Update `.groups` message after `group_by() |> summarize()`

[mine-cetinkaya-rundel]

Currently this is the message dplyr emits for summarize() after group_by() with multiple variables.

library(dplyr)

mtcars |>
  group_by(vs, am) |>
  summarize(mean_mpg = mean(mpg))
#> `summarise()` has grouped output by 'vs'. You can override using the `.groups`
#> argument.
#> # A tibble: 4 × 3
#> # Groups:   vs [2]
#>      vs    am mean_mpg
#>   <dbl> <dbl>    <dbl>
#> 1     0     0     15.0
#> 2     0     1     19.8
#> 3     1     0     20.7
#> 4     1     1     28.4

^{Created on 2024-01-29 with reprex v2.0.2}

I think this message is still confusing and would be more clear if the grouping message was about the output and it explicitly stated .groups is an argument in summarize(), e.g.,

The output is grouped by `vs`. You can specify grouping structure of the output using the `.groups` argument in `summarize()`.

If going this route some things to keep in mind:

• Maybe "result" instead of "output" in two places in the message, or change the description of the .groups argument to say "Grouping structure of the output." Basically, we should match what we're calling the "thing" that the function spits out.
• It would be a nice-to-have if US/UK spelling of the function in the message matched what the spelling in the code that generates the message.

An alternative suggestion by @DavisVaughan was

summarize() has computed your expressions grouped by (foo, bar), and has regrouped the output by (foo).

I think this is an improvement over the current message too, but I'd suggest going with something simpler like the one above.

[janxkoci]

Also, you grouped by two variables (vs, am) but the message only mentions vs. I see the same thing in my session with dplyr 1.1.4.

I have another problem where my results from summarise have duplicated rows for no obvious reason, so I'm looking around here if it's reported...

[telenskyt]

Yes, the warning is very confusing! And it took me a lot of time to figure out why the hell is summarize() telling me it grouped by (x) when I asked to group by (x, y)! Many people are confused by this message, see e.g. here, here, or here.

I completely agree the warning should be changed to something more comprehensible to save a lot of frustration and time of the users. And because of the confusion I experienced (see the bold above), I think it is very important that the message reassures the user that everything was done as expected, and that the new grouping has been applied after the operation, which I think is what @DavisVaughan intended. I would just add that this really unfortunate default behaviour is kept just for backward compatibility reasons and can be changed:

summarize() has grouped by (foo, bar) as requested, and subsequently regrouped the output by (foo) for compatibility reasons. You can override using the .groups argument.

I know it is longer than most warnings, but I see it important that the message is clear and prevents further confusion and hair loss. It is really unfortunate that the warning is needed in the first place - it is a clear sign that the default behaviour was not chosen well. Please, at least try to remedy this with a clear warning message.