with_scope.rb

class A
  attr_accessor :b
  def with_b( new_b, &blk )
    original_b = b
    self.b = new_b
    yield if block_given?
    self.b = original_b
  end
end

a = A.new
a.b = 1

with_b 2 do
  p a.b # outputs 2
end

p a.b # outputs 1, no primary object state changed