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