[How to] Build an API wrapper package in 10 minutes.

5 minute read

Well… documentation not included (of course).

API are cool. They allow to retrieve data from the web, and if ever you’re familiar with {httr}, {jsonlite} and packages like these, you’re able to build requests and retrieve data in a matter of minutes.

But no worry, if you’re not familiar with http requests and web formats like html, JSON and such, you can still go and look for a package wrapper around that specific API: someone had probably been coding it already.

And, if you want to be that someone that code an API wrapper package, here’s a short tutorial that will allow you to create it.

Disclaimer: the 10 minutes workflow does not (of course) include the package documentation.

Find the API

Well, this first step can take a while… but, let’s say we have already found it, and want to build an package around the french database for addresses: https://adresse.data.gouv.fr/api/.

Step 0: be sure you have all the packages you’ll need

Run this if you want to be sure you have all the packages we’ll use here:

install.packages("devtools")
install.packages("roxygen2")
install.packages("usethis")
install.packages("curl")
install.packages("httr")
install.packages("jsonlite")
install.packages("attempt")
install.packages("purrr")
devtools::install_github("r-lib/desc")

Step 1: the project

Create a new project with RStudio, and click on “Create a package with devtools”.

Step 2: devstuffs

  • In your terminal, run usethis::use_data_raw().

  • Open a new R Script, save it into /data_raw as “devstuffs”, or any other name. Copy and paste into this script:

library(devtools)
library(usethis)
library(desc)

# Remove default DESC
unlink("DESCRIPTION")
# Create and clean desc
my_desc <- description$new("!new")

# Set your package name
my_desc$set("Package", "yourpackage")

#Set your name
my_desc$set("Authors@R", "person('Colin', 'Fay', email = 'contact@colinfay.me', role = c('cre', 'aut'))")

# Remove some author fields
my_desc$del("Maintainer")

# Set the version
my_desc$set_version("0.0.0.9000")

# The title of your package
my_desc$set(Title = "My Supper API Wrapper")
# The description of your package
my_desc$set(Description = "A long description of this super package I'm working on.")
# The urls
my_desc$set("URL", "http://this")
my_desc$set("BugReports", "http://that")
# Save everyting
my_desc$write(file = "DESCRIPTION")

# If you want to use the MIT licence, code of conduct, and lifecycle badge
use_mit_license(name = "Colin FAY")
use_code_of_conduct()
use_lifecycle_badge("Experimental")
use_news_md()

# Get the dependencies
use_package("httr")
use_package("jsonlite")
use_package("curl")
use_package("attempt")
use_package("purrr")

# Clean your description
use_tidy_description()

Then, run everything from this script.

Copy and paste at the top of you README.Rmd :

  [![lifecycle](https://img.shields.io/badge/lifecycle-experimental-orange.svg)](https://www.tidyverse.org/lifecycle/#experimental)

And at the bottom :

  Please note that this project is released with a [Contributor Code of Conduct](CODE_OF_CONDUCT.md).
  By participating in this project you agree to abide by its terms.

Step 3: get the API url

There are two ways you can request on an API :

  • By building url: url.and/path/to/the/data

  • With parameters: url.that/q=this&data=that

This is the case for https://adresse.data.gouv.fr/api/. We’ve got the base url: http 'https://api-adresse.data.gouv.fr/search/?q=8 bd du port', and search parameters.

Here, the base url is everything before the ?, and the parameters are the key-value pairs after the .

Step 4: utils

Before creating the main calling functions, we’ll create some utilitary functions that will run some tests: is the internet connexion running? Does the {httr} result return the right http code?

