Python - Public, Protected, Private Members
Classical object-oriented languages, such as C++ and Java, control the access to class resources by public, private, and protected keywords. Private members of the class are denied access from the environment outside the class. They can be handled only from within the class.
Public members (generally methods declared in a class) are accessible from outside the class. The object of the same class is required to invoke a public method. This arrangement of private instance variables and public methods ensures the principle of data encapsulation.
All members in a Python class are public by default. Any member can be accessed from outside the class environment.
class Student: schoolName = 'XYZ School' # class attribute def __init__(self, name, age): self.name=name # instance attribute self.age=age # instance attribute
You can access the
Student class's attributes and also modify their values, as shown below.
>>> std = Student("Steve", 25) >>> std.schoolName 'XYZ School' >>> std.name 'Steve' >>> std.age = 20 >>> std.age 25
Protected members of a class are accessible from within the class and are also available to its sub-classes. No other environment is permitted access to it. This enables specific resources of the parent class to be inherited by the child class.
Python's convention to make an instance variable protected is to add a prefix _ (single underscore) to it. This effectively prevents it from being accessed unless it is from within a sub-class.
class Student: _schoolName = 'XYZ School' # protected class attribute def __init__(self, name, age): self._name=name # protected instance attribute self._age=age # protected instance attribute
In fact, this doesn't prevent instance variables from accessing or modifying the instance. You can still perform the following operations:
>>> std = Student("Swati", 25) >>> std._name 'Swati' >>> std._name = 'Dipa' >>> std._name 'Dipa'
However, you can define a property using property decorator and make it protected, as shown below.
class Student: def __init__(self,name): self._name = name @property def name(self): return self._name @name.setter def name(self,newname): self._name = newname
Above, @property decorator is used to make the
name() method as property and
@name.setter decorator to another overloads of the
name() method as property setter method. Now,
_name is protected.
>>> std = Student("Swati") >>> std.name 'Swati' >>> std.name = 'Dipa' >>> std.name 'Dipa' >>> std._name # still accessible
Above, we used
std.name property to modify
_name attribute. However, it is still accessible in Python.
Hence, the responsible programmer would refrain from accessing and modifying instance variables prefixed with
_ from outside its class.
Python doesn't have any mechanism that effectively restricts access to any instance variable or method. Python prescribes a convention of prefixing the name of the variable/method with a single or double underscore to emulate the behavior of protected and private access specifiers.
The double underscore
__ prefixed to a variable makes it private.
It gives a strong suggestion not to touch it from outside the class. Any attempt to do so will result in an AttributeError:
class Student: __schoolName = 'XYZ School' # private class attribute def __init__(self, name, age): self.__name=name # private instance attribute self.__salary=age # private instance attribute def __display(self): # private method print('This is private method.')
>>> std = Student("Bill", 25) >>> std.__schoolName AttributeError: 'Student' object has no attribute '__schoolName' >>> std.__name AttributeError: 'Student' object has no attribute '__name' >>> std.__display() AttributeError: 'Student' object has no attribute '__display'
Python performs name mangling of private variables. Every member with a double underscore will be changed to
_object._class__variable. So, it can still be accessed from outside the class, but the practice should be refrained.
>>> std = Student("Bill", 25) >>> std._Student__name 'Bill' >>> std._Student__name = 'Steve' >>> std._Student__name 'Steve' >>> std._Student__display() 'This is private method.'
Thus, Python provides conceptual implementation of public, protected, and private access modifiers, but not like other languages like C#, Java, C++.