Date 05/03/2014 Series Part 1 of "OOP concepts in Python 2.x" Tags Python2 / Python / OOP

Abstract

Object-oriented programming (OOP) has been the leading programming paradigm for several decades now, starting from the initial attempts back in the 60s to some of the most important languages used nowadays. Being a set of programming concepts and design methodologies, OOP can never be said to be "correctly" or "fully" implemented by a language: indeed there are as many implementations as languages.

So one of the most interesting aspects of OOP languages is to understand how they implement those concepts. In this post I am going to try and start analyzing the OOP implementation of the Python language. Due to the richness of the topic, however, I consider this attempt just like a set of thoughts for Python beginners trying to find their way into this beautiful (and sometimes peculiar) language.

This first post covers the following topics:

Objects and types

Classes and instances

Object members: methods and attributes

Delegation: inheritance and composition

This post refers to the internals of Python 2.x - please note that Python 3.x changes (improves!) some of the features shown here. You can find the updated version here.

Back to the Object

Computer science deals with data and with procedures to manipulate that data. Everything, from the earliest Fortran programs to the latest mobile apps is about data and their manipulation.

So if data are the ingredients and procedures are the recipes, it seems (and can be) reasonable to keep them separate.

Let's do some procedural programming in Python

# This is some data data = ( 13 , 63 , 5 , 378 , 58 , 40 ) # This is a procedure that computes the average def avg ( d ): return sum ( d ) / len ( d ) print avg ( data )

As you can see the procedure is quite good and general: the procedure is run on a sequence of data, and it returns the average of the sequence items. So far, so good: computing the average of some numbers leaves the numbers untouched and creates new data.

The observation of the everyday world, however, shows that complex data mutate: an electrical device is on or off, a door is open or closed, the content of a bookshelf in your room changes as you buy new books.

You can still manage it keeping data and procedures separate, for example

# These are two numbered doors, initially closed door1 = [ 1 , 'closed' ] door2 = [ 2 , 'closed' ] # This procedure opens a door def open_door ( door ): door [ 1 ] = 'open' open_door ( door1 ) print door1

I described a door as a structure containing a number and the status of the door. The procedure knows how this structure is made and may alter it.

This also works like a charm. Some problems arise, however, when we start building specialized types of data. What happens, for example, when I introduce a "lockable door" data type, which can be opened only when it is not locked? Let's see

# These are two standard doors, initially closed door1 = [ 1 , 'closed' ] door2 = [ 2 , 'closed' ] # This is a lockable door, initially closed and unlocked ldoor1 = [ 1 , 'closed' , 'unlocked' ] # This procedure opens a standard door def open_door ( door ): door [ 1 ] = 'open' # This procedure opens a lockable door def open_ldoor ( door ): if door [ 2 ] == 'unlocked`: door [ 1 ] = `open` open_door ( door1 ) print door1 open_ldoor ( ldoor1 ) print ldoor1

Everything still works, no surprises in this code. However, as you can see, I had to find a different name for the procedure that opens a locked door since its implementation differs from the procedure that opens a standard door. But, wait... I'm still opening a door, the action is the same, and it just changes the status of the door itself. So why shall I remember that a locked door shall be opened with open_ldoor() instead of open_door() if the verb is the same?

Chances are that this separation between data and procedures doesn't perfectly fit some situations. The key problem is that the "open" action is not actually using the door; rather it is changing its state. So, just like the volume control buttons of your phone, which are on your phone, the "open" procedure should stick to the "door" data.

This is exactly what leads to the concept of object: an object, in the OOP context, is a structure holding data and procedures operating on them.

What About Type?

When you talk about data you immediately need to introduce the concept of type. This concept may have two meanings that are worth being mentioned in computer science: the behavioural and the structural one.

The behavioural meaning represents the fact that you know what something is by describing how it acts. This is the foundation of the so-called "duck typing" (here "typing" means "to give a type" and not "to type on a keyboard"): if it types acts like a duck, it is a duck.

