Object-oriented programming in Python (2)

Transcription

1 Programming Languages Week 9 Object-oriented programming in Python (2) College of Information Science and Engineering Ritsumeikan University

2 plan last week: dictionaries vs. objects classes, instances, constructors and methods this week: sharing behaviour and state: inheritance class vs. instance behaviour and state next week: meta-programming, metaclasses, method resolution inheritance vs. delegation prototype-based programming finally: Smalltalk (everything is an object) message-passing syntax, messages as objects closures as objects, control flow without primitives, user-defined control structures Metaclass hierarchies 2

3 sharing behaviour and state: inheritance class CartesianPoint(): def init (self, x, y): self._x = x; self._y = y def x(self): return self._x def y(self): return self._y def r(self): return sqrt(self._x ** 2 + self._y ** 2) def a(self): return degrees(atan2(self._y, self._x)) class PolarPoint(): def init (self, r, a): self._r = r; self._a = a def x(self): return self._r * cos(radians(self._a)) def y(self): return self._r * sin(radians(self._a)) def r(self): return self._r def a(self): return self._a point addition: implement it twice? once in CartesianPoint once in PolarPoint 3

4 sharing behaviour and state: inheritance both kinds of point are specialisations of a more general idea point which can implement all behaviour not specific to a representation class Point add (self, other): return self.getx() + other.getx(), self.gety() + other.gety()... representation-independent methods class CartesianPoint class PolarPoint _x 3 _r _y 4 _a 45 getx(self): return self._x getx(self): return self._r * cos(self._a)... representation-dependent methods... representation-dependent methods 4

5 sharing behaviour and state: inheritance class CartesianPoint(Point): # inherit methods from Point def getx(self): return self.x p = CartesianPoint(3, 4) subclass CartesianPoint inherits methods from superclass Point when calling a method of p look for the method in CartesianPoint, run it if found, otherwise... look for the method in Point, run it if found searches for methods invoked on self begin again in CartesianPoint class Point(): def add (self, other): return CartesianPoint(self.getX() + other.getx(), self.gety() + other.gety()) add? class CartesianPoint(Point): def getx(self): return self.x class PolarPoint(Point): def getx(self): return self.r * cos(radians(self.a)) acartesianpoint + apolarpoint 5

6 sharing behaviour and state: inheritance class Base(): # base class is another name for superclass def foo(self): print "Base.foo" def bar(self): print "Base.bar" def baz(self): print "Base.baz" def qux(self): print "Base.qux", class Derived(Base): # derived class is another name for subclass def foo(self): print "Derived.foo" # override base method def baz(self): print "Derived.baz", Base.baz(self) # call base method explicitly def qux(self): Base.qux(self) print "Derived.qux" # call base method explicitly Base().foo() Derived().foo() Derived().bar() Derived().baz() Derived().qux() #=> Base.foo #=> Derived.foo #=> Base.bar #=> Derived.baz Base.foo #=> Base.qux Derived.qux 6

7 sharing behaviour and state: inheritance methods defined inside a class are shared between all instances of that class class Obj(): def greet(self): print "hello" a, b = Obj(), Obj() a.greet() b.greet() #=> hello #=> hello class variables defined inside class are shared between all instances of that class class Obj(): data = "initial" def set(self, value): data = value def get(self): return data a, b = Obj(), Obj() a.get() b.get() b.set("final") a.get() b.get() #=> initial #=> initial #=> final #=> final 7

8 classes are objects too sharing behaviour and state: inheritance class objects can have their own methods written inside the class with a preceding their definition class Obj(): data = def get(cls): # cls == Obj within this method return data Obj.get() #=> initial class methods useful for initialising instances from different data class def withxy(cls, x, y): cls(x, def withra(cls, r, a): cls(r * cos(a), r * sin(a)) 8

9 class heirarchy class inheritance structure is a tree called the class hierarchy object Point CartesianPoint PolarPoint seemingly unrelated ideas can sometimes end up with a common superclass in Smalltalk, these four classes have a common superclass: Date, Time, Number, Character what is it? 9

10 methods are objects too methods as first-class values can be obtained like any other property of a class but object used to access them is bound permanently to the method returned the method obtained behaves like a function self is always the object used to obtain the function cannot be changed class Obj(): def init (self, x): self.x = x def get(self): return self.x a, b = Obj(3), Obj(4) print a.get(), b.get() #=> 3 4 aget, bget = a.get, b.get print aget(), bget() #=>

11 static methods you can, of course, continue to put normal functions in instance variables def hello(): print "hail!" class Obj(): def init (self): self.greet = hello obj = Obj() obj.greet() #=> hail! if every object needs a hello function, this wastes a lot of space static methods are just like normal functions except they are shared between all instances class def greet(): # note: no implicit self argument print "hail!" MyClass().greet() #=> hail! 11

12 exercises practice inheritance refactoring shared methods class methods shared variables 12

13 glossary base class another name for superclass. class method a method associated with, and invoked on, a class object. class variable a variable stored in a class object, shared between the class and all of its instances. derived class another name for subclass. generalisation a kind of object that has less specific state or behaviour than another. A superclass is a generalisation of its subclasses. inheritance methods (and/or state) from a superclass being made available in the instances of its subclasses. inheritance hierarchy the tree-structure created by the subclass/superclass relationships between classes. At the top of the inheritance hierarchy is object, the most general class of all from which all other classes inherit. instance method a method that can be invoked on an instance of a class. specialisation an extension of a class that adds additional state and/or behaviour. A sublass is a specialisation of its superclass. subclass a class that specialises another, and which inherits methods from it. superclass a class that is specialised by another, and whose methods are inherited by it. 13