foo.rb

class Foo
  def self.with_context
    old = self.context #save current context
    yield
    @@context = old    #return context back after method was run
  end

  def self.context=(value)
    @@context = value
  end

  def self.context
    @@context
  end

  def self.bar
    with_context do
      Foo.context = "baz"
      puts Foo.context
    end
  end
end

Foo.context = "bar"
puts Foo.context
Foo.bar
puts Foo.context

foo.stdout

$ ruby foo.rb

bar
baz
bar