Python Note4 (OOP)

Labels: Python, Class,Object

Ref:
Python Classes/Objects https://www.tutorialspoint.com/python/python_classes_objects.htm

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Classes

• Creating Class
  The name of the class immediately follows the keyword class followed by a colon as follow:

class ClassName:
   'Optional class documentation string'
   class_components

Example

class Employee:
   'Common base class for all employees'
   empCount = 0
   def __init__(self, name, salary):
      self.name = name
      self.salary = salary
      Employee.empCount += 1
   def displayEmployee(self):
      print "Name : ", self.name,  ", Salary: ", self.salary

__attribute: Private attribute ('__'). Not be visible outside the class definition.

• Creating instance objects

# This would create first object of Employee class
emp1 = Employee("Zara", 2000)

• Accessing attributes and functions

emp1.displayEmployee()
print "Total Employee %d" % Employee1.empCount

object._className__attrName.: Access the private (hided) attributes.

• Attributes accessing functions

Function	Description
getattr(obj, name[, default])	access the attribute of object.
hasattr(obj,name)	check if an attribute exists or not.
setattr(obj,name,value)	set an attribute. If attribute does not exist, then it would be created.
delattr(obj, name)	delete an attribute.

Examples:

hasattr(emp1, 'age')    # Returns true if 'age' attribute exists
getattr(emp1, 'age')    # Returns value of 'age' attribute
setattr(emp1, 'age', 8) # Set attribute 'age' at 8
delattr(empl, 'age')    # Delete attribute 'age'

• Class Built-in Attributes
  It can be accessed using dot operator.

Attribute	Description
__dict__	Dictionary containing the class's namespace.
__doc__	Class documentation string or none, if undefined.
__name__	Class name.
__module__	Module name in which the class is defined. This attribute is __main__ in interactive mode.
__bases__	A possibly empty tuple containing the base classes, in the order of their occurrence in the base class list.

Example:

class Employee:
  'Common base class for all employees'
  empCount = 0
  def displayEmployee(self):
     print "Name : ", self.name,  ", Salary: ", self.salary
print "Employee.__doc__:", Employee.__doc__
print "Employee.__name__:", Employee.__name__
print "Employee.__module__:", Employee.__module__
print "Employee.__bases__:", Employee.__bases__
print "Employee.__dict__:", Employee.__dict__

Result:

Employee.__doc__: Common base class for all employees
Employee.__name__: Employee
Employee.__module__: __main__
Employee.__bases__: ()
Employee.__dict__: {'__module__': '__main__',  'empCount': 2, 
'displayEmployee': , 
'__doc__': 'Common base class for all employees'}

• Class Inheritance
  The child class inherits the attributes of its parent class.
  A child class can also override data members and methods from the parent.

class SubClassName (ParentClass1[, ParentClass2, ...]):
   'Optional class documentation string'
   class_suite

A class from multiple parent classes.

class A:        # define your class A
.....
class B:         # define your class B
.....
class C(A, B):   # subclass of A and B
.....

Check a relationships of two classes and instances:

• issubclass(sub, sup) returns true if the given subclass sub is indeed a subclass of the superclass sup.

• isinstance(obj, Class) returns true if obj is an instance of class Class or is an instance of a subclass of Class

• Methods Overloading

• Base Overloading Methods

Method	Description	Sample Call
__init__(self [,args...])	Constructor (with any optional arguments)	obj = className(args)
__del__( self )	Destructor, deletes an object	del obj
__repr__( self )	Evaluatable string representation	repr(obj)
__str__( self )	Printable string representation	str(obj)
__cmp__ ( self, x )	Object comparison	cmp(obj, x)