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C.6 Chapter 6

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

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. Copying the initial value in the constructor is important, because it may be mutable; you don't want to change or have a reference to an object that's possibly shared somewhere outside the class. The routes method __getattr__ calls to the wrapped list:

% cat mylist.py

class MyList:
    def __init__(self, start):
        #self.wrapped = start[:]               # copy start: no side effects
        self.wrapped = []                      # make sure it's a list here
        for x in start: self.wrapped.append(x)
    def __add__(self, other):
        return MyList(self.wrapped + other)
    def __mul__(self, time):
        return MyList(self.wrapped * time)
    def __getitem__(self, offset):
        return self.wrapped[offset]
    def __len__(self):
        return len(self.wrapped)
    def __getslice__(self, low, high):
        return MyList(self.wrapped[low:high])
    def append(self, node):
        self.wrapped.append(node)
    def __getattr__(self, name):           # other members--sort/reverse/etc.
        return getattr(self.wrapped, name)
    def __repr__(self):
        return `self.wrapped`

if __name__ == '__main__':
    x = MyList('spam')
    print x
    print x[2]
    print x[1:]
    print x + ['eggs']
    print x * 3
    x.append('a')
    x.sort()
    for c in x: print c,

% python mylist.py
['s', 'p', 'a', 'm']
a
['p', 'a', 'm']
['s', 'p', 'a', 'm', 'eggs']
['s', 'p', 'a', 'm', 's', 'p', 'a', 'm', 's', 'p', 'a', 'm']
a a m p s

Subclassing. Our solution appears below. Your solution should appear similar.

% cat mysub.py 

from mylist import MyList

class MyListSub(MyList):
    calls = 0                                     # shared by instances

    def __init__(self, start):
        self.adds = 0                             # varies in each instance
        MyList.__init__(self, start)

    def __add__(self, other):
        MyListSub.calls = MyListSub.calls + 1     # class-wide counter
        self.adds = self.adds + 1                 # per instance counts
        return MyList.__add__(self, other)

    def stats(self):
        return self.calls, self.adds                  # all adds, my adds

if __name__ == '__main__':
    x = MyListSub('spam')
    y = MyListSub('foo')
    print x[2]
    print x[1:]
    print x + ['eggs']
    print x + ['toast']
    print y + ['bar']
    print x.stats()

% python mysub.py
a
['p', 'a', 'm']
['s', 'p', 'a', 'm', 'eggs']
['s', 'p', 'a', 'm', 'toast']
['f', 'o', 'o', 'bar']
(3, 2)

Metaclass methods. We worked through this exercise as follows. Notice that operators try to fetch attributes through _ _getattr__ too; you need to return a value to make them work.

>>> class Meta:
...     def __getattr__(self, name):        print 'get', name
...     def __setattr__(self, name, value): print 'set', name, value
...
>>> x = Meta()
>>> x.append
get append
>>> x.spam = "pork"
set spam pork
>>>
>>> x + 2
get __coerce__
Traceback (innermost last):
  File "<stdin>", line 1, in ?
TypeError: call of non-function
>>>
>>> x[1]
get __getitem__
Traceback (innermost last):
  File "<stdin>", line 1, in ?
TypeError: call of non-function

>>> x[1:5]
get __len__
Traceback (innermost last):
  File "<stdin>", line 1, in ?
TypeError: call of non-function

Set objects. Here's the sort of interaction you should get; comments explain which methods are called.

