , `:=`(y, "j"), keyby = "x y"): := keyby not straightforward character column names or list() of column names, treating as a by:`x y`

^{Created on 2019-02-09 by the reprex package (v0.2.1)}

It should work as it does with by.

[MichaelChirico]

Hugh what monstrous column names are you concocting over there

[HughParsonage]

You don't want to know.

But in all seriousness, for things like printable tables and plots you don't want . or _ in your names. And it's safer to use the displayed name rather than using a placeholder and replacing it.