Our first doctest

The following is the current version of the price method in the Stock class:

    def price(self):
        try:
            return self.history[-1].value
        except IndexError:
            return None

Now, in the docstring, we add an example of how this method might be used. The examples are basically a copy-paste of a Python interactive shell. Hence, the lines containing input to be executed are prefixed with >>> prompt, and the lines without the prompt indicate output, as shown in the following:

    def price(self):
        """Returns the current price of the Stock

        >>> from datetime import datetime
        >>> stock = Stock("GOOG")
        >>> stock.update(datetime(2011, 10, 3), 10)
        >>> stock.price
        10
        """
        try:
            return self.history[-1].value
        except IndexError:
            return None

Now that we have the docstring, we need a way to execute it. Add the following lines...