How to deal with not knowing what exceptions can be raised by a library method in Ruby?

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This is somewhat of a broad question, but it is one that I continue to come across when programming in Ruby. I am from a largely C and Java background, where when I use a library function or method, I look at the documentation and see what it returns on error (usually in C) or which exceptions it can throw (in Java).

In Ruby, the situation seems completely different. Just now I need to parse some JSON I receive from a server:

data = JSON.parse(response)

Naturally, the first thing I think after writing this code is, what if the input is bad? Is parse going to return nil on error, or raise some exception, and if so, which ones?

I check the documentation (http://flori.github.com/json/doc/JSON.html#M000022) and see, simply:

"Parse the JSON string source into a Ruby data structure and return it."

This is just an example of a pattern I have run into repeatedly in Ruby. Originally, I figured it was some shortcoming of the documentation of whatever library I was working with, but now I am starting to feel this is standard practice and I am in a somewhat different mindset than Ruby programmers. Is there some convention I am unaware of?

How do developers deal with this?

(And yes I did look at the code of the library method, and can get some idea of what exceptions are raised but I cannot be 100% sure and if it is not documented I feel uncomfortable relying on it.)

EDIT: After looking at the first two answers, let me continue the JSON parsing example from above.

I suspect I should not do:

begin
  data = JSON.parse(response)
  raise "parse error" if data.nil?
rescue Exception => e
  # blahblah
end

because I can look at the code/tests and see it seems to raise a ParserError on error (returning nil seems to not be standard practice in Ruby). Would I be correct in saying the recommended practice is to do:

begin
  data = JSON.parse(response)
rescue JSON::ParserError => e
  # blahblah
end

...based upon what I learned about ParserError by looking through the code and tests?

(I also edited the example to clarify it is a response from a server that I am parsing.)

exception-handling rescue exception ruby
5 Answers

Your question boils down to basically two questions: is there a convention or standard for finding possible exceptions, and also where is the documentation related to such a convention?

For the first question, the closest thing to a convention or standard is the location and existence of an exceptions.rb file. For libraries or gems where the source code is publicly available, you can typically find the types of exceptions in this file. (Ref here).

If source code is not available or easily accessed, the documentation is your next best source of information. This brings us to your second question. Unfortunately documentation does not have a consistent format concerning potential exceptions, even in standard libraries. For example, the Net::Http documentation doesn't make it obvious what exceptions are available, although if you dig through it you will find that all exceptions inherit from Net::HTTPExceptions.

As another example (again from the standard library documentation), JSON documentation shows an Exception class (which is indeed in an exceptions.rb file, albeit in the source at json/lib/json/add/exceptions.rb). The point here is that it's inconsistent; the documentation for the Exception class is not listed in a way similar to that of Net::HTTPException.

Moreover, in documentation for most methods there is no indication of the exception that may be raised. Ref for example parse, one of the most-used methods of the JSON module already mentioned: exceptions aren't mentioned at all.

The lack of standard and consistency is also found in core modules. Documentation for Math doesn't contain any reference to an exceptions.rb file. Same with File, and its parent IO.

And so on, and so on.

A web search will turn up lots of information on how to rescue exceptions, and even multiple types of exceptions (ref here, here, here, here, etc). However, none of these indicate an answer to your questions of what is the standard for finding exceptions that can be raised, and where is the documentation for this.

As a final note, it has been suggested here that if all else fails you can rescue StandardError. This is a less-than-ideal practice in many cases (ref this SO answer), although I'm assuming you already understand this based on your familiarity with Java and the way you've asked this question. And of course coming from a Java world, you'll need to remember to rescue StandardError and not Exception.

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