A constructor is a special type of method (function) which is used to initialize the instance members of the class.
In C++ or Java, the constructor has the same name as its class, but it treats constructor differently in Python. It is used to create an object.
Constructors can be of two types.
- Parameterized Constructor
- Non-parameterized Constructor
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.
Creating the constructor in python
In Python, the method the __init__() simulates the constructor of the class. This method is called when the class is instantiated. It accepts the self-keyword as a first argument which allows accessing the attributes or method of the class.
We can pass any number of arguments at the time of creating the class object, depending upon the __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.
Example
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()Output:ID: 101 Name: John ID: 102 Name: David
Counting the number of objects of a class
The constructor is called automatically when we create the object of the class. Consider the following example.
Example
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)Output:The number of students: 3
Python Non-Parameterized Constructor
The non-parameterized constructor uses when we do not want to manipulate the value or the constructor that has only self as an argument. Consider the following example.
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
The parameterized constructor has multiple parameters along with the self. Consider the following example.
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()Output:This is parametrized constructor Hello John
Python Default Constructor
When we do not include the constructor in the class or forget to declare it, then that becomes the default constructor. It does not perform any task but initializes the objects. Consider the following example.
Example
class Student:
roll_num = 101
name = "Joseph"
def display(self):
print(self.roll_num,self.name)
st = Student()
st.display()Output:101 Joseph
More than One Constructor in Single class
Let’s have a look at another scenario, what happen if we declare the two same constructors in the class.
Example
class Student:
def __init__(self):
print("The First Constructor")
def __init__(self):
print("The second contructor")
st = Student()Output:
In the above code, the object st called the second constructor whereas both have the same configuration. The first method is not accessible by the st object. Internally, the object of the class will always call the last constructor if the class has multiple constructors.
Note: The constructor overloading is not allowed in Python.
Python built-in class functions
The built-in functions defined in the class are described in the following table.
| SN | Function | Description |
|---|---|---|
| 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. |
Example
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)Output:John 23 True AttributeError: ‘Student’ object has no attribute ‘age’
Built-in class attributes
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.
| SN | Attribute | Description |
|---|---|---|
| 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. |
Example
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__)Output:None {‘name’: ‘John’, ‘id’: 101, ‘age’: 22} __main__
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