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Type-based dispatch

8 September, 2015 - 10:43

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.

If other is a Time object, __add__ invokes 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 + duration11:20:00>>> print start + 133710:07:17

Unfortunately, this implementation of addition is not commutative. If the integer is the first operand, you get

>>> print 1337 + startTypeError: 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 to do that. 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 + start10:07:17

Exercise 17.5. Write anaddmethod 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.