Where are instance variables defined in a Python class
In this article, we will learn about the types of variables offered by Python’s object model i.e. class variables and instance variables. OOPs allows for variables to be used as a class variable or as instance variable. But both have their own pros and cons. Show
Class VariablesClass variables are those variables that are defined within a class and before the class constructor i,e, __init__ method. Because class variables are shared by all instances of the class and owned by the class itself. Class variables are typically placed below the class name and outside of instance methods. Let’s take an example: class Person: person_name = "Albert" #Class Variable new_person = Person() print(new_person.person_name) So in the above example, we declared a class Person and inside that class, we declared a variable. That variable is known as Class Variable. OutPut: Albert This program returns the value of the variable. Class Variables can also contain any data type ( Like any other variable, ). Instance VariablesInstance variables are declared inside the constructor method of the class i.e the __init__ method. Every object has its own value in an instance variable. Let’s take an example: class Person: def __init__(self, name, age): self.name = name #Instance Variable self.age = age #Instance Variable new_person = Person("Diptam", 20) print(new_person.name) print(new_person.age) In this example, we created a Person class, and name and age are instance variables OutPut: Diptam 20 The output we receive is made up of the values of the variables that we initialized for the object instance of the new_person. as the Person class assigned the values to the instance variables. So it is clear that instance variables are owned by Objects of the class and each object can assign different values to those variables. Differences Between Class Variable and Instance Variable in PythonLet’s again start with a short code: class Bike: wheels = 2 # Class variable def __init__(self, name): self.name = name # Instance variable The above code has a class name and one class variable and one instance variable. Since each bike model will have the same number of wheels, hence we declare that as a class variable. So now let’s call the class: suzuki = Bike("Suzuki") bmw = Bike("BMW") print(suzuki.name, bmw.name) Output: Suzuki BMW So accessing the instance variable name is pretty straightforward. However, accessing the class variable has a little more flexibility. We can call the class variable with the object name as well as with the class name: print(suzuki.wheels) print(bmw.wheels) print(Bike.wheels) Output: 2 2 2 But calling the instance variable with the class name (E.g. Bike.name) will through an attribute error since instance variables are object-oriented and are created when __init__ is invoked. Now assume BMW has 1 wheel, in that case, if you modify the wheel of BMW, (e.g bmw.wheels = 1) it will change all bike’s wheel to 1. So changing or modifying a class variable affects all instances of the class. But this will not happen if we call wheels as an instance variable. Python has been an object-oriented language since it existed. Because of this, creating and using classes and objects are downright easy. This chapter helps you become an expert in using Python's object-oriented programming support. If you do not have any previous experience with object-oriented (OO) programming, you may want to consult an introductory course on it or at least a tutorial of some sort so that you have a grasp of the basic concepts. However, here is small introduction of Object-Oriented Programming (OOP) to bring you at speed − Overview of OOP Terminology
Creating ClassesThe class statement creates a new class definition. The name of the class immediately follows the keyword class followed by a colon as follows − class ClassName: 'Optional class documentation string' class_suite
ExampleFollowing is the example of a simple Python class − 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 displayCount(self): print "Total Employee %d" % Employee.empCount def displayEmployee(self): print "Name : ", self.name, ", Salary: ", self.salary
Creating Instance ObjectsTo create instances of a class, you call the class using class name and pass in whatever arguments its __init__ method accepts. "This would create first object of Employee class" emp1 = Employee("Zara", 2000) "This would create second object of Employee class" emp2 = Employee("Manni", 5000) Accessing AttributesYou access the object's attributes using the dot operator with object. Class variable would be accessed using class name as follows − emp1.displayEmployee() emp2.displayEmployee() print "Total Employee %d" % Employee.empCount Now, putting all the concepts together − #!/usr/bin/python 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 displayCount(self): print "Total Employee %d" % Employee.empCount def displayEmployee(self): print "Name : ", self.name, ", Salary: ", self.salary "This would create first object of Employee class" emp1 = Employee("Zara", 2000) "This would create second object of Employee class" emp2 = Employee("Manni", 5000) emp1.displayEmployee() emp2.displayEmployee() print "Total Employee %d" % Employee.empCount When the above code is executed, it produces the following result − Name : Zara ,Salary: 2000 Name : Manni ,Salary: 5000 Total Employee 2 You can add, remove, or modify attributes of classes and objects at any time − emp1.age = 7 # Add an 'age' attribute. emp1.age = 8 # Modify 'age' attribute. del emp1.age # Delete 'age' attribute. Instead of using the normal statements to access attributes, you can use the following functions −
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' Built-In Class AttributesEvery Python class keeps following built-in attributes and they can be accessed using dot operator like any other attribute −
For the above class let us try to access all these attributes − #!/usr/bin/python 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 displayCount(self): print "Total Employee %d" % Employee.empCount 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__ When the above code is executed, it produces the following result − Employee.__doc__: Common base class for all employees Employee.__name__: Employee Employee.__module__: __main__ Employee.__bases__: () Employee.__dict__: {'__module__': '__main__', 'displayCount': , 'empCount': 2, 'displayEmployee': , '__doc__': 'Common base class for all employees', '__init__': } Destroying Objects (Garbage Collection)Python deletes unneeded objects (built-in types or class instances) automatically to free the memory space. The process by which Python periodically reclaims blocks of memory that no longer are in use is termed Garbage Collection. Python's garbage collector runs during program execution and is triggered when an object's reference count reaches zero. An object's reference count changes as the number of aliases that point to it changes. An object's reference count increases when it is assigned a new name or placed in a container (list, tuple, or dictionary). The object's reference count decreases when it's deleted with del, its reference is reassigned, or its reference goes out of scope. When an object's reference count reaches zero, Python collects it automatically. 