Temporarily reassigning a constant

redefine_constant.rb

module A
  module B
    C = Class.new
  end
end

def redefine_constant full_const_name, value
  const_path = full_const_name.to_s.split('::')
  namespace = const_path[0..-2].inject(Kernel) { |scope, name| scope.const_get(name) }
  const = const_path[-1]
  orig_value = namespace.const_get(const)

  namespace.send :remove_const, const
  namespace.const_set(const, value)
  yield
  namespace.send :remove_const, const
  namespace.const_set(const, orig_value)
end

describe 'redefining constants' do
  it 'redefines a nested constant' do
    orig_value = A::B::C
    new_value = double('New Value')

    redefine_constant(A::B::C, new_value) do
      A::B::C.should be new_value
    end
    A::B::C.should be orig_value
  end
end