Refactoring the design

In the previous section, we talked about the new requirement of having to check if a stock has an increasing trend.

Let us start by writing a test first:

class StockTrendTest(unittest.TestCase):
    def setUp(self):
        self.goog = Stock("GOOG")
    def test_increasing_trend_is_true_if_price_increase_for_3_updates(self):
        timestamps = [datetime(2014, 2, 11), datetime(2014, 2, 12), datetime(2014, 2, 13)]
        prices = [8, 10, 12]
        for timestamp, price in zip(timestamps, prices):
            self.goog.update(timestamp, price)
        self.assertTrue(self.goog.is_increasing_trend())

The test takes three timestamps and prices and performs an update for each one. Since all three prices are increasing, the is_increasing_trend method should return True.

To make this test pass, we need to first add support for storing price history.

In the initializer, let us replace the price attribute with a price_history list. This list will store the history of price updates...