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 displayCount(self): print "Total Employee %d" % Employee.empCount def displayEmployee(self): print "Name : ", self.name, ", Salary: ", self.salary0 You normally will not notice when the garbage collector destroys an orphaned instance and reclaims its space. But a class can implement the special method __del__(), called a destructor, that is invoked when the instance is about to be destroyed. This method might be used to clean up any non memory resources used by an instance. ExampleThis __del__() destructor prints the class name of an instance that is about to be destroyed − 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 displayCount(self): print "Total Employee %d" % Employee.empCount def displayEmployee(self): print "Name : ", self.name, ", Salary: ", self.salary1 When the above code is executed, it produces following result − 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 displayCount(self): print "Total Employee %d" % Employee.empCount def displayEmployee(self): print "Name : ", self.name, ", Salary: ", self.salary2 Note − Ideally, you should define your classes in separate file, then you should import them in your main program file using import statement. Class InheritanceInstead of starting from scratch, you can create a class by deriving it from a preexisting class by listing the parent class in parentheses after the new class name. The child class inherits the attributes of its parent class, and you can use those attributes as if they were defined in the child class. A child class can also override data members and methods from the parent. SyntaxDerived classes are declared much like their parent class; however, a list of base classes to inherit from is given after the class name − 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 displayCount(self): print "Total Employee %d" % Employee.empCount def displayEmployee(self): print "Name : ", self.name, ", Salary: ", self.salary3 Exampleclass Employee: 'Common base class for all employees' empCount = 0 def __init__(self, name, salary): self.name = name self.salary = salary Employee.empCount += 1 def displayCount(self): print "Total Employee %d" % Employee.empCount def displayEmployee(self): print "Name : ", self.name, ", Salary: ", self.salary4 When the above code is executed, it produces the following result − 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 displayCount(self): print "Total Employee %d" % Employee.empCount def displayEmployee(self): print "Name : ", self.name, ", Salary: ", self.salary5 Similar way, you can drive a class from multiple parent classes as follows − 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 displayCount(self): print "Total Employee %d" % Employee.empCount def displayEmployee(self): print "Name : ", self.name, ", Salary: ", self.salary6 You can use issubclass() or isinstance() functions to check a relationships of two classes and instances.
Overriding MethodsYou can always override your parent class methods. One reason for overriding parent's methods is because you may want special or different functionality in your subclass. Exampleclass Employee: 'Common base class for all employees' empCount = 0 def __init__(self, name, salary): self.name = name self.salary = salary Employee.empCount += 1 def displayCount(self): print "Total Employee %d" % Employee.empCount def displayEmployee(self): print "Name : ", self.name, ", Salary: ", self.salary7 When the above code is executed, it produces the following result − 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 displayCount(self): print "Total Employee %d" % Employee.empCount def displayEmployee(self): print "Name : ", self.name, ", Salary: ", self.salary8 Base Overloading MethodsFollowing table lists some generic functionality that you can override in your own classes − Sr.No.Method, Description & Sample Call1__init__ ( self [,args...] ) Constructor (with any optional arguments) Sample Call : obj = className(args) 2__del__( self ) Destructor, deletes an object Sample Call : del obj 3__repr__( self ) Evaluable string representation Sample Call : repr(obj) 4__str__( self ) Printable string representation Sample Call : str(obj) 5__cmp__ ( self, x ) Object comparison Sample Call : cmp(obj, x) Overloading OperatorsSuppose you have created a Vector class to represent two-dimensional vectors, what happens when you use the plus operator to add them? Most likely Python will yell at you. You could, however, define the __add__ method in your class to perform vector addition and then the plus operator would behave as per expectation − Exampleclass Employee: 'Common base class for all employees' empCount = 0 def __init__(self, name, salary): self.name = name self.salary = salary Employee.empCount += 1 def displayCount(self): print "Total Employee %d" % Employee.empCount def displayEmployee(self): print "Name : ", self.name, ", Salary: ", self.salary9 When the above code is executed, it produces the following result − "This would create first object of Employee class" emp1 = Employee("Zara", 2000) "This would create second object of Employee class" emp2 = Employee("Manni", 5000)0 Data HidingAn object's attributes may or may not be visible outside the class definition. You need to name attributes with a double underscore prefix, and those attributes then are not be directly visible to outsiders. Example"This would create first object of Employee class" emp1 = Employee("Zara", 2000) "This would create second object of Employee class" emp2 = Employee("Manni", 5000)1 When the above code is executed, it produces the following result − "This would create first object of Employee class" emp1 = Employee("Zara", 2000) "This would create second object of Employee class" emp2 = Employee("Manni", 5000)2 Python protects those members by internally changing the name to include the class name. You can access such attributes as object._className__attrName. If you would replace your last line as following, then it works for you − Where are instance variables declared in a class?Instance variables are declared at the same level as methods within a class definition. They are usually given private access to restrict visibility. They can receive initial values either when they are declared or in a constructor.
Where is an instance variable defined?An instance variable is a variable which is declared in a class but outside of constructors, methods, or blocks. Instance variables are created when an object is instantiated, and are accessible to all the constructors, methods, or blocks in the class. Access modifiers can be given to the instance variable.
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