Learning Python

Chapter 6

1. The basics. Here’s the solution we coded up for this exercise, along with some interactive tests. The __add__ overload has to appear only once, in the superclass. Notice that you get an error for expressions where a class instance appears on the right of a +; to fix this, use __radd__ methods also (an advanced topic we skipped; see other Python books and/or Python reference manuals for more details). You could also write the add method to take just two arguments, as shown in the chapter’s examples.

   % cat adder.py
   
   class Adder:
       def add(self, x, y):
           print 'not implemented!'
       def __init__(self, start=[]):
           self.data = start
       def __add__(self, other):
           return self.add(self.data, other)   # or in subclasses--return type?
   
   class ListAdder(Adder):
       def add(self, x, y):
           return x + y
   
   class DictAdder(Adder):
       def add(self, x, y):
           new = {}
           for k in x.keys(): new[k] = x[k]
           for k in y.keys(): new[k] = y[k]
           return new
   
   % python
   >>> from adder import *
   >>> x = Adder()
   >>> x.add(1, 2)
   not implemented!
   >>> x = ListAdder()
   >>> x.add([1], [2])
   [1, 2]
   >>> x = DictAdder()
   >>> x.add({1:1}, {2:2})
   {1: 1, 2: 2}
   
   >>> x = Adder([1])
   >>> x + [2]
   not implemented!
   >>>
   >>> x = ListAdder([1])
   >>> x + [2]
   [1, 2]
   >>> [2] + x
   Traceback (innermost last):
     File "<stdin>", line 1, in ?
   TypeError: __add__ nor __radd__ defined for these operands

2. Operator overloading. Here’s what we came up with for this one. It uses a few operator overload methods we didn’t say much about, but they should be straightforward to understand. ...