Object Oriented Programming in Python

1. Object Oriented Programming in Python Juan Manuel Gimeno Illa jmgimeno@diei.udl.cat Curs 2009-2010 J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 1 / 49

2. Outline 1 Introduction 2 Classes 3 Instances I 4 Descriptors Referencing Attributes Bound and Unbound Methods Properties Class-Level Methods 5 Inheritance Method Resolution Order Cooperative Superclasses 6 Instances II J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 2 / 49

3. Introduction Programming Paradigms A programming paradigm consists in the basic concepts into which our programs are made of Procedural Modules, data structures and procedures that operate upon them Objectural Objects which encapsulate state and behaviour and messages passed between these objects Functional Functions and closures, recursion, lists, ... Python is a multiparadigm programming language this allows the programmer to choose the paradigm that best suits the problem this allows the program to mix paradigms this allows the program to evolve switching paradigm if necessary J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 3 / 49

11. Classes Python classes A class is a python object with several characteristics: You can call a class as it where a function and this call returns a new instance of the class A class has arbitrary named attributes that can be bound, unbound an referenced The class attributes can be descriptors (including functions) or normal data objects Class attributes bound to functions are also known as methods A method can have special python-deﬁned meaning (they’re named with two leading and trailing underscores) A class clan inherit from other classes, meaning it delegates to other classes the look-up of attributes that are not found in the class itself J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 4 / 49

17. Classes Object models Since Python2.2 there co-exist two slightly diﬀerent object models in the language Old-style (classic) classes This is the model existing prior to Python2.2 New-style classes This is the preferred model for new code Old-style New-style >>> class A: pass >>> class A(object): pass >>> class B: pass >>> class B(object): pass >>> a, b = A(), B() >>> a, b = A(), B() >>> type(a) == type(b) >>> type(a) == type(b) True False >>> type(a) >>> type(a) <type ’instance’> <class ’ main .A’> J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 5 / 49

22. Classes New-style classes Defined in the type and class unification effort in python2.2 (Introduced without breaking backwards compatibility) Simpler, more regular and more powerful Built-in types (e.g. dict) can be subclassed Properties: attributes managed by get/set methods Static and class methods (via descriptor API) Cooperative classes (sane multiple inheritance) Meta-class programming It will be the default (and unique) in the future Documents: Unifying types and classes in Python 2.2 PEP-252: Making types look more like classes PEP-253: Subtyping built-in types J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 6 / 49

35. Classes The class statement class classname(base-classes): statement(s) classname is a variable that gets (re)bound to the class object after the class statement ﬁnishes executing base-classes is a comma separated series of expressions whose values must be classes if it does not exists, the created class is old-style if all base-classes are old-style, the created class is old-style otherwise it is a new-style class1 since every type subclasses built-in object, we can use object to mark a class as new-style when no true bases exist The statements (a.k.a. the class body) deﬁne the set of class attributes which will be shared by all instances of the class 1 We are not considering metaclass now J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 7 / 49

43. Classes Attributes of class objects Attributes can be bound inside or outside the class body. >>> class C1(object): >>> class C2(object): pass ... x = 23 >>> C2.x = 23 >>> print C1.x >>> print C2.x 23 23 Some attributes are implicitly set: >>> print C1. name , C1. bases C1, (<type ’object’>,) >>> C1. dict [’z’] = 42 >>> print C1.z 42 >>> print C1. dict [’x’] 23 J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 8 / 49

50. Classes Accessing class attributes In statements directly inside the class’ body: >>> class C3(object): ... x = 23 ... y = x + 19 In statements in methods of the class: >>> class C4(object): ... x = 23 ... def amethod(self): ... print C4.x In statements outside the class: >>> class C3(object): ... x = 23 >>> C3.x = 42 J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 9 / 49

59. Classes Class-private attributes When a statement in the body (or in a method in the body) uses an identiﬁer starting with two underscores (but not ending with them) such as private, the Python compiler changes it to classname private This lets classes to use private names reducing the risk of accidentally duplicating names used elsewhere By convention all identiﬁers starting with a single underscore are meant to be private in the scope that binds them >>> class C5(object): ... private = 23 >>> print C5.__private AttributeError: class A has no attribute ’ private’ >>> print C5. C5 private 23 J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 10 / 49