% python
>>> from set import Set
>>> x = Set([1,2,3,4])          # runs __init__
>>> y = Set([3,4,5])

>>> x & y                       # __and__, intersect, then __repr__
Set:[3, 4]
>>> x | y                       # __or__, union, then __repr__
Set:[1, 2, 3, 4, 5]

>>> z = Set("hello")            # __init__ removes duplicates
>>> z[0], z[-1]                 # __getitem__ 
('h', 'o')

>>> for c in z: print c,        # __getitem__ 
...
h e l o
>>> len(z), z                   # __len__, __repr__
(4, Set:['h', 'e', 'l', 'o'])

>>> z & "mello", z | "mello"
(Set:['e', 'l', 'o'], Set:['h', 'e', 'l', 'o', 'm'])

Our solution to the multiple-operand extension subclass looks like the class below. It needs only to replace two methods in the original set. The class's documentation string explains how it works:

from set import Set

class MultiSet(Set):
    """
    inherits all Set names, but extends intersect
    and union to support multiple operands; note
    that "self" is still the first argument (stored
    in the *args argument now); also note that the
    inherited & and | operators call the new methods
    here with 2 arguments, but processing more than 
    2 requires a method call, not an expression:
    """

    def intersect(self, *others):
        res = []
        for x in self:                     # scan first sequence
            for other in others:           # for all other args
                if x not in other: break   # item in each one?
            else:                          # no:  break out of loop
                res.append(x)              # yes: add item to end
        return Set(res)

    def union(*args):                      # self is args[0]
        res = []
        for seq in args:                   # for all args
            for x in seq:                  # for all nodes
                if not x in res:
                    res.append(x)          # add new items to result
        return Set(res)

Assuming the new set is stored in a module called multiset.py, your interaction with the extension will be something along these lines; note that you can intersect by using & or calling intersect, but must call intersect for three or more operands; & is a binary (two-sided) operator:

>>> from multiset import *
>>> x = MultiSet([1,2,3,4])
>>> y = MultiSet([3,4,5])
>>> z = MultiSet([0,1,2])

>>> x & y, x | y                               # 2 operands
(Set:[3, 4], Set:[1, 2, 3, 4, 5])

>>> x.intersect(y, z)                          # 3 operands
Set:[]
>>> x.union(y, z)
Set:[1, 2, 3, 4, 5, 0]

>>> x.intersect([1,2,3], [2,3,4], [1,2,3])     # 4 operands 
Set:[2, 3]
>>> x.union(range(10))                         # non-MultiSets work too
Set:[1, 2, 3, 4, 0, 5, 6, 7, 8, 9]

Class tree links. Here's the way we extended the Lister class and a rerun of the test to show its format. To display class attributes too, you'd need to do something like what the attrnames method currently does, but recursively, at each class reached by climbing __bases__ links.

class Lister:
    def __repr__(self):
        return ("<Instance of %s(%s), address %s:\n%s>" %
                          (self.__class__.__name__,      # my class's name
                           self.supers(),                # my class's supers
                           id(self),                     # my address
                           self.attrnames()) )           # name=value list
    def attrnames(self):
            Unchanged...

def supers(self):
        result = ""
        first = 1
        for super in self.__class__.__bases__:      # one level up from class
            if not first:
                result = result + ", "
            first = 0
            result = result + super.__name__
        return result

C:\python\examples> python testmixin.py
<Instance of Sub(Super, Lister), address 7841200:
        name data3=42
        name data2=eggs
        name data1=spam
>

Composition. Our solution is below, with comments from the description mixed in with the code. This is one case where it's probably easier to express a problem in Python than it is in English:

class Lunch:
    def __init__(self):
        # make/embed Customer and Employee
        self.cust = Customer()
        self.empl = Employee()
    def order(self, foodName):          
        # start a Customer order simulation
        self.cust.placeOrder(foodName, self.empl)
    def result(self):
        # ask the Customer what kind of Food it has
        self.cust.printFood()

class Customer:
    def __init__(self):                        
        # initialize my food to None
        self.food = None
    def placeOrder(self, foodName, employee):
        # place order with an Employee
        self.food = employee.takeOrder(foodName)
    def printFood(self):                       
        # print the name of my food
        print self.food.name

class Employee:
    def takeOrder(self, foodName):    
        # return a Food, with requested name
        return Food(foodName)

class Food:
    def __init__(self, name):         
        # store food name
        self.name = name

if __name__ == '__main__':
    x = Lunch()
    x.order('burritos')
    x.result()
    x.order('pizza')
    x.result()

% python lunch.py
burritos
pizza

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