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)
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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)
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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__()
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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)
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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)
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Output :
<__main__.Test instance at 0x7fa079da6710>