17.8: Type-based dispatch
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In the previous section we added two Time objects, but you also might want to add an integer to a Time object. The following is a version of
__add__ that checks the type of
other and invokes either
add_time or increment:
# inside class Time: def __add__(self, other): if isinstance(other, Time): return self.add_time(other) else: return self.increment(other) def add_time(self, other): seconds = self.time_to_int() + other.time_to_int() return int_to_time(seconds) def increment(self, seconds): seconds += self.time_to_int() return int_to_time(seconds)
The built-in function
isinstance takes a value and a class object, and returns
True if the value is an instance of the class.
other is a Time object,
add_time. Otherwise it assumes that the parameter is a number and invokes
increment. This operation is called a type-based dispatch because it dispatches the computation to different methods based on the type of the arguments.
Here are examples that use the + operator with different types:
>>> start = Time(9, 45) >>> duration = Time(1, 35) >>> print(start + duration) 11:20:00 >>> print(start + 1337) 10:07:17
Unfortunately, this implementation of addition is not commutative. If the integer is the first operand, you get
>>> print(1337 + start) TypeError: unsupported operand type(s) for +: 'int' and 'instance'
The problem is, instead of asking the Time object to add an integer, Python is asking an integer to add a Time object, and it doesn’t know how. But there is a clever solution for this problem: the special method
__radd__, which stands for “right-side add”. This method is invoked when a Time object appears on the right side of the + operator. Here’s the definition:
# inside class Time: def __radd__(self, other): return self.__add__(other)
And here’s how it’s used:
>>> print(1337 + start) 10:07:17
As an exercise, write an
add method for Points that works with either a Point object or a tuple:
- If the second operand is a Point, the method should return a new Point whose x coordinate is the sum of the x coordinates of the operands, and likewise for the y coordinates.
- If the second operand is a tuple, the method should add the first element of the tuple to the x coordinate and the second element to the y coordinate, and return a new Point with the result.