How can one mock API errors?

[JBGruber]

Quick question (and reporting my confusion about the behaviour): How could I test a custom error message for an API function? It looks like with_mock() gobbles errors. Is there a way around that? I checked your tests, but could not find one that tests the actual error output.

Here is a reproducible example:

library(testthat)
library(httr2)
#> 
#> Attaching package: 'httr2'
#> The following objects are masked from 'package:testthat':
#> 
#>     local_mock, with_mock

api_fun <- function(url, api_key = NULL) {
  req <- httr2::request(url) |>
    httr2::req_method("POST") |>
    httr2::req_headers(
      "Content-Type" = "application/json",
      "Authorization" = glue::glue("Bearer {api_key}")
    ) |>
    httr2::req_error(body = error_body)
  
  httr2::req_perform(req)
}

error_body <- function(resp) {
  msg <- httr2::resp_status(resp)
  if (resp_status(resp) == 429) {
    msg <- c(msg, "This error can also mean that you ran out of API credit.")
  }
  return(msg)
}

# shows correct error
api_fun("https://httpbin.org/status/429")
#> Error:
#> ! Timeout was reached: [httpbin.org] Failed to connect to httpbin.org port 443 after 10001 ms: Timeout was reached
#> Backtrace:
#>     ▆
#>  1. └─global api_fun("https://httpbin.org/status/429")
#>  2.   └─httr2::req_perform(req)
#>  3.     └─base::tryCatch(...)
#>  4.       └─base (local) tryCatchList(expr, classes, parentenv, handlers)
#>  5.         └─base (local) tryCatchOne(expr, names, parentenv, handlers[[1L]])
#>  6.           └─value[[3L]](cond)

mock_error <- function(...) {
  httr2::response(429)
}

# does not error with with_mock
expect_error(
  with_mock(mock = mock_error, code = api_fun("https://httpbin.org/status/429")),
  "This error can also mean that you ran out of API credit."
)
#> Error: `with_mock(mock = mock_error, code = api_fun("https://httpbin.org/status/429"))` did not throw an error.

^{Created on 2023-07-11 with reprex v2.0.2}

Thanks!

[hadley]

A slightly more minimal reprex:

library(httr2)

api_fun <- function(url, api_key = NULL) {
  request(url) |>
    req_error(body = error_body) |> 
    req_perform()
}

error_body <- function(resp) {
  if (resp_status(resp) == 429) {
    "You may have run out of API credit."
  }
}
api_fun("https://hb.cran.dev/status/429")
#> Error in `req_perform()`:
#> ! HTTP 429 Too Many Requests.
#> • You may have run out of API credit.
#> Backtrace:
#>     ▆
#>  1. └─global api_fun("https://hb.cran.dev/status/429")
#>  2.   └─httr2::req_perform(req_error(request(url), body = error_body))
#>  3.     └─httr2:::resp_abort(resp, error_body(req, resp), call = error_call)
#>  4.       └─rlang::abort(...)

with_mock(
  mock = \(...) response(429),
  code = api_fun("https://httpbin.org/status/429")
)
#> <httr2_response>
#> GET https://example.com
#> Status: 429 Too Many Requests
#> Body: Empty

^{Created on 2023-08-09 with reprex v2.0.2}