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Comparing cards

8 September, 2015 - 10:43

For built-in types, there are relational operators (<, >, ==, etc.) that compare values and determine when one is greater than, less than, or equal to another. For user-defined types, we can override the behavior of the built-in operators by providing a method named __cmp__.

__cmp__ takes two parameters, self and other, and returns a positive number if the first object is greater, a negative number if the second object is greater, and 0 if they are equal to each other.

The correct ordering for cards is not obvious. For example, which is better, the 3 of Clubs or the 2 of Diamonds? One has a higher rank, but the other has a higher suit. In order to compare cards, you have to decide whether rank or suit is more important.

The answer might depend on what game you are playing, but to keep things simple, we’ll make the arbitrary choice that suit is more important, so all of the Spades outrank all of the Diamonds, and so on.

With that decided, we can write __cmp__:

# inside class Card:
def __cmp__(self, other):
# check the suits
if self.suit > other.suit: return 1
if self.suit < other.suit: return -1
# suits are the same... check ranks
if self.rank > other.rank: return 1
if self.rank < other.rank: return -1
# ranks are the same... it's a tie
return 0

You can write this more concisely using tuple comparison:

# inside class Card:
def __cmp__(self, other):
t1 = self.suit, self.rank
t2 = other.suit, other.rank
return cmp(t1, t2)

The built-in function cmp has the same interface as the method __cmp__: it takes two values and returns a positive number if the first is larger, a negative number if the second is larger, and 0 if they are equal.

In Python 3, cmp no longer exists, and the __cmp__ method is not supported. Instead you should provide __lt__, which returns True if self is less than other. You can implement __lt__ using tuples and the < operator.

Exercise 18.1. Write a__cmp__method for Time objects. Hint: you can use tuple comparison, but you also might consider using integer subtraction.