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.
Python
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 codemyObject = MyClass() myObject.add(2)myObject.add(5) # This line causes errorprint (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:
Python
# A Python program to demonstrate that hidden# members can be accessed outside a classclass MyClass: # Hidden member of MyClass __hiddenVariable = 10 # Driver codemyObject = 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.
Python
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__.
Python
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.
Python
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>
