Is there a general solution for parsing server error messages that aren't returned in the header in httr so that we can print error message for users in a package?

parse_error_messages.md

This is a call to the GBIF API. There is an intential error in the call that gives an error message from the server

com.spatial4j.core.exception.InvalidShapeException: Invalid latitude: latitudes are range -90 to 90: provided lat: [-125.0]

In R with httr:

library(httr)
call <- "http://api.gbif.org/v0.9/occurrence/search?geometry=POLYGON((38.4 -125, 40.9 -125, 40.9 -121.8, 38.4 -121.8, 38.4 -125))"
a <- GET(call)

None of the metadata from the call, as far as I know, gives the error message.

You can parse the message out

library(XML)
doc = htmlParse(content(a, as="text"))
xpathSApply(doc, "//h1", xmlValue)

[1] "HTTP Status 500 - com.spatial4j.core.exception.InvalidShapeException: Invalid latitude: latitudes are range -90 to 90: provided lat: [-125.0]"

But I wonder if there is a general solution available in httr that works beyond just this example.

Perhaps the server error above should be in the header?

All you get from http is a status code - in this case 500, which means "Internal Server Error". The server can return anything it likes in the body, there's no standard for passing more error info except as text after the status code. I'm not sure if compliant servers can vary what they put after the status code...

You can get the status code from a$headers$status and the message in a$headers$statusmessage but thats it.

The "reason phrases" are suggestions: http://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol#Status_codes so if the server wants to say something more explanatory it could!

Hi @spacedman ! Thanks for the comments and the explanation. That makes sense. Right, I'm familiar with a$header$statusmessage, but was curious if that more detailed message could be collected. Thanks!

As @spacedman points out, this is something that varies from API to API. I think you need to implement something by hand for each API to hook up to.

Okay, thanks Hadley