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What is the difference - technical, philosophical, conceptual, or otherwise - between
raise "foo" and
raise Exception.new("foo") ?
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The raise keyword is used to raise an exception. You can define what kind of error to raise, and the text to print to the user.
raise e would still have a reference to the exception caught a few lines above, while the raise shorthand would try to raise None , which is an error. Follow this answer to receive notifications.
How do I manually throw/raise an exception in Python? Use the most specific Exception constructor that semantically fits your issue. Be specific in your message, e.g.: raise ValueError('A very specific bad thing happened.
Ruby actually gives you the power to manually raise exceptions yourself by calling Kernel#raise. This allows you to choose what type of exception to raise and even set your own error message. If you do not specify what type of exception to raise, Ruby will default to RuntimeError (a subclass of StandardError ).
Technically, the first raises a RuntimeError with the message set to "foo", and the second raises an Exception with the message set to "foo".
Practically, there is a significant difference between when you would want to use the former and when you want to use the latter.
Simply put, you probably want a RuntimeError not an Exception. A rescue block without an argument will catch RuntimeErrors, but will NOT catch Exceptions. So if you raise an Exception in your code, this code will not catch it:
begin rescue end In order to catch the Exception you will have to do this:
begin rescue Exception end This means that in a sense, an Exception is a "worse" error than a RuntimeError, because you have to do more work to recover from it.
So which you want depends on how your project does its error handling. For instance, in our daemons, the main loop has a blank rescue which will catch RuntimeErrors, report them, and then continue. But in one or two circumstances, we want the daemon to really really die on an error, and in that case we raise an Exception, which goes straight through our "normal error handling code" and out.
And again, if you are writing library code, you probably want a RuntimeError, not an Exception, as users of your library will be surprised if it raises errors that a blank rescue block can't catch, and it will take them a moment to realize why.
Finally, I should say that the RuntimeError is a subclass of the StandardError class, and the actual rule is that although you can raise any type of object, the blank rescue will by default only catch anything that inherits from StandardError. Everything else has to be specific.
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From the offical documentation:
raise raise( string ) raise( exception [, string [, array ] ] ) With no arguments, raises the exception in $! or raises a RuntimeError if $! is nil. With a single String argument, it raises a RuntimeError with the string as a message. Otherwise, the first parameter should be the name of an Exception class (or an object that returns an Exception when sent exception). The optional second parameter sets the message associated with the exception, and the third parameter is an array of callback information. Exceptions are caught by the rescue clause of begin...end blocks.
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