Testing for exceptions

The update method should also raise a ValueError when the price is less than zero. The following is how we verify this in the doctest:

    def update(self, timestamp, price):
        """Updates the stock with the price at the given timestamp

        >>> from datetime import datetime
        >>> stock = Stock("GOOG")
        >>> stock.update(datetime(2014, 10, 2), 10)
        >>> stock.price
        10

        The method raises a ValueError exception if the price is negative

        >>> stock.update(datetime(2014, 10, 2), -1)
        Traceback (most recent call last):
            ...
        ValueError: price should not be negative
        """

        if price < 0:
            raise ValueError("price should not be negative")
        self.history.update(timestamp, price)
        self.updated.fire(self)

The next section shows the expectation that doctest looks at:

Traceback (most recent call last):
    ...
ValueError: price...