The structural meaning identifies the type of something by looking at its internal structure. So two things that act in the same way but are internally different are of different type.

Both points of view can be valid, and different languages may implement and emphasize one meaning of type or the other, and even both.

Class Games

Objects in Python may be built describing their structure through a class. A class is the programming representation of a generic object, such as "a book", "a car", "a door": when I talk about "a door" everyone can understand what I'm saying, without the need of referring to a specific door in the room.

In Python, the type of an object is represented by the class used to build the object: that is, in Python the word type has the same meaning of the word class.

For example, one of the built-in classes of Python is int , which represents an integer number

>>> a = 6 >>> print a 6 >>> print type ( a ) <type 'int'> >>> print a . __class__ <type 'int'>

As you can see, the built-in function type() returns the content of the magic attribute __class__ (magic here means that its value is managed by Python itself offstage). The type of the variable a , or its class, is int . (This is a very inaccurate description of this rather complex topic, so remember that we are just scratching the surface).

Once you have a class you can instantiate it to get a concrete object (an instance) of that type, i.e. an object built according to the structure of that class. The Python syntax to instantiate a class is the same of a function call

>>> b = int () >>> type ( b ) <type 'int'>

When you create an instance, you can pass some values, according to the class definition, to initialize it.

>>> b = int () >>> print b 0 >>> c = int ( 7 ) >>> print c 7

In this example, the int class creates an integer with value 0 when called without arguments, otherwise it uses the given argument to initialize the newly created object.

Let us write a class that represents a door to match the procedural examples done in the first section

class Door ( object ): def __init__ ( self , number , status ): self . number = number self . status = status def open ( self ): self . status = 'open' def close ( self ): self . status = 'closed'

The class keyword defines a new class named Door ; everything indented under class is part of the class. The functions you write inside the object are called methods and don't differ at all from standard functions; the name changes only to highlight the fact that those functions now are part of an object.

Methods of a class must accept as first argument a special value called self (the name is a convention but please never break it).

The class can be given a special method called __init__() which is run when the class is instantiated, receiving the arguments passed when calling the class; the general name of such a method, in the OOP context, is constructor, even if the __init__() method is not the only part of this mechanism in Python.

The self.number and self.status variables are called attributes of the object. In Python, methods and attributes are both members of the object and are accessible with the dotted syntax; the difference between attributes and methods is that the latter can be called (in Python lingo you say that a method is a callable).

As you can see the __init__() method shall create and initialize the attributes since they are not declared elsewhere.

The class can be used to create a concrete object

>>> door1 = Door ( 1 , 'closed' ) >>> type ( door1 ) <class '__main__.Door'> >>> print door1 . number 1 >>> print door1 . status closed

Now door1 is an instance of the Door class; type() returns the class as __main__.Door since the class was defined directly in the interactive shell, that is in the current main module. To call a method of an object, that is to run one of its internal functions, you just access it as an attribute with the dotted syntax and call it like a standard function.

>>> door1 . open () >>> print door1 . number 1 >>> print door1 . status open

In this case, the open() method of the door1 instance has been called. No arguments have been passed to the open() method, but if you review the class declaration, you see that it was declared to accept an argument ( self ). When you call a method of an instance, Python automatically passes the instance itself to the method as the first argument.

You can create as many instances as needed and they are completely unrelated each other. That is the changes you make on one instance do not reflect on another instance of the same class.

Recap

Objects are described by a class, which can generate one or more instances, unrelated each other. A class contains methods, which are functions, and they accept at least one argument called self , which is the actual instance on which the method has been called. A special method, __init__() deals with the initialization of the object, setting the initial value of the attributes.

Python Classes Strike Again

The Python implementation of classes has some peculiarities. The bare truth is that in Python the class of an object is an object itself. You can check this by issuing type() on the class

>>> type ( door1 ) <class '__main__.Door'> >>> print type ( Door ) <type 'type'>

This shows that the Door class is an object, an instance of the type class.

