gistfile1.rb

class PaymentProcessor
  def initlialize(stripe = Stripe.new(api_key: 'whatever'))
    self.stripe = stripe
  end
  
  def void_transaction(transaction)
    stripe.void_transaction(transaction)
  end
  
  def process(*payment_attributes)
    stripe.process(payment_attributes)
  end
  
  private
  
  attr_reader stripe
end

class SuccessPaymentProcessorDouble
  def process(*_)
    double(Stripe::Transaction, success?: true)
  end
  
  def void_transaction(_)
    true
  end
end

class FailurePaymentProcessorDouble
  def process(*_)
    raise Stripe::PaymentFailed
  end
  
  def void_transaction()
    raise Stripe::TransactionVoidFailed
  end
end

### in my real application
payment_processor = PaymentProcessor.new


### in my happy path test code
payment_processor = SuccessPaymentProcessorDouble.new
payment_processor.process # yay!

### in my fail test code
payment_processor = FailurePaymentProcessorDouble.new
payment_processor.process # oh no!

#### you could also inject a bad stripe object to simulate failures

bad_stripe = double(Stripe)
bad_stripe.stub(:process).and_raise(TimeoutError)

payment_processor = PaymentProcessor.new(bad_stripe)
payment_processor.process # boom!