63. Classes Function definitions in a class body Most class bodies include def statements since functions (called methods in this context) are important attributes for most class objects A method defined in a class body has a mandatory first parameter (conventionally called self) that refers to the instance on which the method is called (staticmethods and classmethods are not considered now) A class can define a variety of special methods (two leading and two trailing underscores) relating to specific operation on its instances >>> class C5(object): ... """This is the docstring of the class. ... It can be accessed by C5. doc """ ... def hello(self): ... "And this the docstring of the method" ... print "Hello!!" J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 11 / 49

67. Instances I Creating Instances 1 To create an instance of a >>> anInstance = C5() class, call the class object as >>> isinstance(anInstance, C5) if it were a function True >>> class C6(object): If it deﬁnes or inherits ... def init (self, n): init , calling the class ... self.x = n object implicitly calls it to >>> anInstance = C6(42) perform any needed >>> print anInstance.x instance-speciﬁc 42 initialisation >>> anInstance.z = 8 You can give an instance an >>> print anInstance.z attribute by binding a value 8 to an attribute reference J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 12 / 49

80. Instances I Attributes of Instance Objects Attributes can be bound inside or outside class methods >>> class C1(object): >>> class C2(object): ... def amethod(self, n=8): ... pass ... self.n = n >>> d = C2() >>> c = C1() >>> d.n = 15 >>> c.amethod() >>> print d.n >>> print c.n 15 8 Some attributes are implicitly set (both can be rebound but not unbound): >>> print d. class <class ’ main .C2’> >>> d. dict [’z’] = 42 >>> print d.z 42 >>> print d. dict [’n’] 15 J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 13 / 49

87. Descriptors Descriptors A descriptor is any new-style object whose class supplies a special method named get Descriptors that are class attributes control the semantics of accessing and setting attributes on instances of that class If a descriptor’s class also supplies method set then it is called an overriding descriptor (a.k.a. data descriptor) If not, it is called non-overriding (a.k.a. non-data) descriptor Function objects (and methods) are non-overriding descriptors Descriptors are the mechanism behind properties, methods, static methods, class methods, and super (cooperative super-classes) The descriptor protocol also contains method delete for unbinding attributes but it is seldom used J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 14 / 49

94. Descriptors A Descriptor Example >>> class Area(object): ... """An overriding descriptor""" ... def get (self, obj, klass): ... return obj.x * obj.y ... def set (self, obj, value): ... raise AttributeError >>> class Rectangle(object): ... """A new-style class for representing rectangles""" ... area = Area() ... def init (self, x, y): ... self.x = x ... self.y = y >>> r = Rectangle(5, 10) >>> print r.area 50 J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 15 / 49

95. Descriptors Referencing Attributes Attribute Reference Basics An attribute reference is an expression of the form x.name, where x is an expression and name is an identifier Many kinds of Python objects have attributes, but an attribute reference when x refers to a class or an instance has special rich semantics The mechanics of attribute getting is defined in the special method getattribute The predefined behaviour is defined in the implementation of this method in the type (for class attributes) and object (for instance attributes) built-in types J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 16 / 49

99. Descriptors Referencing Attributes Getting an attribute from a class When you use the syntax C.name to refer to an attribute on a class object C, the look-up proceeds in two steps: 1 When ’name’ is a key in C. dict , C.name fetches the value v from C. dict [’name’]. If v is a descriptor (i.e. type(v) supplies a get method), then type(v). get (v,None,C) is returned Otherwise, ir returns v 2 Otherwise, it delegates the look-up to its base classes (in method resolution order) When these look-ups steps do not ﬁnd an attribute, Python raises an AttributeError exception. J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 17 / 49

106. Descriptors Referencing Attributes Getting an attribute from an instance I When you use the syntax x.name to refer to an attribute of instance x of class C, the look-up proceeds in three steps: 1 When ’name’ is found in C (or in one of C’s ancestor classes) as the name of an overriding descriptor v (i.e. type(v) supplies both get and set ), then the value of x.name is type(v). get (v,x,C) 2 Otherwise, when ’name’ is key in x. dict , x.name fetches and returns x. dict [’name’] 3 Otherwise, x.name delegates the look-up to x’s class (looking into C. dict or delegating to C’s bases) and if a descriptor v is found, the overall result is again type(v). get (v,x,C). if a nondescriptor value v is found, the result is v. J.M.Gimeno (jmgimeno@diei.udl.cat) OOP in Python Curs 2009-2010 18 / 49

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