This concept is not so difficult to grasp as it can seem at first sight: in the real world we deal with concepts using them like things: for example we can talk about the concept of "door", telling people how a door looks like and how it works. So in our everyday experience the type of an object is an object itself. In Python this can be expressed by saying that everything is an object.

If the class is an instance it is a concrete object and is stored somewhere in memory. Let us leverage the inspection capabilities of Python and its id() function to check the status of our objects. The id() built-in function returns the memory position of an object.

First of all, let's check that the two objects are stored at different addresses

>>> hex ( id ( door1 )) '0x7fa4c818bad0' >>> hex ( id ( door2 )) '0x7fa4c818b890'

This confirms that the two instances are separate and unrelated. Please note that your values are very likely to be different from the ones I got.

However if we use id() on the class of the two instances we discover that the class is exactly the same

>>> hex ( id ( door1 . __class__ )) '0x766800' >>> hex ( id ( door2 . __class__ )) '0x766800'

Well this is very important. In Python, a class is not just the schema used to build an object. Rather, the class is a shared living object, which code is accessed at run time.

As we already tested, however, attributes are not stored in the class but in every instance, due to the fact that __init__() works on self when creating them. Classes, however, can be given attributes like any other object; with a terrific effort of imagination, let's call them class attributes.

As you can expect, class attributes are shared among the class instances just like their container

class Door ( object ): colour = 'brown' def __init__ ( self , number , status ): self . number = number self . status = status def open ( self ): self . status = 'open' def close ( self ): self . status = 'closed'

The colour attribute here is not created using self , and any change of its value reflects on all instances

>>> door1 = Door ( 1 , 'closed' ) >>> door2 = Door ( 2 , 'closed' ) >>> Door . colour 'brown' >>> door1 . colour 'brown' >>> door2 . colour 'brown' >>> Door . colour = 'white' >>> Door . colour 'white' >>> door1 . colour 'white' >>> door2 . colour 'white' >>> hex ( id ( Door . colour )) '0xb74c5420L' >>> hex ( id ( door1 . colour )) '0xb74c5420L' >>> hex ( id ( door2 . colour )) '0xb74c5420L'

Raiders of the Lost Attribute

Any Python object is automatically given a __dict__ attribute, which contains its list of attributes. Let's investigate what this dictionary contains for our example objects:

>>> Door . __dict__ dict_proxy({'__module__': '__main__', 'colour': 'brown', '__weakref__': <attribute '__weakref__' of 'Door' objects>, '__dict__': <attribute '__dict__' of 'Door' objects>, 'close': <function close at 0xb6a8a56c>, 'open': <function open at 0xb6a8a534>, '__doc__': None, '__init__': <function __init__ at 0xb6a8a48c>}) >>> door1 . __dict__ {'status': 'closed', 'number': 1}

Leaving aside the difference between a dictionary and a dict_proxy object, you can see that the colour attribute is listed among the Door class attributes, while status and number are listed for the instance.

How comes that we can call door1.colour , if that attribute is not listed for that instance? This is a job performed by the magic __getattribute__() method; in Python the dotted syntax automatically invokes this method so when we write door1.colour , Python executes door1.__getattribute__('colour') . That method performs the attribute lookup action, i.e. finds the value of the attribute by looking in different places.

The standard implementation of __getattribute__() searches first the internal dictionary ( __dict__ ) of an object, then the type of the object itself; in this case door1.__getattribute__('colour') executes first door1.__dict__['colour'] and then door1.__class__.__dict__['colour']

>>> door1 . __dict__ [ 'colour' ] Traceback (most recent call last): File "<stdin>", line 1, in <module> KeyError: 'colour' >>> door1 . __class__ . __dict__ [ 'colour' ] 'brown'

Indeed, if we check the objects equality through the is operator we can confirm that both door1.colour and Door.colour are exactly the same object

>>> door1 . colour is Door . colour True

When we try to assign a value to a class attribute directly on an instance, we just put in the __dict__ of the instance a value with that name, and this value masks the class attribute since it is found first by __getattribute__() . As you can see from the examples of the previous section, this is different from changing the value of the attribute on the class itself.

