Rubyでの例外処理コードの記述例

  begin
   fail 'Oops'
  rescue => error
   raise if error.message ! = 'Oops'
  end

 # bad
  fail RuntimeError, 'message'

  # good - signals a RuntimeError by default
  fail 'message'

 # bad
  fail SomeException.new('message')
  # Note that there is no way to do `fail SomeException.new('message'), backtrace`.

  # good
  fail SomeException, 'message'
  # Consistent with `fail SomeException, 'message', backtrace`.

 def foo
   begin
    fail
   ensure
    return 'very bad idea'
   end
  end

 # bad
  def foo
   begin
    # main logic goes here
   rescue
    # failure handling goes here
   end
  end

  # good
  def foo
   # main logic goes here
  rescue
   # failure handling goes here
  end

  # bad
  begin
   something_that_might_fail
  rescue IOError
   # handle IOError
  end

  begin
   something_else_that_might_fail
  rescue IOError
   # handle IOError
  end

  # good
  def with_io_error_handling
    yield
  rescue IOError
   # handle IOError
  end

  with_io_error_handling { something_that_might_fail }

  with_io_error_handling { something_else_that_might_fail }

  # bad
  begin
   # an exception occurs here
  rescue SomeError
   # the rescue clause does absolutely nothing
  end

  # bad
  do_something rescue nil

 # bad - this catches exceptions of StandardError class and its descendant classes
  read_file rescue handle_error($!)

  # good - this catches only the exceptions of Errno::ENOENT class and its descendant classes
  def foo
   read_file
  rescue Errno::ENOENT => ex
   handle_error(ex)
  end

 # bad
  begin
   n / d
  rescue ZeroDivisionError
   puts "Cannot divide by 0!"
  end

  # good
  if d.zero?
   puts "Cannot divide by 0!"
  else
   n / d
  end

  # bad
  begin
   # calls to exit and kill signals will be caught (except kill -9)
   exit
  rescue Exception
   puts "you didn't really want to exit, right?"
   # exception handling
  end

  # good
  begin
   # a blind rescue rescues from StandardError, not Exception as many
   # programmers assume.
  rescue => e
   # exception handling
  end

  # also good
  begin
   # an exception occurs here

  rescue StandardError => e
   # exception handling
  end

  # bad
  begin
   # some code
  rescue Exception => e
   # some handling
  rescue StandardError => e
   # some handling
  end

  # good
  begin
   # some code
  rescue StandardError => e
   # some handling
  rescue Exception => e
   # some handling
  end

 f = File.open('testfile')
  begin
   # ... process
  rescue
   # ... handle error
  ensure
   f.close unless f.nil?
  end