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PORuby exception inheritance with dynamically generated classes
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copied!<p>I'm new to Ruby, so I'm having some trouble understanding this weird exception problem I'm having. I'm using the ruby-aaws gem to access Amazon ECS: <a href="http://www.caliban.org/ruby/ruby-aws/" rel="nofollow noreferrer">http://www.caliban.org/ruby/ruby-aws/</a>. This defines a class Amazon::AWS:Error:</p> <pre><code>module Amazon module AWS # All dynamically generated exceptions occur within this namespace. # module Error # An exception generator class. # class AWSError attr_reader :exception def initialize(xml) err_class = xml.elements['Code'].text.sub( /^AWS.*\./, '' ) err_msg = xml.elements['Message'].text unless Amazon::AWS::Error.const_defined?( err_class ) Amazon::AWS::Error.const_set( err_class, Class.new( StandardError ) ) end ex_class = Amazon::AWS::Error.const_get( err_class ) @exception = ex_class.new( err_msg ) end end end end end </code></pre> <p>This means that if you get an errorcode like <code>AWS.InvalidParameterValue</code>, this will produce (in its exception variable) a new class <code>Amazon::AWS::Error::InvalidParameterValue</code> which is a subclass of <code>StandardError</code>.</p> <p>Now here's where it gets weird. I have some code that looks like this:</p> <pre><code>begin do_aws_stuff rescue Amazon::AWS::Error => error puts "Got an AWS error" end </code></pre> <p>Now, if <code>do_aws_stuff</code> throws a <code>NameError</code>, my rescue block gets triggered. It seems that Amazon::AWS::Error isn't the superclass of the generated error - I guess since it's a module everything is a subclass of it? Certainly if I do:</p> <pre><code>irb(main):007:0> NameError.new.kind_of?(Amazon::AWS::Error) => true </code></pre> <p>It says <code>true</code>, which I find confusing, especially given this:</p> <pre><code>irb(main):009:0> NameError.new.kind_of?(Amazon::AWS) => false </code></pre> <p>What's going on, and how am I supposed to separate out AWS errors from other type of errors? Should I do something like:</p> <pre><code>begin do_aws_stuff rescue => error if error.class.to_s =~ /^Amazon::AWS::Error/ puts "Got an AWS error" else raise error end end </code></pre> <p>That seems exceptionally janky. The errors thrown aren't class AWSError either - they're raised like this:</p> <pre><code>error = Amazon::AWS::Error::AWSError.new( xml ) raise error.exception </code></pre> <p>So the exceptions I'm looking to <code>rescue</code> from are the generated exception types that only inherit from StandardError.</p> <p>To clarify, I have two questions:</p> <ol> <li><p>Why is NameError, a Ruby built in exception, a <code>kind_of?(Amazon::AWS::Error)</code>, which is a module?<br> <strong>Answer:</strong> I had said <code>include Amazon::AWS::Error</code> at the top of my file, thinking it was kind of like a Java import or C++ include. What this actually did was add everything defined in <code>Amazon::AWS::Error</code> (present and future) to the implicit Kernel class, which is an ancestor of every class. This means <strong>anything</strong> would pass <code>kind_of?(Amazon::AWS::Error)</code>.</p></li> <li><p>How can I best distinguish the dynamically-created exceptions in <code>Amazon::AWS::Error</code> from random other exceptions from elsewhere?</p></li> </ol>

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