rbindlist silently dropping NA factor level

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
38/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
r
Domain
data

Research direction

Start by reproducing the issue with the two data frames and rbindlist() example in the report, then read the rbindlist() factor-handling documentation. Trace the implementation and existing tests for factor level merging, and determine whether NA levels should be preserved or the behavior documented; done means the intended behavior is covered by a regression test.

Written by the indexing model from the issue text.

Description

[rbind|cbind|merge]list 
> df1 <- data.frame(col = factor(c('A', NA, 'B'), exclude = NULL))
> levels(df1$col)
[1] "A" "B" NA

> df2 <- data.frame(col = factor('C'))

> df <- data.table::rbindlist(list(df1, df2))
> levels(df$col)
[1] "A" "B" "C"

The documentation for data.table::rbindlist() reads:

If a column contains factors then a factor is created. [...] Any strings in regular factor and character columns which are missing from the longest ordered levels are added at the end.

Unfortunately, NA factor levels are silently dropped. Is this by design and intended? Because I don't expect any factor levels to be dropped.

I am using data.table_1.12.4.

Dominant language
R
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