For this, we’ll create a file called utils.R, save it in the R/ folder, and put into it:

#' @importFrom attempt stop_if_not
#' @importFrom curl has_internet
check_internet <- function(){
  stop_if_not(.x = has_internet(), msg = "Please check your internet connexion")
}

#' @importFrom httr status_code
check_status <- function(res){
  stop_if_not(.x = status_code(res), 
              .p = ~ .x == 200,
              msg = "The API returned an error")
}

In this same file we’ll also create two objects that will contain the base API urls:

base_url <- "https://api-adresse.data.gouv.fr/search/"
reverse_url <- "https://api-adresse.data.gouv.fr/reverse/"

Step 5 : the function that will call on the API

To call the API, we’ll use GET function from {httr}. Let’s first try just this:

httr::GET(url = base_url, query = list(q = "Yeaye"))
## Response [https://api-adresse.data.gouv.fr/search/?q=Yeaye]
##   Date: 2018-02-04 21:40
##   Status: 200
##   Content-Type: application/json; charset=utf-8
##   Size: 574 B

As you can see, the status is 200. Which is a good sign: no error from the API.

In the API we have chosen, there are 4 entry points: search, reverse, and their counterparts with csv. These counterparts work by POSTing a csv to the API, so let’s forget them for now.

The search entrypoint can take 8 parameters:

  • q: text search
  • limit: maximum number of results to return
  • autocomplete: autocompletion
  • lat: latitude
  • lon: longitude
  • type: type of elements to return (housenumber, street,place, municipality)
  • postcode: Postal code
  • citycode: INSEE code

These are the elements which will be passed as a list in the query parameter from httr::GET.

Note: if you’re going for an url based request (url.and/path/to/the/data or url.and/?path=this&to=that), you don’t need to set the query parameter, you can simply paste the url with the elements (url <- paste0("url.and/?path=", this, "&to=", that)).

Create a new R script and write the functions used to call the API:

#' Search the BAN
#' 
#' @param q text search
#' @param limit maximum number of results to return 
#' @param autocomplete autocompletion
#' @param lat latitude
#' @param lon longitude
#' @param type type of elements to return (housenumber, street,place, municipality)
#' @param postcode Postal code
#' @param citycode INSEE code
#'
#' @importFrom attempt stop_if_all
#' @importFrom purrr compact
#' @importFrom jsonlite fromJSON
#' @importFrom httr GET
#' @export
#' @rdname searchban
#'
#' @return the results from the search
#' @examples 
#' \dontrun{
#' search_ban("Rennes")
#' reverse_search_ban("48.11", "-1.68")
#' }

search_ban <- function(q = NULL, limit = NULL, autocomplete = NULL, lat = NULL, lon = NULL, type = NULL, postcode = NULL, citycode = NULL){
  args <- list(q = q, limit = limit, autocomplete = autocomplete, lat = lat, 
               lon = lon, type = type, postcode = postcode, citycode = citycode)
  # Check that at least one argument is not null
  stop_if_all(args, is.null, "You need to specify at least one argument")
  # Chek for internet
  check_internet()
  # Create the 
  res <- GET(base_url, query = compact(args))
  # Check the result
  check_status(res)
  # Get the content and return it as a data.frame
  fromJSON(rawToChar(res$content))$features
}

#' @export
#' @rdname searchban
reverse_search_ban <- function(lat = NULL, lon = NULL){
  args <- list(lat = lat, lon = lon)
  # Check that at least one argument is not null
  stop_if_all(args, is.null, "You need to specify at least one argument")
  # Chek for internet
  check_internet()
  # Create the 
  res <- GET(reverse_url, query = compact(args))
  # Check the result
  check_status(res)
  # Get the content and return it as a data.frame
  fromJSON(rawToChar(res$content))$features
}

Note: you’ll need to change the arguments and documentation for your specific API (obviously).

If ever you want to a part of this roxygen filling programmatically, you should check the excellent {sinew} package by Jonathan Sidi.

Step 6 : Roxygenise

Now, run in your console :

roxygen2::roxygenise()

And that’s it! You’ve got a working package :)

Step 7 : build your package

You can test that everything is ok with:

devtools::check()
# Then build it with:
devtools::build()

What’s left

As I said, the 10 minutes workflow does not include the doc, so here’s what left for you to do:

  • Write the Readme
  • Write a Vignette
  • Write tests

For that, go back to your dev file, add, and run:

use_testthat()
use_vignette("{your-package-name}")
use_readme_rmd()

And now, grab you best pen and write documentation ;)

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