gistfile1.rb

class Hash
  def except(*blacklist)
    self.reject {|key, value| blacklist.include?(key) }
  end
  
  def only(*whitelist)
    self.reject {|key, value| !whitelist.include?(key) }
  end
end

h = {:a => 10, :b => 34, :c => "hello"}

h.only :a, :b # => {:a=>10, :b=>34}
h.except :a, :b # => {:c=>"hello"}