>>> door1 . colour = 'white' >>> door1 . __dict__ [ 'colour' ] 'white' >>> door1 . __class__ . __dict__ [ 'colour' ] 'brown' >>> Door . colour = 'red' >>> door1 . __dict__ [ 'colour' ] 'white' >>> door1 . __class__ . __dict__ [ 'colour' ] 'red'

Revenge of the Methods

Let's play the same game with methods. First of all you can see that, just like class attributes, methods are listed only in the class __dict__ . Chances are that they behave the same as attributes when we get them

>>> door1 . open is Door . open False

Whoops. Let us further investigate the matter

>>> Door . __dict__ [ 'open' ] <function open at 0xb73ee10c> >>> Door . open <unbound method Door.open> >>> door1 . open <bound method Door.open of <__main__.Door object at 0xb73f956c>>

So, the class method is listed in the members dictionary as function. So far, so good. The same method, taken directly from the class is returned as unbound method, while taking it from the instance it changes to bound method. Well, a function is a procedure you named and defined with the def statement. When you refer to a function as part of a class you get an unbound method. The name method simply means "a function inside a class", according to the usual OOP definitions, while unbound signals that the method is not bound to any instance. As you can see, as soon as you access the method from an instance, the method becomes bound.

Why does Python bother with methods being bound or unbound? And how does Python transform an unbound method into a bound one?

First of all, if you try to call an unbound method you get an error

>>> Door . open () Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unbound method open() must be called with Door instance as first argument (got nothing instead)

so by default Python considers methods as functions that shall operate on instances, and calling them from the class leaves the interpreter puzzled. Let us try to pass the instance as suggested by the exception message

>>> Door . open ( door1 ) >>> door1 . status 'open'

Python does not complain here, and the method works as expected. So Door.open(door1) is the same as door1.open() . Again, under the hood, __getattribute__() is working to make everything work and when we call door1.open() , Python actually calls door1.__class__.open(door1) . However, door1.__class__.open is an unbound method, so there is something more that converts it into a bound method that Python can safely call.

When you access a member of an object, Python calls __getattribute__() to satisfy the request. This magic method, however, conforms to a procedure known as descriptor protocol. For the read access __getattribute__() checks if the object has a __get__() method and calls this latter. So for function the conversion from an unbound into a bound method is made by such a mechanism. Let us review it by means of an example.

>>> door1 . __class__ . __dict__ [ 'open' ] <function open at 0xb73ee10c>

This syntax retrieves the function defined in the class; the function knows nothing about objects, but it is an object (remember "everything is an object"). So we can look inside it with the dir() built-in function

>>> dir ( door1 . __class__ . __dict__ [ 'open' ]) ['__call__', '__class__', '__closure__', '__code__', '__defaults__', '__delattr__', '__dict__', '__doc__', '__format__', '__get__', '__getattribute__', '__globals__', '__hash__', '__init__', '__module__', '__name__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'func_closure', 'func_code', 'func_defaults', 'func_dict', 'func_doc', 'func_globals', 'func_name'] >>> door1 . __class__ . __dict__ [ 'open' ] . __get__ <method-wrapper '__get__' of function object at 0xb73ee10c>

As you can see, a __get__ method is listed among the members of the function, and Python recognizes it as a method-wrapper. This method shall connect the open function to the door1 instance, so we can call it passing the instance alone

>>> door1 . __class__ . __dict__ [ 'open' ] . __get__ ( door1 ) <bound method ?.open of <__main__.Door object at 0xb73f956c>>

Ok, something is missing. Indeed __get__() in this case also accepts the owner class, i.e. the class we are trying to get the attribute from. So the correct form is

