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Keep trying that api call with purrr::possibly()

Sometimes you need to call an api to get some result from a web service, but sometimes this call might fail. You might get an error 500 for example, or maybe you’re making too many calls too fast. Regarding this last point, I really encourage you to read Ethics in Web Scraping.

In this blog post I will show you how you can keep trying to make this api call using purrr::possibly().

For this, let’s use this function that will simulate an api call:

get_data = function(){
  number = rbinom(1, 1, 0.9)
  ifelse(number == 0, "OK", stop("Error: too many calls!"))
}

This function simply returns a random draw from a binomial distribution. If this number equals 0 with probability 0.1, the function returns “OK”, if not, it throws an error. Because the probability of success is only 10%, your api call might be unsuccessful:

get_data()
Error in ifelse(number == 0, "OK", stop("Error: too many calls!")) :
  Error: too many calls!

How to keep trying until it works? For this, we’re going to use purrr::possibly(); this function takes another function as argument and either returns the result, or another output in case of error, that the user can define:

possibly_get_data = purrr::possibly(get_data, otherwise = NULL)

Let’s try it:

set.seed(12)
possibly_get_data()
## NULL

With set.seed(12), the function returns a number different from 0, and thus throws an error: but because we’re wrapping the function around purrr::possibly(), the function now returns NULL. The first step is done; now we can use this to our advantage:

definitely_get_data = function(func, n_tries, sleep, ...){

  possibly_func = purrr::possibly(func, otherwise = NULL)

  result = NULL
  try_number = 1

  while(is.null(result) && try_number <= n_tries){
    print(paste("Try number: ", try_number))
    try_number = try_number + 1
    result = possibly_func(...)
    Sys.sleep(sleep)
  }

  return(result)
}

definitely_get_data() is a function that takes any function as argument, as well as a user provided number of tries (as well as ... to pass further arguments to func()). Remember, if func() fails, it will return NULL; the while loop ensures that while the result is NULL, and the number of tries is below what you provided, the function will keep getting called. I didn’t talk about sleep; this argument is provided to Sys.sleep() which introduces a break between calls that is equal to sleep seconds. This ensures you don’t make too many calls too fast. Let’s try it out:

set.seed(123)
definitely_get_data(get_data, 10, 1)
## [1] "Try number:  1"
## [1] "Try number:  2"
## [1] "Try number:  3"
## [1] "Try number:  4"
## [1] "Try number:  5"
## [1] "OK"

It took 5 tries to get the result! However, if after 10 tries get_data() fails to return what you need it will stop (but you can increase the number of tries…).

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