Raise custom Exception with arguments

Question

I'm defining a custom Exception on a model in rails as kind of a wrapper Exception: (begin[code]rescue[raise custom exception]end)

When I raise the Exception, I'd like to pass it some info about a) the instance of the model whose internal functions raise the error, and b) the error that was caught.

This is going on an automated import method of a model that gets populated by POST request to from foreign datasource.

tldr; How can one pass arguments to an Exception, given that you define the Exception yourself? I have an initialize method on that Exception but the raise syntax seems to only accept an Exception class and message, no optional parameters that get passed into the instantiation process.

Answer 1

create an instance of your exception with new:

class CustomException < StandardError   def initialize(data)     @data = data   end end # => nil  raise CustomException.new(bla: "blupp") # CustomException: CustomException 

Answer 2

Solution:

class FooError < StandardError   attr_reader :foo    def initialize(foo)    super    @foo = foo   end end 

This is the best way if you follow the Rubocop Style Guide and always pass your message as the second argument to raise:

raise FooError.new('foo'), 'bar' 

You can get foo like this:

rescue FooError => error   error.foo     # => 'foo'   error.message # => 'bar' 

If you want to customize the error message then write:

class FooError < StandardError   attr_reader :foo    def initialize(foo)    super    @foo = foo   end    def message     "The foo is: #{foo}"   end end 

This works well if foo is required. If you want foo to be an optional argument, then keep reading.

---

Explanation:

Pass your message as the second argument to raise

As the Rubocop Style Guide says, the message and the exception class should be provided as separate arguments because if you write:

raise FooError.new('bar') 

And want to pass a backtrace to raise, there is no way to do it without passing the message twice:

raise FooError.new('bar'), 'bar', other_error.backtrace 

As this answer says, you will need to pass a backtrace if you want to re-raise an exception as a new instance with the same backtrace and a different message or data.

Implementing FooError

The crux of the problem is that if foo is an optional argument, there are two different ways of raising exceptions:

raise FooError.new('foo'), 'bar', backtrace # case 1 

and

raise FooError, 'bar', backtrace # case 2 

and we want FooError to work with both.

In case 1, since you've provided an error instance rather than a class, raise sets 'bar' as the message of the error instance.

In case 2, raise instantiates FooError for you and passes 'bar' as the only argument, but it does not set the message after initialization like in case 1. To set the message, you have to call super in FooError#initialize with the message as the only argument.

So in case 1, FooError#initialize receives 'foo', and in case 2, it receives 'bar'. It's overloaded and there is no way in general to differentiate between these cases. This is a design flaw in Ruby. So if foo is an optional argument, you have three choices:

(a) accept that the value passed to FooError#initialize may be either foo or a message.

(b) Use only case 1 or case 2 style with raise but not both.

(c) Make foo a keyword argument.

If you don't want foo to be a keyword argument, I recommend (a) and my implementation of FooError above is designed to work that way.

If you raise a FooError using case 2 style, the value of foo is the message, which gets implicitly passed to super. You will need an explicit super(foo) if you add more arguments to FooError#initialize.

If you use a keyword argument (h/t Lemon Cat's answer) then the code looks like:

class FooError < StandardError   attr_reader :foo    def initialize(message, foo: nil)    super(message)    @foo = foo   end end 

And raising looks like:

raise FooError, 'bar', backtrace raise FooError(foo: 'foo'), 'bar', backtrace