After realising you can't point the gun at the foot, you pass a reference to the gun to a foot object. After the foot is blown up, the gun object remains alive for eternity, ready to shoot all future feet that may happen to appear. Java: You find that Microsoft and Sun have released incompatible class libraries both implementing Gun objects. You then find that although there are plenty of feet objects implemented in the past in many other languages, you cannot get access to one. But seeing as Java is so cool, you don't care and go around shooting anything else you can find. So why not attend one of the live in Paris, London, Berlin, Munich or Lake Constance by Bernd Klein, the author of this tutorial. Onsite Training Courses Let us come to your company or institute and train your employees, as we've done it many times in Amsterdam The NetherlandsBerlin GermanyBern SwitzerlandBasel SwitzerlandZurich SwitzerlandLocarno SwitzerlandDen Haag The HagueHamburg GermanyFrankfurt GermanyToronto CanadaEdmonton CanadaMunich Germany and many other cities. This way you will get a perfect training up to your needs and it will be extremely cost efficient as well. Skilled Python Programmers You can hire skilled Python programmers or even a skilled team of Python developers to work exclusively on your project. Quote of the Day: Man is the best computer we can put aboard a spacecraft. In this chapter, we will catch up on what has been missing so far. The first programming language to use objects was Simula 67. As the name implies, Simula 67 was introduced in the year 1967. A major breakthrough for object-oriented programming came with the programming language Smalltalk in the 1970s. For this purpose, we would like to draw your attention to a public library. If it helps, you can imagine the libraries in Paris, Berlin, Ottawa or Toronto 1 as well. Each of these contain an organized collection of books, periodicals, newspapers, audiobooks, films and so on. Generally, there are two opposed ways of keeping the stock in a library. In this system, trained staff brings the books and other publications to the users on demand. In this case the books are openly displayed. Imperative languages like C could be seen as open-access shelving libraries. The user can do everything. It's up to the user to find the books and to put them back at the right shelf. Even though this is great for the user, it might lead to serious problems in the long run. For example some books will be misplaced, so it's hard to find them again. The analogy can be seen like this: The books and other publications, which a library offers, are like the data in an object-oriented program. Getting or returning a book is only possible via the staff. So a library can be regarded as a class and a book is an instance or an object of this class. Generally speaking, an object is defined by a class. Python 3 object oriented programming pdf class is a formal description of how an object is designed, i. These objects are called instances as well. The expressions are in most cases used synonymously. A class should not be confused with an object. In fact, everything is a class in Python. That is, they can be assigned to variables, placed in lists, stored in dictionaries, passed as arguments, and so forth. Each of these are instances of their corresponding classes. We will not explain how Python has implemented lists internally. We don't need this information, because the list class provides us with all the necessary methods to access the data indirectly. This means that the encapsulation details are encapsulated. We will learn about encapsulation later. A Minimal Class in Python We will design and use a robot class in Python as an example to demonstrate the most important terms and ideas of object orientation. We will start with the simplest class in Python. The header usually consists of just python 3 object oriented programming pdf line of code. The class name is followed by a listing of other class names, which are classes from which the defined class inherits from. These classes are called superclasses, base classes or sometimes parent classes. If you look at our example, you will see that this listing of superclasses is not obligatory. You don't have to bother about inheritance and superclasses for the time being. We will introduce them later. The body of a class consists of an indented block of statements. A class object is created, when the definition is left normally, i. This is basically a wrapper around the contents of the namespace created by the class definition. It's hard to believe, especially for C++ or Python 3 object oriented programming pdf programmers, but we have already defined a complete class with just three words and two lines of code. Besides this, we have created a reference y2 to y, i. The output of this example program looks like this: True False Attributes Those who have learned already another object-oriented language, will have realized that the terms attributes and properties are usually used synonymously. It may also refer to or set the specific value for a given instance of such. So, think a while about your outstanding attributes. Great, if one of your strong points is your ability to quickly understand and adapt to new situations. Otherwise, you would not learn Python. Let's get back to Python: We will learn later that properties and attributes are essentially different things in Python. This subsection of our tutorial is about attributes in Python. So far our robots have no attributes. Not even a name, like it is customary for ordinary robots, isn't it. So, let's implement a name attribute. We can dynamically create arbitrary