group_modify error with empty data frame

[clemenshug]

I came across an edge case today where group_modify gives a misleading error message if the input data frame is empty. It reports that the grouping variable was not found. I think the expected behavior would be to return an empty data frame, similar to summarize.

group_map returns an empty list given the same input.

library(dplyr)

mtcars %>%
  group_by(cyl) %>%
  filter(hp > 1000) %>%
  group_modify(~.x)
#> Error: Column `cyl` is unknown

mtcars %>%
  group_by(cyl) %>%
  filter(hp > 1000) %>%
  group_map(~.x)
#> list()

^{Created on 2019-06-12 by the reprex package (v0.3.0)}

Session info

devtools::session_info()
#> ─ Session info ──────────────────────────────────────────────────────────
#>  setting  value                       
#>  version  R version 3.6.0 (2019-04-26)
#>  os       macOS Mojave 10.14.4        
#>  system   x86_64, darwin18.5.0        
#>  ui       unknown                     
#>  language (EN)                        
#>  collate  en_US.UTF-8                 
#>  ctype    en_US.UTF-8                 
#>  tz       America/New_York            
#>  date     2019-06-12                  
#> 
#> ─ Packages ──────────────────────────────────────────────────────────────
#>  package     * version date       lib source        
#>  assertthat    0.2.1   2019-03-21 [1] CRAN (R 3.6.0)
#>  backports     1.1.4   2019-04-10 [1] CRAN (R 3.6.0)
#>  callr         3.2.0   2019-03-15 [1] CRAN (R 3.6.0)
#>  cli           1.1.0   2019-03-19 [1] CRAN (R 3.6.0)
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#>  desc          1.2.0   2018-05-01 [1] CRAN (R 3.6.0)
#>  devtools      2.0.2   2019-04-08 [1] CRAN (R 3.6.0)
#>  digest        0.6.18  2018-10-10 [1] CRAN (R 3.6.0)
#>  dplyr       * 0.8.1   2019-05-14 [1] CRAN (R 3.6.0)
#>  evaluate      0.13    2019-02-12 [1] CRAN (R 3.6.0)
#>  fansi         0.4.0   2018-10-05 [1] CRAN (R 3.6.0)
#>  fs            1.3.1   2019-05-06 [1] CRAN (R 3.6.0)
#>  glue          1.3.1   2019-03-12 [1] CRAN (R 3.6.0)
#>  highr         0.8     2019-03-20 [1] CRAN (R 3.6.0)
#>  htmltools     0.3.6   2017-04-28 [1] CRAN (R 3.6.0)
#>  knitr         1.22    2019-03-08 [1] CRAN (R 3.6.0)
#>  magrittr      1.5     2014-11-22 [1] CRAN (R 3.6.0)
#>  memoise       1.1.0   2017-04-21 [1] CRAN (R 3.6.0)
#>  pillar        1.4.0   2019-05-11 [1] CRAN (R 3.6.0)
#>  pkgbuild      1.0.3   2019-03-20 [1] CRAN (R 3.6.0)
#>  pkgconfig     2.0.2   2018-08-16 [1] CRAN (R 3.6.0)
#>  pkgload       1.0.2   2018-10-29 [1] CRAN (R 3.6.0)
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#>  processx      3.3.1   2019-05-08 [1] CRAN (R 3.6.0)
#>  ps            1.3.0   2018-12-21 [1] CRAN (R 3.6.0)
#>  purrr         0.3.2   2019-03-15 [1] CRAN (R 3.6.0)
#>  R6            2.4.0   2019-02-14 [1] CRAN (R 3.6.0)
#>  Rcpp          1.0.1   2019-03-17 [1] CRAN (R 3.6.0)
#>  remotes       2.0.4   2019-04-10 [1] CRAN (R 3.6.0)
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#>  rmarkdown     1.12    2019-03-14 [1] CRAN (R 3.6.0)
#>  rprojroot     1.3-2   2018-01-03 [1] CRAN (R 3.6.0)
#>  sessioninfo   1.1.1   2018-11-05 [1] CRAN (R 3.6.0)
#>  stringi       1.4.3   2019-03-12 [1] CRAN (R 3.6.0)
#>  stringr       1.4.0   2019-02-10 [1] CRAN (R 3.6.0)
#>  testthat      2.1.1   2019-04-23 [1] CRAN (R 3.6.0)
#>  tibble        2.1.1   2019-03-16 [1] CRAN (R 3.6.0)
#>  tidyselect    0.2.5   2018-10-11 [1] CRAN (R 3.6.0)
#>  usethis       1.5.0   2019-04-07 [1] CRAN (R 3.6.0)
#>  utf8          1.1.4   2018-05-24 [1] CRAN (R 3.6.0)
#>  vctrs         0.1.0   2018-11-29 [1] CRAN (R 3.6.0)
#>  withr         2.1.2   2018-03-15 [1] CRAN (R 3.6.0)
#>  xfun          0.7     2019-05-14 [1] CRAN (R 3.6.0)
#>  yaml          2.2.0   2018-07-25 [1] CRAN (R 3.6.0)
#>  zeallot       0.1.0   2018-01-28 [1] CRAN (R 3.6.0)
#> 
#> [1] /usr/local/lib/R/3.6/site-library
#> [2] /usr/local/Cellar/r/3.6.0_2/lib/R/library

[lock]

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