A constructor is a special type of method (function) which is used to initialize the instance members of the class.
Constructors can be of two types.
Constructor definition is executed when we create the object of this class. Constructors also verify that there are enough resources for the object to perform any start-up task.
In python, the method __init__ simulates the constructor of the class. This method is called when the class is instantiated. We can pass any number of arguments at the time of creating the class object, depending upon __init__ definition. It is mostly used to initialize the class attributes. Every class must have a constructor, even if it simply relies on the default constructor.
Consider the following example to initialize the Employee class attributes.
class Employee: def __init__(self,name,id): self.id = id; self.name = name; def display (self): print("ID: %d \nName: %s"%(self.id,self.name)) emp1 = Employee("John",101) emp2 = Employee("David",102) #accessing display() method to print employee 1 information emp1.display(); #accessing display() method to print employee 2 information emp2.display();
Example: Counting the number of objects of a class
class Student: count = 0 def __init__(self): Student.count = Student.count + 1 s1=Student() s2=Student() s3=Student() print("The number of students:",Student.count)
Python Non-Parameterized Constructor Example
class Student: # Constructor - non parameterized def __init__(self): print("This is non parametrized constructor") def show(self,name): print("Hello",name) student = Student() student.show("John")
Python Parameterized Constructor Example
class Student: # Constructor - parameterized def __init__(self, name): print("This is parametrized constructor") self.name = name def show(self): print("Hello",self.name) student = Student("John") student.show()
The in-built functions defined in the class are described in the following table.
|1||getattr(obj,name,default)||It is used to access the attribute of the object.|
|2||setattr(obj, name,value)||It is used to set a particular value to the specific attribute of an object.|
|3||delattr(obj, name)||It is used to delete a specific attribute.|
|4||hasattr(obj, name)||It returns true if the object contains some specific attribute.|
class Student: def __init__(self,name,id,age): self.name = name; self.id = id; self.age = age #creates the object of the class Student s = Student("John",101,22) #prints the attribute name of the object s print(getattr(s,'name')) # reset the value of attribute age to 23 setattr(s,"age",23) # prints the modified value of age print(getattr(s,'age')) # prints true if the student contains the attribute with name id print(hasattr(s,'id')) # deletes the attribute age delattr(s,'age') # this will give an error since the attribute age has been deleted print(s.age)
Along with the other attributes, a python class also contains some built-in class attributes which provide information about the class.
The built-in class attributes are given in the below table.
|1||__dict__||It provides the dictionary containing the information about the class namespace.|
|2||__doc__||It contains a string which has the class documentation|
|3||__name__||It is used to access the class name.|
|4||__module__||It is used to access the module in which, this class is defined.|
|5||__bases__||It contains a tuple including all base classes.|
class Student: def __init__(self,name,id,age): self.name = name; self.id = id; self.age = age def display_details(self): print("Name:%s, ID:%d, age:%d"%(self.name,self.id)) s = Student("John",101,22) print(s.__doc__) print(s.__dict__) print(s.__module__)