new attributes for existing instances of a class. We introduced this example, because we think that it may help to make the following explanations easier to understand. In this case, each instance will possess this name as well. If so, the value can be retrieved. If an attribute name is not in included in either of the dictionary, the attribute name is not defined. Even function names can be attributed. We use a counter attribute in the following example: def f x : f. There may some uncertainty arise at this point. It is possible to assign attributes to most class instances, but this has nothing to do with defining classes. We will see soon how to assign attributes when we define a class. To properly create instances of classes we also need methods. You will learn in the following subsection of our Python tutorial, how you can define methods. Methods We will demonstrate now, how we can define methods in classes. Usually, it will be called like this: x. It's just a naming convention. Most other object-oriented programming languages pass the reference to the object self as a hidden parameter to the methods. You saw before that the calls Robot. Python automatically binds it to the instance name. Before you proceed with the following text, you may mull over the previous example for awhile. Can you figure out, what is wrong in the design. There is more than one thing about this code, which may disturb you, but the essential problem at the moment is the fact that we create a robot and that after the creation, we shouldn't forget about naming it. We need a mechanism to initialize an instance right after its creation. This name is fixed and it is not possible to chose another name. There is no explicit constructor or destructor method in Python, as they are known in C++ and Java. python 3 object oriented programming pdf But there is a difference. Encapsulation is seen as the bundling of data with the methods that operate on that data. Data encapsulation via methods doesn't necessarily mean that the data is hidden. You might be capable of accessing and seeing the data anyway, but using the methods is recommended. Finally, data abstraction is present, if both data hiding and data encapsulation is used. Getter methods do not change the values of attributes, they just return the values. The methods used for changing the values of attributes are called setter methods. We will define now a Robot class with a Getter and a Setter for the name attribute. There is still something wrong with our Robot class. This can be prevented by using private attributes, which we will explain later. It also produces a string representation. If the method does exist, it will be called. The result of this parsing is in an equal object. The String created by str can't be turned into a datetime. We will extend our robot class with a repr method. Shooting as soon as somebody enters their property. Maybe he hasn't seen the sign, maybe the sign is hard to be seen. Trespassing is a criminal offence in Massachusetts. He was innocent anyway, because the signage was inadequate in the area. Should the fence keep the dog in the yard or the burglar in the street. Choose your fence: Wood panel fencing, post-and-rail fencing, chain-link fencing with or without barbed wire and so on. We have a similar situation in the design of object-oriented programming languages. The first decision to take is how to protect the data which should be private. The second decision is what to do if trespassing, i. Of course, the private data may be protected in a way that it can't be accessed under no circumstances. Some owners allow a restricted access to their property. A third kind of property might be public property like streets or parks, where it is perfectly legal to be. Essentially, this means that they should only be used under certain conditions. Python uses a special naming scheme for attributes to control the accessibility of the attributes. So far, we have used attribute names, which can be freely used inside or outside of a class definition, as we have seen. This corresponds to public attributes of course. This marks the attribute as protected. It tells users of the class not to use this attribute unless, somebody writes a subclass. We will learn about inheritance and subclassing in the next chapter of our tutorial. The attribute is now inaccessible and invisible from outside. It's neither possible to read nor write to those attributes except inside of the class definition itself. It's neither possible to read nor write to those attributes, except inside of the class definition itself. There is such an attribute, but we are told that there isn't. This is perfect information hiding. Telling a user that an attribute name is private, means that we make some information visible, i. Our next task consists in rewriting our Robot class. Data Encapsulation means, that we should only be able to access private attributes via getters and setters. We have to replace each occurrence of self. Hi, I am Caliban I was built in the year 1993. Every private attribute of our class has a getter and a setter. First of all not every private attribute needs to be accessed from outside. Second, we will create non-pythonic Code this way, as you will learn soon. Destructor What we said about constructors holds true for destructors as well. It is called when the instance is about to be destroyed and if there is no other reference to this instance. We will learn later, why this is the case. All data in a Python program is represented by objects or by relations between objects. Every object has an identity, a type and a value. In our example, we can do it like this: x.