>>> door1 . __class__ . __dict__ [ 'open' ] . __get__ ( door1 , Door ) <bound method Door.open of <__main__.Door object at 0xb73f956c>>

When methods met classes

If we use type() on an unbound method, we get an interesting result

>>> door1 . __class__ . __dict__ [ 'open' ] <function open at 0xb6aa548c> >>> type ( door1 . __class__ . open ) <type 'instancemethod'>

The method is an instance method; as we discovered, it shall be bound to an instance to work, so the name is a good choice. Does this imply that we can also define a class method? Indeed we can, through the classmethod decorator

class Door ( object ): colour = 'brown' def __init__ ( self , number , status ): self . number = number self . status = status @classmethod def knock ( cls ): print "Knock!" def open ( self ): self . status = 'open' def close ( self ): self . status = 'closed'

Such a definition makes the method callable on both the instance and the class

>>> door1 . knock () Knock! >>> Door . knock () Knock! >>> door1 . knock <bound method type.knock of <class '__main__.Door'>> >>> Door . knock <bound method type.knock of <class '__main__.Door'>>

And this is possible since, as you can see, both the class method and the instance method are bound. The class method is bound to the class itself, while the instance method is bound to the instance of the class. What about the type of the method?

>>> door1 . __class__ . __dict__ [ 'knock' ] <classmethod object at 0xb6a8db6c> >>> type ( door1 . __class__ . knock ) <type 'instancemethod'>

Puzzled? Don't be confused! When you look in the __dict__ you are not going through the __getattribute__() and __get__() machinery, so you get the plain unprocessed attribute. With standard methods you find function objects in the members dictionary, while for class methods you find classmethod objects.

On the other side, when you check the type of door1.__class__.knock you implicitly invoke __get__() , which binds the method to the class.

>>> type ( door1 . __class__ . __dict__ [ 'knock' ] . __get__ ( door1 , Door )) <type 'instancemethod'> >>> type ( door1 . __class__ . __dict__ [ 'knock' ]) <type 'classmethod'>

After all, it is no surprise that a class method is bound since a class is an instance of type .

Intermezzo

I realize now that I introduced this post as "a set of thoughts for Python beginners". Sorry. If you are reading this, however, chances are that you are still alive. Calm down. Breathe. Relaxed? Ready to go? Let's dive into delegation!

The Delegation Run

If classes are objects what is the difference between types and instances? When I talk about "my cat" I am referring to a concrete instance of the "cat" concept, which is a subtype of "animal". So, despite being both objects, while types can be specialized, instances cannot.

Usually an object B is said to be a specialization of an object A when:

B has all the features of A

B can provide new features

B can perform some or all the tasks performed by A in a different way

Those targets are very general and valid for any system and the key to achieve them with the maximum reuse of already existing components is delegation. Delegation means that an object shall perform only what it knows best, and leave the rest to other objects.

Delegation can be implemented with two different mechanisms: composition and inheritance. Sadly, very often inheritance is listed among the pillars of OOP techniques, forgetting that it is an implementation of the more generic and fundamental mechanism of delegation; perhaps a better nomenclature for the two techniques could be explicit delegation (composition) and implicit delegation (inheritance). Please note that, again, when talking about composition and inheritance we are talking about focusing on a behavioural or structural delegation. Another way to think about the difference between composition and inheritance is to consider if the object knows who can satisfy your request or if the object is the one that satisfy the request.

Please, please, please do not forget composition: in many cases, composition can lead you to a simpler system, with benefits on maintainability and changeability.

Usually composition is said to be a very generic technique that needs no special syntax, while inheritance and its rules are strongly dependent on the language of choice. Actually, the strong dynamic nature of Python softens the boundary line between the two techniques.

