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 programmer-defined types, we can override the behavior of the built-in operators by providing a method named
__lt__, which stands for “less than”.
__lt__ takes two parameters,
other, and returns
self is strictly less than
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
# inside class Card: def __lt__(self, other): # check the suits if self.suit < other.suit: return True if self.suit > other.suit: return False # suits are the same... check ranks return self.rank < other.rank
You can write this more concisely using tuple comparison:
# inside class Card: def __lt__(self, other): t1 = self.suit, self.rank t2 = other.suit, other.rank return t1 < t2
As an exercise, write an
__lt__ method for Time objects. You can use tuple comparison, but you also might consider comparing integers.