some_spec.rb

RSpec.describe 'Something' do
  subject { described_class.new([item]) }
  before { subject.update_args }
  
  context 'this item' do
    let(:item) { Item.new('item', 1, 0) }
    
    it 'foos' do
      expect(item.sell_in).to eq(0)
    end
  end
  
  context 'that other item' do
    let(:item) { Item.new(1, 1) }
    
      it 'bars' do
        # ...
      end
    end
  end
end