Inheritance Now

In Python a class can be declared as an extension of one or more different classes, through the class inheritance mechanism. The child class (the one that inherits) internally has the same structure of the parent class (the one that is inherited), and for the case of multiple inheritance the language has very specific rules to manage possible conflicts or redefinitions among the parent classes. A very simple example of inheritance is

class SecurityDoor ( Door ): pass

where we declare a new class SecurityDoor that, at the moment, is a perfect copy of the Door class. Let us investigate what happens when we access attributes and methods. First we instance the class

>>> sdoor = SecurityDoor ( 1 , 'closed' )

The first check we can do is that class attributes are still global and shared

>>> SecurityDoor . colour is Door . colour True >>> sdoor . colour is Door . colour True

This shows us that Python tries to resolve instance members not only looking into the class the instance comes from, but also investigating the parent classes. In this case sdoor.colour becomes SecurityDoor.colour , that in turn becomes Door.colour . SecurityDoor is a Door .

If we investigate the content of __dict__ we can catch a glimpse of the inheritance mechanism in action

>>> sdoor . __dict__ {'status': 'closed', 'number': 1} >>> sdoor . __class__ . __dict__ dict_proxy({'__module__': '__main__', '__doc__': None}) >>> Door . __dict__ dict_proxy({'knock': <classmethod object at 0xb6a8db6c>, '__module__': '__main__', '__weakref__': <attribute '__weakref__' of 'Door' objects>, '__dict__': <attribute '__dict__' of 'Door' objects>, 'close': <function close at 0xb6aa5454>, 'colour': 'brown', 'open': <function open at 0xb6aa53e4>, '__doc__': None, '__init__': <function __init__ at 0xb6aa51ec>})

As you can see the content of __dict__ for SecurityDoor is very narrow compared to that of Door . The inheritance mechanism takes care of the missing elements by climbing up the classes tree. Where does Python get the parent classes? A class always contains a __bases__ tuple that lists them

>>> SecurityDoor . __bases__ (<class '__main__.Door'>,)

So an example of what Python does to resolve a class method call through the inheritance tree is

>>> sdoor . __class__ . __bases__ [ 0 ] . __dict__ [ 'knock' ] . __get__ ( sdoor , SecurityDoor ) <bound method type.knock of <class '__main__.SecurityDoor'>> >>> sdoor . knock <bound method type.knock of <class '__main__.SecurityDoor'>>

Please note that this is just an example that does not consider multiple inheritance.

Let us try now to override some methods and attributes. In Python you can override (redefine) a parent class member simply by redefining it in the child class.

class SecurityDoor ( Door ): colour = 'gray' locked = True def open ( self ): if not self . locked : self . status = 'open'

As you can forecast, the overridden members now are present in the __dict__ of the SecurityDoor class

>>> SecurityDoor . __dict__ dict_proxy({'locked': True, '__module__': '__main__', 'open': <function open at 0xb73d8844>, 'colour': 'gray', '__doc__': None})

So when you override a member, the one you put in the child class is used instead of the one in the parent class simply because the former is found before the latter while climbing the class hierarchy. This also shows you that Python does not implicitly call the parent implementation when you override a method. So, overriding is a way to block implicit delegation.

If we want to call the parent implementation we have to do it explicitly. In the former example we could write

class SecurityDoor ( Door ): colour = 'gray' locked = True def open ( self ): if self . locked : return Door . open ( self )

You can easily test that this implementation is working correctly. This form of explicit parent delegation is heavily discouraged, however.

The first reason is because of the very high coupling that results from explicitly naming the parent class again when calling the method; if you decide to use a new parent class you have to manually propagate the change to every method that calls it. Moreover, since in Python the class hierarchy can be dynamically changed (i.e. at runtime), this form of explicit delegation could be not only annoying but wrong.

The second reason is that in general you need to deal with multiple inheritance, where you do not know a priori which parent class implements the original form of the method you are overriding.

To solve these issues, Python supplies the super() built-in function, that climbs the class hierarchy and returns the correct class that shall be called. The syntax for calling super() is

class SecurityDoor ( Door ): colour = 'gray' locked = True def open ( self ): if self . locked : return super ( SecurityDoor , self ) . open ( self )

As you can see you have to explicitly pass the class you are in and the current instance. This is a (indeed very light) form of repetition that has been fixed in Python 3.x.

