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Safari | Python Developer's Handbook -> Python Classes and Instances





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Python Classes and Instances

In Python, a class is a user-defined data type, and as in most other languages, you define Python classes using the keyword class.

					
class <class name>:
    <class statements>

				

The class statements section contains any valid Python statement that defines class constants or class methods. Note that the contents of the variable namespace formed by executing the commands in the class statement make up the class dictionary.

Two ways to create classes are

  • You can define it from scratch.

    							
    class <class name>:
        ["documentation text"]
        <class statements>
    
    						
  • You can create a new class that inherits properties of other classes. This is called subclassing, and you will learn more about it later in this chapter.

    							
    class <class name> [(baseclass1, baseclass2, )]:
        ["documentation text"]
        <statements>
    
    						

A class definition starts at the keyword class and ends at the last line of the indented block of code that goes underneath.

Methods and class constants define a class namespace. Usually, a class has several methods, and they must all start with the keyword def.

Tip

Methods are how to call functions in a class.



All methods have the additional argument self as the first argument in the method headerThe convention is to call it self because it could be any other name. Python's self argument is similar to the this keyword in C++. Its function is to transport a reference of the object in a way that when a method is called, it knows which object should be used.

					
>>> class a:
     def __init__(self):
         print self

>>> b = a()
>>> b
<__main__.a instance at 795420>

				

In order to reference an attribute within a class, you need to use either self.attribute or classname.attribute. Note that the self.attribute syntax is to remove ambiguities between instance variables and function local variables. Also, self.attribute and classname.attribute are different. The second sets class attributes, which will affect all instances of the class.

					
>>> class c:
     def __init__(self, value=None):
         self.name = value


				

To reference an attribute while using a class instance, you have to use instancename.attribute.

					
>>> obj.name

				

A class can also contain class variable assignments. These variables are shared by all the class instances. Class variables are useful when the assignment of default values to instances is required. Class variables do not have the self. prefix.

For example

					
>>> class Student:
    default_age = 20                      # class variable
    def __init__ (self):
        self.age = Student.default_age    # instance variable

				

Note that in the previous example, we had to use Student.default_age instead of using only default_age because the global namespace for a method is the module in which it was definednot the class namespace.

The next example creates an instance variable that has the same name of the class variable.

					
>>> class Student:
     default_age = 20                 # class variable
     def __init__ (self, age):
         self.default_age = age       # instance variable

				

Suppose that you have the following code stored in a file called c:\ python\ studentfile.py. This code defines three different variables named default_age (at lines 2, 4, and 9).

					
1: class Student:
2:     default_age = 20               # base class variable
3:     def __init__(self, age):
4:         self.default_age = age     # base class instance variable
5:
6: class Newstudent(Student):
7:     "New student class"
8:     def __init__(self, age=20):
9:         self.default_age = age     # instance variable

				

The following code imports the previous module. Which variable is being used by the instance call at line 5?

					
1: >>> import sys
2: >>> sys.path = sys.path + ['c:\ \ python']
3: >>> import studentfile
4: >>> Joao = studentfile.Newstudent(15)
5: >>> Joao.default_age
6: 15

				

Tip

In order for Python to find your modules, the directory where you save them must be an entry of the sys.path list.



The answer is the instance variable of the newstudent class (line 9 from the first listing). In cases like this, the search order is defined as

  1. instance variables

  2. class variables

  3. base classes variablesnote that the search order for base classes makes the deepest-level classes used first

    							
    >>> Renata = studentfile.newstudent()
    >>> print Renata.default_age
    20
    
    						

The following variation is slightly different than the previous code. This example shows what you need to do to make the class Newstudent call the superclass's __init__ method.

					
6: class Newstudent(Student):
7:     "New student class"
8:     def __init__(self):
9:         Student.__init__(self, Student.default_age)

				

Note that we are calling the __init__ method of the Student class (the superclass). The class constant Student.default_age is also used in this example. It is important to say that when calling unbound methods (methods that are not tied to an instance) like this one, you must explicitly say that the first argument is self.

					
1: >>> Joao = studentfile.Newstudent()
2: >>> Joao.default_age
3: 20

				

Attributes of a Class

Next, I list the attributes that classes expose to programmers.

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