Access named nested list elements with string name in R

Question

I am trying to find a way to access named nested list elements by passing their names as a string (or list of strings). Something like you can do with attr(the_thing, "the_attr") but I want to do attr(the_thing, "$attr1$attr2$attr3"). Seems like this should be possible but I'm at a loss.

For example, I'm writing some code to consume responses from the Github API. There are a bunch of functions like (these are very simplified just for example's sake):

get_milestone <- function(org, repo) {
        response <- graphql_query(org = org, repo = repo)
        return(response$repository$milestone$issues)
}
get_pull_request <- function(org, repo) {
        response <- graphql_query(org = org, repo = repo)
        return(response$repository$pull_request$issues)
}
get_issues <- function(org, repo) {
        response <- graphql_query(org = org, repo = repo)
        return(response$repository$issues)
}

and you'll notice the only difference is what attributes you pull out of the response.

I'd like to make simple helper function like

get_something <- function(org, repo, attr_to_get) {
        response <- graphql_query(org = org, repo = repo)
        return(attr(response, attr_to_get))
}

and then call it each time like

get_something(org, repo, attr_to_get="$repository$milestone$issues")
get_something(org, repo, attr_to_get="$repository$pull_request$issues")
get_something(org, repo, attr_to_get="$repository$issues")

but that syntax doesn't work.

Is there any way to pass a string to specify nested attributes to extract from an object? I feel like rlang or something like that may be relevant, but I can't seem to figure it out.

My initial idea was just to do something like this

param_list <- unlist(strsplit(attr_to_get, "\\$"))
for (p in param_list) {
    if (p != "") {
       response <- response[[p]]
    }
}

But that feels very ugly and silly to me. Like, there must be a one-liner way of doing this without manually iterating. But maybe I should just do it that way? Any help is much appreciated.

For reproducibility you can replace the response <- graphql_query(org = org, repo = repo) line in my function with something like response <- list(repository = list(milestone = list(issues=c("issue1", "issue2", "issue3")))) and then attempt to extract "$repository$milestone$issues" from the response. Thanks!

first $repository$milestone$issues is not an attribute. it is just a nested list. Are you dealing with nested lists or with attributes?? — Onyambu
– Onyambu, Commented Jan 13, 2020 at 20:41
well just use a vector instead of a string eg response[[c("repository","milestone","issues")]] — Onyambu
– Onyambu, Commented Jan 13, 2020 at 20:44
perfect. That works. Feel free to make an answer and I'll accept. Although I guess I should modify the question to be explicitly about nested lists. — seth127
– seth127, Commented Jan 13, 2020 at 20:48
The purrr:pluck function is also very helpful for extracting elements from nested lists. — MrFlick
– MrFlick, Commented Jan 13, 2020 at 20:52

Onyambu · Accepted Answer · 2020-01-13 21:04:13Z

4

you can do

 nm <- c("repository","milestone","issues")
 response[[nm]]

also we can use the purrr::pluck function

  purrr::pluck(response, !!!nm)

edited Jan 13, 2020 at 21:04

answered Jan 13, 2020 at 20:56

Onyambu

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Dave Gruenewald Over a year ago

Similarly, purrr::pluck(response, "repository", "milestone", "issues")

Anna Nevison · Accepted Answer · 2020-01-13 21:54:48Z

2

If you feel like using rlang here's another possible answer:

# Something generic that produces a nested list with the names you specified
graphql_query <- function(org, repo){
list(repository = list(milestone = list(issues = 'hi')))
}

get_something <- function(org, repo, ...) {
response <- graphql_query(org, repo)
attr_to_get <- paste0('response$', paste(rlang::list2(...), collapse = '$'))
rlang::eval_tidy(rlang::parse_expr(attr_to_get))
}

get_something('org', 'repo', 'repository', 'milestone', 'issues')
>>> [1] "hi"

or you can do:

att_lst <- c('repository', 'milestone', 'issues')
get_something('org', 'repo', !!!att_lst)
>>> [1] "hi"

edited Jan 13, 2020 at 21:54

answered Jan 13, 2020 at 21:37

Anna Nevison

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1 Comment

MrFlick Over a year ago

I would strongly recommend against using parse_expr. Building strings of code and executing them can be messy and dangerous. Especially given that the other answers already show safe, built-in ways to accomplish this.

ThomasIsCoding · Accepted Answer · 2020-01-13 21:11:15Z

0

Another base R solution is to use Reduce() + [[

Reduce(`[[`,list(response,"repository","milestone","issues"))

or

Reduce(`[[`,list(response,c("repository","milestone","issues")))

edited Jan 13, 2020 at 21:11

answered Jan 13, 2020 at 21:05

ThomasIsCoding

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