thisisnic commented on a change in pull request #11184:
URL: https://github.com/apache/arrow/pull/11184#discussion_r712104449
##########
File path: r/R/dplyr-summarize.R
##########
@@ -97,6 +93,35 @@ do_arrow_summarize <- function(.data, ..., .groups = NULL) {
ctx$post_mutate
)[c(.data$group_by_vars, names(exprs))]
}
+
+ # Handle .groups argument
+ if (length(.data$group_by_vars)) {
+ if (is.null(.groups)) {
+ # dplyr docs say:
+ # When ‘.groups’ is not specified, it is chosen based on the
+ # number of rows of the results:
+ # • If all the results have 1 row, you get "drop_last".
+ # • If the number of rows varies, you get "keep".
+ #
+ # But we don't support anything that returns multiple rows now
+ .groups <- "drop_last"
+ } else {
+ assert_that(is.string(.groups))
Review comment:
If the length of `.groups` is > 1, it's caught here, which raises an
error and drops to the R level, which also raises an error.
However, the error message from `dplyr` isn't quite fit for our purposes -
it mentions that `rowwise` is a valid value for `.groups`, which is true in
`dplyr` but not in `arrow`. Is it worth catching this case ourselves and
providing our own error message, which doesn't mention `rowwise`?
[Edited after reading the tests below] Or is this something worth living
with, as the alternative would require significant work to implement for little
actual benefit?
```
mtcars %>%
Table$create() %>%
group_by(mpg) %>%
summarise(.groups = c("drop", "keep")) %>%
collect()
# Warning: Error : .groups is not a string (a length one character vector).;
pulling data into R
# Error: `.groups` can't be <chr>
# ℹ Possible values are NULL (default), "drop_last", "drop", "keep", and
"rowwise"
# Run `rlang::last_error()` to see where the error occurred.
```
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