From: Brian Candler <b.candler@...>
Date: 2009-12-09T00:31:49+09:00
Subject: Re: Newbie question: what exceptions are thrown by FTP class

The Net::FTP documentation does tell you some exceptions that it can 
raise, and if you really care about the detail you can just look at the 
source, since it's all written in Ruby, and at the top you'll find

module Net

  # :stopdoc:
  class FTPError < StandardError; end
  class FTPReplyError < FTPError; end
  class FTPTempError < FTPError; end
  class FTPPermError < FTPError; end
  class FTPProtoError < FTPError; end
  # :startdoc:

But often you don't care. Exceptions form a hierarchy, and the ones you 
want to catch are usually either under StandardError or RuntimeError 
(itself a subclass of StandardError)

A simple "rescue" by itself is the same as "rescue StandardError".

If you want to capture *all* exceptions then rescue Exception, but this 
is usually not a good idea as it will also capture fatal errors - e.g. 
NoMemoryError, SyntaxError, and Interrupt (ctrl-C)

For Net::FTP I would suggest:

  begin
    ...
  rescue IOError, SystemCallError, Net::FTPError
    ...
  end

to catch everything socket-related and any protocol errors raised within 
Net::FTP, without catching things which are normally programmer bugs 
like NameError or ArgumentError.

Google "ruby exception hierarchy" for more info.
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