Collections and delegating format specifications

When formatting a complex object that includes a collection, we have two formatting issues: how to format the overall object and how to format the items in the collection. When we look at Hand, for example, we see that we have a collection of individual Cards class. We'd like to have Hand delegate some formatting details to the individual Card instances in the Hand collection.

The following is a __format__() method that applies to Hand:

    def __format__( self, format_specification ):
        if format_specification == "":
            return str(self)
        return ", ".join( "{0:{fs}}".format(c, fs=format_specification)
            for c in self.cards )

The format_specification parameter will be used for each individual Card instance within the Hand collection. The format specification of "{0:{fs}}" uses the nested format specification technique to push the format_specification string to create a format that applies to each Card instance. Given this method, we can format a Hand object, player_hand, as follows:

"Player: {hand:%r%s}".format(hand=player_hand)

This will apply the %r%s format specification to each Card instance of the Hand object.