Enter the Composition

Composition means that an object knows another object, and explicitly delegates some tasks to it. While inheritance is implicit, composition is explicit: in Python, however, things are far more interesting than this =).

First of all let us implement classic composition, which simply makes an object part of the other as an attribute

class SecurityDoor ( object ): colour = 'gray' locked = True def __init__ ( self , number , status ): self . door = Door ( number , status ) def open ( self ): if self . locked : return self . door . open () def close ( self ): self . door . close ()

The primary goal of composition is to relax the coupling between objects. This little example shows that now SecurityDoor is an object and no more a Door , which means that the internal structure of Door is not copied. For this very simple example both Door and SecurityDoor are not big classes, but in a real system objects can very complex; this means that their allocation consumes a lot of memory and if a system contains thousands or millions of objects that could be an issue.

As you can see, however, our solution is far from perfect.

The composed SecurityDoor has to redefine the colour attribute since the concept of delegation applies only to methods and not to attributes, doesn't it? Well, no. Python provides a very high degree of indirection for objects manipulation and attribute access is one of the most useful. As you already discovered, accessing attributes is ruled by a special method called __getattribute__() that is called whenever an attribute of the object is accessed. Overriding __getattribute__() , however, is overkill; it is a very complex method, and, being called on every attribute access, any change makes the whole thing slower.

The method we have to leverage to delegate attribute access is __getattr__() , which is a special method that is called whenever the requested attribute is not found in the object. So basically it is the right place to dispatch all attribute and method access our object cannot handle. In the previous example

class SecurityDoor ( object ): locked = True def __init__ ( self , number , status ): self . door = Door ( number , status ) def open ( self ): if self . locked : return self . door . open () def __getattr__ ( self , attr ): return getattr ( self . door , attr )

Using __getattr__() blends the separation line between inheritance and composition since after all the former is a form of automatic delegation of every member access.

class ComposedDoor ( object ): def __init__ ( self , number , status ): self . door = Door ( number , status ) def __getattr__ ( self , attr ): return getattr ( self . door , attr )

As this last example shows, delegating every member access through __getattr__() is very simple. Pay attention to getattr() which is different from __getattr__() . The former is a built-in that is equivalent to the dotted syntax, i.e. getattr(obj, 'someattr') is the same as obj.someattr , but you have to use it since the name of the attribute is contained in a string.

Composition provides a superior way to manage delegation since it can selectively delegate the access, even mask some attributes or methods, while inheritance cannot. In Python you also avoid the memory problems that might arise when you put many objects inside another; Python handles everything through its reference, i.e. through a pointer to the memory position of the thing, so the size of an attribute is constant and very limited.

Final words

There is much more that can be said about the Python implementation of objects, classes, and friends. Just to name a couple of things, the object creation mechanism and the relationship between object and type have not been explained. Hopefully, there will be space in a later post. In the second post, however, I'll start talking about polymorphism, Abstract Base Classes, and metaclasses. Stay tuned!

Movie Trivia

If you are wondering why section titles are so weird, chances are that you missed some good movies to watch while you are not coding =). The movies are: Back to the Future, What About Bob?, Wargames, The Empire Strikes Back, Raiders of the Lost Ark, Revenge of the Nerds, When Harry Met Sally, The Cannonball Run, Apocalypse Now, Enter the Dragon.

Sources

Some sources for the content of this post. Thank you authors!

Updates

2014-03-08: "When methods met classes" section had a typo: the sentence "The class method is bound to the class itself, while the instance method is bound to the class of the instance." is "while the instance method is bound to instance of the class.". Thanks Mohcin Shah for spotting it!

2014-03-10: Fixed link to Alex Martelli's presentation.

2014-04-07: A typo when discussing door1.colour has been fixed. Thanks to pujuma.

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