Object Oriented Programming in Python | Set 2 (Data Hiding and Object Printing)

Prerequisite: Object-Oriented Programming in Python | Set 1 (Class, Object and Members)

 
Data hiding 

In Python, we use double underscore (Or __) before the attributes name and those attributes will not be directly visible outside. 

class MyClass:

    # Hidden member of MyClass
    __hiddenVariable = 0
  
    # A member method that changes 
    # __hiddenVariable 
    def add(self, increment):
        self.__hiddenVariable += increment
        print (self.__hiddenVariable)
 
# Driver code
myObject = MyClass()     
myObject.add(2)
myObject.add(5)

# This line causes error
print (myObject.__hiddenVariable)

Output : 

2
7
Traceback (most recent call last):
  File "filename.py", line 13, in 
    print (myObject.__hiddenVariable)
AttributeError: MyClass instance has 
no attribute '__hiddenVariable' 

In the above program, we tried to access a hidden variable outside the class using an object and it threw an exception.
We can access the value of a hidden attribute by a tricky syntax: 

# A Python program to demonstrate that hidden
# members can be accessed outside a class
class MyClass:

    # Hidden member of MyClass
    __hiddenVariable = 10

# Driver code
myObject = MyClass()     
print(myObject._MyClass__hiddenVariable)

Output : 

10

Private methods are accessible outside their class, just not easily accessible. Nothing in Python is truly private; internally, the names of private methods and attributes are mangled and unmangled on the fly to make them seem inaccessible by their given names [See this for source ]. 

 Printing Objects 

Printing objects give us information about objects we are working with. In C++, we can do this by adding a friend ostream& operator << (ostream&, const Foobar&) method for the class. In Java, we use toString() method.
In python, this can be achieved by using __repr__ or __str__ methods.

class Test:
    def __init__(self, a, b):
        self.a = a
        self.b = b

    def __repr__(self):
        return "Test a:%s b:%s" % (self.a, self.b)

    def __str__(self):
        return "From str method of Test: a is %s," \
              "b is %s" % (self.a, self.b)

# Driver Code        
t = Test(1234, 5678)
print(t) # This calls __str__()
print([t]) # This calls __repr__()

Output : 

From str method of Test: a is 1234,b is 5678
[Test a:1234 b:5678]

Important Points about Printing: 

• If no __str__ method is defined, print t (or print str(t)) uses __repr__. 

class Test:
    def __init__(self, a, b):
        self.a = a
        self.b = b

    def __repr__(self):
        return "Test a:%s b:%s" % (self.a, self.b)

# Driver Code        
t = Test(1234, 5678)
print(t) 

Output :

Test a:1234 b:5678

• If no __repr__ method is defined then the default is used. 

class Test:
    def __init__(self, a, b):
        self.a = a
        self.b = b

# Driver Code        
t = Test(1234, 5678)
print(t) 

Output :

<__main__.Test instance at 0x7fa079da6710>