In an object oriented python program, you can restrict access to methods and variables. This can prevent the data from being modified by accident and is known as encapsulation. Let’s start with an example.
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We create a class Car which has two methods: drive() and updateSoftware(). When a car object is created, it will call the private methods __updateSoftware().
This function cannot be called on the object directly, only from within the class.
#!/usr/bin/env python class Car: def __init__(self): self.__updateSoftware() def drive(self): print 'driving' def __updateSoftware(self): print 'updating software' redcar = Car() redcar.drive() #redcar.__updateSoftware() not accesible from object.
This program will output:
updating software driving
The private method __updateSoftware() can only be called within the class itself. It can never be called from outside the class.
Variables can be private which can be useful on many occasions. A private variable can only be changed within a class method and not outside of the class.
Objects can hold crucial data for your application and you do not want that data to be changeable from anywhere in the code.
#!/usr/bin/env python class Car: __maxspeed = 0 __name = "" def __init__(self): self.__maxspeed = 200 self.__name = "Supercar" def drive(self): print 'driving. maxspeed ' + str(self.__maxspeed) redcar = Car() redcar.drive() redcar.__maxspeed = 10 # will not change variable because its private redcar.drive()
If you want to change the value of a private variable, a setter method is used. This is simply a method that sets the value of a private variable.
#!/usr/bin/env python class Car: __maxspeed = 0 __name = "" def __init__(self): self.__maxspeed = 200 self.__name = "Supercar" def drive(self): print 'driving. maxspeed ' + str(self.__maxspeed) def setMaxSpeed(self,speed): self.__maxspeed = speed redcar = Car() redcar.drive() redcar.setMaxSpeed(320) redcar.drive()
Why would you create them? Because some of the private values you may want to change after creation of the object while others may not need to be changed at all.
To summarize, in Python there are:
|public methods||Accessible from anywhere|
|private methods||Accessible only in their own class. starts with two underscores|
|public variables||Accessible from anywhere|
|private variables||Accesible only in their own class or by a method if defined. starts with two underscores|
Other programming languages have protected class methods too, but Python does not.
Encapsulation gives you more control over the degree of coupling in your code, it allows a class to change its implementation without affecting other parts of the code.